code:
const SemVer = require('../classes/semver')
const Comparator = require('../classes/comparator')
const { ANY } = Comparator
const Range = require('../classes/range')
const satisfies = require('../functions/satisfies')
const gt = require('../functions/gt')
const lt = require('../functions/lt')
const lte = require('../functions/lte')
const gte = require('../functions/gte')

const outside = (version, range, hilo, options) => {
  version = new SemVer(version, options)
  range = new Range(range, options)

  let gtfn, ltefn, ltfn, comp, ecomp
  switch (hilo) {
    case '>':
      gtfn = gt
      ltefn = lte
      ltfn = lt
      comp = '>'
      ecomp = '>='
      break
    case '<':
      gtfn = lt
      ltefn = gte
      ltfn = gt
      comp = '<'
      ecomp = '<='
      break
    default:
      throw new TypeError('Must provide a hilo val of "<" or ">"')
  }

  // If it satisfies the range it is not outside
  if (satisfies(version, range, options)) {
    return false
  }

  // From now on, variable terms are as if we're in "gtr" mode.
  // but note that everything is flipped for the "ltr" function.

  for (let i = 0; i < range.set.length; ++i) {
    const comparators = range.set[i]

    let high = null
    let low = null

    comparators.forEach((comparator) => {
      if (comparator.semver === ANY) {
        comparator = new Comparator('>=0.0.0')
      }
      high = high || comparator
      low = low || comparator
      if (gtfn(comparator.semver, high.semver, options)) {
        high = comparator
      } else if (ltfn(comparator.semver, low.semver, options)) {
        low = comparator
      }
    })

    // If the edge version comparator has a operator then our version
    // isn't outside it
    if (high.operator === comp || high.operator === ecomp) {
      return false
    }

    // If the lowest version comparator has an operator and our version
    // is less than it then it isn't higher than the range
    if ((!low.operator || low.operator === comp) &&
        ltefn(version, low.semver)) {
      return false
    } else if (low.operator === ecomp && ltfn(version, low.semver)) {
      return false
    }
  }
  return true
}

module.exports = outside


code:
// given a set of versions and a range, create a "simplified" range
// that includes the same versions that the original range does
// If the original range is shorter than the simplified one, return that.
const satisfies = require('../functions/satisfies.js')
const compare = require('../functions/compare.js')
module.exports = (versions, range, options) => {
  const set = []
  let first = null
  let prev = null
  const v = versions.sort((a, b) => compare(a, b, options))
  for (const version of v) {
    const included = satisfies(version, range, options)
    if (included) {
      prev = version
      if (!first) {
        first = version
      }
    } else {
      if (prev) {
        set.push([first, prev])
      }
      prev = null
      first = null
    }
  }
  if (first) {
    set.push([first, null])
  }

  const ranges = []
  for (const [min, max] of set) {
    if (min === max) {
      ranges.push(min)
    } else if (!max && min === v[0]) {
      ranges.push('*')
    } else if (!max) {
      ranges.push(`>=${min}`)
    } else if (min === v[0]) {
      ranges.push(`<=${max}`)
    } else {
      ranges.push(`${min} - ${max}`)
    }
  }
  const simplified = ranges.join(' || ')
  const original = typeof range.raw === 'string' ? range.raw : String(range)
  return simplified.length < original.length ? simplified : range
}

license: mit


code:
const Range = require('../classes/range.js')
const Comparator = require('../classes/comparator.js')
const { ANY } = Comparator
const satisfies = require('../functions/satisfies.js')
const compare = require('../functions/compare.js')

// Complex range `r1 || r2 || ...` is a subset of `R1 || R2 || ...` iff:
// - Every simple range `r1, r2, ...` is a null set, OR
// - Every simple range `r1, r2, ...` which is not a null set is a subset of
//   some `R1, R2, ...`
//
// Simple range `c1 c2 ...` is a subset of simple range `C1 C2 ...` iff:
// - If c is only the ANY comparator
//   - If C is only the ANY comparator, return true
//   - Else if in prerelease mode, return false
//   - else replace c with `[>=0.0.0]`
// - If C is only the ANY comparator
//   - if in prerelease mode, return true
//   - else replace C with `[>=0.0.0]`
// - Let EQ be the set of = comparators in c
// - If EQ is more than one, return true (null set)
// - Let GT be the highest > or >= comparator in c
// - Let LT be the lowest < or <= comparator in c
// - If GT and LT, and GT.semver > LT.semver, return true (null set)
// - If any C is a = range, and GT or LT are set, return false
// - If EQ
//   - If GT, and EQ does not satisfy GT, return true (null set)
//   - If LT, and EQ does not satisfy LT, return true (null set)
//   - If EQ satisfies every C, return true
//   - Else return false
// - If GT
//   - If GT.semver is lower than any > or >= comp in C, return false
//   - If GT is >=, and GT.semver does not satisfy every C, return false
//   - If GT.semver has a prerelease, and not in prerelease mode
//     - If no C has a prerelease and the GT.semver tuple, return false
// - If LT
//   - If LT.semver is greater than any < or <= comp in C, return false
//   - If LT is <=, and LT.semver does not satisfy every C, return false
//   - If GT.semver has a prerelease, and not in prerelease mode
//     - If no C has a prerelease and the LT.semver tuple, return false
// - Else return true

const subset = (sub, dom, options = {}) => {
  if (sub === dom) {
    return true
  }

  sub = new Range(sub, options)
  dom = new Range(dom, options)
  let sawNonNull = false

  OUTER: for (const simpleSub of sub.set) {
    for (const simpleDom of dom.set) {
      const isSub = simpleSubset(simpleSub, simpleDom, options)
      sawNonNull = sawNonNull || isSub !== null
      if (isSub) {
        continue OUTER
      }
    }
    // the null set is a subset of everything, but null simple ranges in
    // a complex range should be ignored.  so if we saw a non-null range,
    // then we know this isn't a subset, but if EVERY simple range was null,
    // then it is a subset.
    if (sawNonNull) {
      return false
    }
  }
  return true
}

const minimumVersionWithPreRelease = [new Comparator('>=0.0.0-0')]
const minimumVersion = [new Comparator('>=0.0.0')]

const simpleSubset = (sub, dom, options) => {
  if (sub === dom) {
    return true
  }

  if (sub.length === 1 && sub[0].semver === ANY) {
    if (dom.length === 1 && dom[0].semver === ANY) {
      return true
    } else if (options.includePrerelease) {
      sub = minimumVersionWithPreRelease
    } else {
      sub = minimumVersion
    }
  }

  if (dom.length === 1 && dom[0].semver === ANY) {
    if (options.includePrerelease) {
      return true
    } else {
      dom = minimumVersion
    }
  }

  const eqSet = new Set()
  let gt, lt
  for (const c of sub) {
    if (c.operator === '>' || c.operator === '>=') {
      gt = higherGT(gt, c, options)
    } else if (c.operator === '<' || c.operator === '<=') {
      lt = lowerLT(lt, c, options)
    } else {
      eqSet.add(c.semver)
    }
  }

  if (eqSet.size > 1) {
    return null
  }

  let gtltComp
  if (gt && lt) {
    gtltComp = compare(gt.semver, lt.semver, options)
    if (gtltComp > 0) {
      return null
    } else if (gtltComp === 0 && (gt.operator !== '>=' || lt.operator !== '<=')) {
      return null
    }
  }

  // will iterate one or zero times
  for (const eq of eqSet) {
    if (gt && !satisfies(eq, String(gt), options)) {
      return null
    }

    if (lt && !satisfies(eq, String(lt), options)) {
      return null
    }

    for (const c of dom) {
      if (!satisfies(eq, String(c), options)) {
        return false
      }
    }

    return true
  }

  let higher, lower
  let hasDomLT, hasDomGT
  // if the subset has a prerelease, we need a comparator in the superset
  // with the same tuple and a prerelease, or it's not a subset
  let needDomLTPre = lt &&
    !options.includePrerelease &&
    lt.semver.prerelease.length ? lt.semver : false
  let needDomGTPre = gt &&
    !options.includePrerelease &&
    gt.semver.prerelease.length ? gt.semver : false
  // exception: <1.2.3-0 is the same as <1.2.3
  if (needDomLTPre && needDomLTPre.prerelease.length === 1 &&
      lt.operator === '<' && needDomLTPre.prerelease[0] === 0) {
    needDomLTPre = false
  }

  for (const c of dom) {
    hasDomGT = hasDomGT || c.operator === '>' || c.operator === '>='
    hasDomLT = hasDomLT || c.operator === '<' || c.operator === '<='
    if (gt) {
      if (needDomGTPre) {
        if (c.semver.prerelease && c.semver.prerelease.length &&
            c.semver.major === needDomGTPre.major &&
            c.semver.minor === needDomGTPre.minor &&
            c.semver.patch === needDomGTPre.patch) {
          needDomGTPre = false
        }
      }
      if (c.operator === '>' || c.operator === '>=') {
        higher = higherGT(gt, c, options)
        if (higher === c && higher !== gt) {
          return false
        }
      } else if (gt.operator === '>=' && !satisfies(gt.semver, String(c), options)) {
        return false
      }
    }
    if (lt) {
      if (needDomLTPre) {
        if (c.semver.prerelease && c.semver.prerelease.length &&
            c.semver.major === needDomLTPre.major &&
            c.semver.minor === needDomLTPre.minor &&
            c.semver.patch === needDomLTPre.patch) {
          needDomLTPre = false
        }
      }
      if (c.operator === '<' || c.operator === '<=') {
        lower = lowerLT(lt, c, options)
        if (lower === c && lower !== lt) {
          return false
        }
      } else if (lt.operator === '<=' && !satisfies(lt.semver, String(c), options)) {
        return false
      }
    }
    if (!c.operator && (lt || gt) && gtltComp !== 0) {
      return false
    }
  }

  // if there was a < or >, and nothing in the dom, then must be false
  // UNLESS it was limited by another range in the other direction.
  // Eg, >1.0.0 <1.0.1 is still a subset of <2.0.0
  if (gt && hasDomLT && !lt && gtltComp !== 0) {
    return false
  }

  if (lt && hasDomGT && !gt && gtltComp !== 0) {
    return false
  }

  // we needed a prerelease range in a specific tuple, but didn't get one
  // then this isn't a subset.  eg >=1.2.3-pre is not a subset of >=1.0.0,
  // because it includes prereleases in the 1.2.3 tuple
  if (needDomGTPre || needDomLTPre) {
    return false
  }

  return true
}

// >=1.2.3 is lower than >1.2.3
const higherGT = (a, b, options) => {
  if (!a) {
    return b
  }
  const comp = compare(a.semver, b.semver, options)
  return comp > 0 ? a
    : comp < 0 ? b
    : b.operator === '>' && a.operator === '>=' ? b
    : a
}

// <=1.2.3 is higher than <1.2.3
const lowerLT = (a, b, options) => {
  if (!a) {
    return b
  }
  const comp = compare(a.semver, b.semver, options)
  return comp < 0 ? a
    : comp > 0 ? b
    : b.operator === '<' && a.operator === '<=' ? b
    : a
}

module.exports = subset


code:
const Range = require('../classes/range')

// Mostly just for testing and legacy API reasons
const toComparators = (range, options) =>
  new Range(range, options).set
    .map(comp => comp.map(c => c.value).join(' ').trim().split(' '))

module.exports = toComparators


code:
const Range = require('../classes/range')
const validRange = (range, options) => {
  try {
    // Return '*' instead of '' so that truthiness works.
    // This will throw if it's invalid anyway
    return new Range(range, options).range || '*'
  } catch (er) {
    return null
  }
}
module.exports = validRange


code:
// Types that are compatible with all supported TypeScript versions.
// It's shared between all TypeScript version-specific definitions.

// Basic
export * from './source/basic';

// Utilities
export {Except} from './source/except';
export {Mutable} from './source/mutable';
export {Merge} from './source/merge';
export {MergeExclusive} from './source/merge-exclusive';
export {RequireAtLeastOne} from './source/require-at-least-one';
export {RequireExactlyOne} from './source/require-exactly-one';
export {PartialDeep} from './source/partial-deep';
export {ReadonlyDeep} from './source/readonly-deep';
export {LiteralUnion} from './source/literal-union';
export {Promisable} from './source/promisable';
export {Opaque} from './source/opaque';
export {SetOptional} from './source/set-optional';
export {SetRequired} from './source/set-required';
export {ValueOf} from './source/value-of';
export {PromiseValue} from './source/promise-value';
export {AsyncReturnType} from './source/async-return-type';
export {ConditionalExcept} from './source/conditional-except';
export {ConditionalKeys} from './source/conditional-keys';
export {ConditionalPick} from './source/conditional-pick';
export {UnionToIntersection} from './source/union-to-intersection';
export {Stringified} from './source/stringified';
export {FixedLengthArray} from './source/fixed-length-array';
export {IterableElement} from './source/iterable-element';
export {Entry} from './source/entry';
export {Entries} from './source/entries';
export {SetReturnType} from './source/set-return-type';
export {Asyncify} from './source/asyncify';

// Miscellaneous
export {PackageJson} from './source/package-json';
export {TsConfigJson} from './source/tsconfig-json';


code:
// These are all the basic types that's compatible with all supported TypeScript versions.
export * from './base';


type-fest

description: A collection of essential TypeScript types

license: (MIT OR CC0-1.0)

repository: sindresorhus/type-fest

//test: xo && tsd && tsc

test: xo && tsc

keywords: typescript ts types utility util utilities omit merge json

@typescript-eslint/ban-types: off

@typescript-eslint/indent: off

node/no-unsupported-features/es-builtins: off


code:
	A collection of essential TypeScript types

[![](https://img.shields.io/badge/unicorn-approved-ff69b4.svg)](https://giphy.com/gifs/illustration-rainbow-unicorn-26AHG5KGFxSkUWw1i)

Many of the types here should have been built-in. You can help by suggesting some of them to the [TypeScript project](https://github.com/Microsoft/TypeScript/blob/master/CONTRIBUTING.md).

Either add this package as a dependency or copy-paste the needed types. No credit required. 👌

PR welcome for additional commonly needed types and docs improvements. Read the [contributing guidelines](.github/contributing.md) first.

## Install

```
$ npm install type-fest
```

*Requires TypeScript >=3.4*

## Usage

```ts
import {Except} from 'type-fest';

type Foo = {
	unicorn: string;
	rainbow: boolean;
};

type FooWithoutRainbow = Except<Foo, 'rainbow'>;
//=> {unicorn: string}
```

## API

Click the type names for complete docs.

### Basic

- [`Primitive`](source/basic.d.ts) - Matches any [primitive value](https://developer.mozilla.org/en-US/docs/Glossary/Primitive).
- [`Class`](source/basic.d.ts) - Matches a [`class` constructor](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Classes).
- [`TypedArray`](source/basic.d.ts) - Matches any [typed array](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/TypedArray), like `Uint8Array` or `Float64Array`.
- [`JsonObject`](source/basic.d.ts) - Matches a JSON object.
- [`JsonArray`](source/basic.d.ts) - Matches a JSON array.
- [`JsonValue`](source/basic.d.ts) - Matches any valid JSON value.
- [`ObservableLike`](source/basic.d.ts) - Matches a value that is like an [Observable](https://github.com/tc39/proposal-observable).

### Utilities

- [`Except`](source/except.d.ts) - Create a type from an object type without certain keys. This is a stricter version of [`Omit`](https://www.typescriptlang.org/docs/handbook/release-notes/typescript-3-5.html#the-omit-helper-type).
- [`Mutable`](source/mutable.d.ts) - Convert an object with `readonly` keys into a mutable object. The inverse of `Readonly<T>`.
- [`Merge`](source/merge.d.ts) - Merge two types into a new type. Keys of the second type overrides keys of the first type.
- [`MergeExclusive`](source/merge-exclusive.d.ts) - Create a type that has mutually exclusive keys.
- [`RequireAtLeastOne`](source/require-at-least-one.d.ts) - Create a type that requires at least one of the given keys.
- [`RequireExactlyOne`](source/require-exactly-one.d.ts) - Create a type that requires exactly a single key of the given keys and disallows more.
- [`PartialDeep`](source/partial-deep.d.ts) - Create a deeply optional version of another type. Use [`Partial<T>`](https://github.com/Microsoft/TypeScript/blob/2961bc3fc0ea1117d4e53bc8e97fa76119bc33e3/src/lib/es5.d.ts#L1401-L1406) if you only need one level deep.
- [`ReadonlyDeep`](source/readonly-deep.d.ts) - Create a deeply immutable version of an `object`/`Map`/`Set`/`Array` type. Use [`Readonly<T>`](https://github.com/Microsoft/TypeScript/blob/2961bc3fc0ea1117d4e53bc8e97fa76119bc33e3/src/lib/es5.d.ts#L1415-L1420) if you only need one level deep.
- [`LiteralUnion`](source/literal-union.d.ts) - Create a union type by combining primitive types and literal types without sacrificing auto-completion in IDEs for the literal type part of the union. Workaround for [Microsoft/TypeScript#29729](https://github.com/Microsoft/TypeScript/issues/29729).
- [`Promisable`](source/promisable.d.ts) - Create a type that represents either the value or the value wrapped in `PromiseLike`.
- [`Opaque`](source/opaque.d.ts) - Create an [opaque type](https://codemix.com/opaque-types-in-javascript/).
- [`SetOptional`](source/set-optional.d.ts) - Create a type that makes the given keys optional.
- [`SetRequired`](source/set-required.d.ts) - Create a type that makes the given keys required.
- [`ValueOf`](source/value-of.d.ts) - Create a union of the given object's values, and optionally specify which keys to get the values from.
- [`PromiseValue`](source/promise-value.d.ts) - Returns the type that is wrapped inside a `Promise`.
- [`AsyncReturnType`](source/async-return-type.d.ts) - Unwrap the return type of a function that returns a `Promise`.
- [`ConditionalKeys`](source/conditional-keys.d.ts) - Extract keys from a shape where values extend the given `Condition` type.
- [`ConditionalPick`](source/conditional-pick.d.ts) - Like `Pick` except it selects properties from a shape where the values extend the given `Condition` type.
- [`ConditionalExcept`](source/conditional-except.d.ts) - Like `Omit` except it removes properties from a shape where the values extend the given `Condition` type.
- [`UnionToIntersection`](source/union-to-intersection.d.ts) - Convert a union type to an intersection type.
- [`Stringified`](source/stringified.d.ts) - Create a type with the keys of the given type changed to `string` type.
- [`FixedLengthArray`](source/fixed-length-array.d.ts) - Create a type that represents an array of the given type and length.
- [`IterableElement`](source/iterable-element.d.ts) - Get the element type of an `Iterable`/`AsyncIterable`. For example, an array or a generator.
- [`Entry`](source/entry.d.ts) - Create a type that represents the type of an entry of a collection.
- [`Entries`](source/entries.d.ts) - Create a type that represents the type of the entries of a collection.
- [`SetReturnType`](source/set-return-type.d.ts) - Create a function type with a return type of your choice and the same parameters as the given function type.
- [`Asyncify`](source/asyncify.d.ts) - Create an async version of the given function type.

### Template literal types

*Note:* These require [TypeScript 4.1 or newer](https://devblogs.microsoft.com/typescript/announcing-typescript-4-1/#template-literal-types).

- [`CamelCase`](ts41/camel-case.d.ts) – Convert a string literal to camel-case (`fooBar`).
- [`KebabCase`](ts41/kebab-case.d.ts) – Convert a string literal to kebab-case (`foo-bar`).
- [`PascalCase`](ts41/pascal-case.d.ts) – Converts a string literal to pascal-case (`FooBar`)
- [`SnakeCase`](ts41/snake-case.d.ts) – Convert a string literal to snake-case (`foo_bar`).
- [`DelimiterCase`](ts41/delimiter-case.d.ts) – Convert a string literal to a custom string delimiter casing.

### Miscellaneous

- [`PackageJson`](source/package-json.d.ts) - Type for [npm's `package.json` file](https://docs.npmjs.com/creating-a-package-json-file).
- [`TsConfigJson`](source/tsconfig-json.d.ts) - Type for [TypeScript's `tsconfig.json` file](https://www.typescriptlang.org/docs/handbook/tsconfig-json.html) (TypeScript 3.7).

## Declined types

*If we decline a type addition, we will make sure to document the better solution here.*

- [`Diff` and `Spread`](https://github.com/sindresorhus/type-fest/pull/7) - The PR author didn't provide any real-world use-cases and the PR went stale. If you think this type is useful, provide some real-world use-cases and we might reconsider.
- [`Dictionary`](https://github.com/sindresorhus/type-fest/issues/33) - You only save a few characters (`Dictionary<number>` vs `Record<string, number>`) from [`Record`](https://github.com/Microsoft/TypeScript/blob/2961bc3fc0ea1117d4e53bc8e97fa76119bc33e3/src/lib/es5.d.ts#L1429-L1434), which is more flexible and well-known. Also, you shouldn't use an object as a dictionary. We have `Map` in JavaScript now.
- [`SubType`](https://github.com/sindresorhus/type-fest/issues/22) - The type is powerful, but lacks good use-cases and is prone to misuse.
- [`ExtractProperties` and `ExtractMethods`](https://github.com/sindresorhus/type-fest/pull/4) - The types violate the single responsibility principle. Instead, refine your types into more granular type hierarchies.

## Tips

### Built-in types

There are many advanced types most users don't know about.

- [`Partial<T>`](https://github.com/Microsoft/TypeScript/blob/2961bc3fc0ea1117d4e53bc8e97fa76119bc33e3/src/lib/es5.d.ts#L1401-L1406) - Make all properties in `T` optional.

			Example

	[Playground](https://www.typescriptlang.org/play/#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)

	```ts
	interface NodeConfig {
			appName: string;
			port: number;
	}

	class NodeAppBuilder {
			private configuration: NodeConfig = {
					appName: 'NodeApp',
					port: 3000
			};

			private updateConfig<Key extends keyof NodeConfig>(key: Key, value: NodeConfig[Key]) {
					this.configuration[key] = value;
			}

			config(config: Partial<NodeConfig>) {
					type NodeConfigKey = keyof NodeConfig;

					for (const key of Object.keys(config) as NodeConfigKey[]) {
							const updateValue = config[key];

							if (updateValue === undefined) {
									continue;
							}

							this.updateConfig(key, updateValue);
					}

					return this;
			}
	}

	// `Partial<NodeConfig>`` allows us to provide only a part of the
	// NodeConfig interface.
	new NodeAppBuilder().config({appName: 'ToDoApp'});
	```

- [`Required<T>`](https://github.com/Microsoft/TypeScript/blob/2961bc3fc0ea1117d4e53bc8e97fa76119bc33e3/src/lib/es5.d.ts#L1408-L1413) - Make all properties in `T` required.

			Example

	[Playground](https://typescript-play.js.org/?target=6#code/AQ4SwOwFwUwJwGYEMDGNgGED21VQGJZwC2wA3gFCjXAzFJgA2A-AFzADOUckA5gNxUaIYjA4ckvGG07c+g6gF8KQkAgCuEFFDA5O6gEbEwUbLm2ESwABQIixACJIoSdgCUYAR3Vg4MACYAPGYuFvYAfACU5Ko0APRxwADKMBD+wFAAFuh2Vv7OSBlYGdmc8ABu8LHKsRyGxqY4oQT21pTCIHQMjOwA5DAAHgACxAAOjDAAdChYxL0ANLHUouKSMH0AEmAAhJhY6ozpAJ77GTCMjMCiV0ToSAb7UJPPC9WRgrEJwAAqR6MwSRQPFGUFocDgRHYxnEfGAowh-zgUCOwF6KwkUl6tXqJhCeEsxDaS1AXSYfUGI3GUxmc0WSneQA)

	```ts
	interface ContactForm {
			email?: string;
			message?: string;
	}

	function submitContactForm(formData: Required<ContactForm>) {
			// Send the form data to the server.
	}

	submitContactForm({
			email: 'ex@mple.com',
			message: 'Hi! Could you tell me more about…',
	});

	// TypeScript error: missing property 'message'
	submitContactForm({
			email: 'ex@mple.com',
	});
	```

- [`Readonly<T>`](https://github.com/Microsoft/TypeScript/blob/2961bc3fc0ea1117d4e53bc8e97fa76119bc33e3/src/lib/es5.d.ts#L1415-L1420) - Make all properties in `T` readonly.

			Example

	[Playground](https://typescript-play.js.org/?target=6#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)

	```ts
	enum LogLevel {
			Off,
			Debug,
			Error,
			Fatal
	};

	interface LoggerConfig {
			name: string;
			level: LogLevel;
	}

	class Logger {
			config: Readonly<LoggerConfig>;

			constructor({name, level}: LoggerConfig) {
					this.config = {name, level};
					Object.freeze(this.config);
			}
	}

	const config: LoggerConfig = {
		name: 'MyApp',
		level: LogLevel.Debug
	};

	const logger = new Logger(config);

	// TypeScript Error: cannot assign to read-only property.
	logger.config.level = LogLevel.Error;

	// We are able to edit config variable as we please.
	config.level = LogLevel.Error;
	```

- [`Pick<T, K>`](https://github.com/Microsoft/TypeScript/blob/2961bc3fc0ea1117d4e53bc8e97fa76119bc33e3/src/lib/es5.d.ts#L1422-L1427) - From `T`, pick a set of properties whose keys are in the union `K`.

			Example

	[Playground](https://typescript-play.js.org/?target=6#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)

	```ts
	interface Article {
			title: string;
			thumbnail: string;
			content: string;
	}

	// Creates new type out of the `Article` interface composed
	// from the Articles' two properties: `title` and `thumbnail`.
	// `ArticlePreview = {title: string; thumbnail: string}`
	type ArticlePreview = Pick<Article, 'title' | 'thumbnail'>;

	// Render a list of articles using only title and description.
	function renderArticlePreviews(previews: ArticlePreview[]): HTMLElement {
			const articles = document.createElement('div');

			for (const preview of previews) {
					// Append preview to the articles.
			}

			return articles;
	}

	const articles = renderArticlePreviews([
			{
				title: 'TypeScript tutorial!',
				thumbnail: '/assets/ts.jpg'
			}
	]);
	```

- [`Record<K, T>`](https://github.com/Microsoft/TypeScript/blob/2961bc3fc0ea1117d4e53bc8e97fa76119bc33e3/src/lib/es5.d.ts#L1429-L1434) - Construct a type with a set of properties `K` of type `T`.

			Example

	[Playground](https://typescript-play.js.org/?target=6#code/AQ4ejYAUHsGcCWAXBMB2dgwGbAKYC2ADgDYwCeeemCaWArgE7ADGMxAhmuQHQBQoYEnJE8wALKEARnkaxEKdMAC8wAOS0kstGuAAfdQBM8ANzxlRjXQbVaWACwC0JPB0NqA3HwGgIwAJJoWozYHCxixnAsjAhStADmwESMMJYo1Fi4HMCIaPEu+MRklHj8gpqyoeHAAKJFFFTAAN4+giDYCIxwSAByHAR4AFw5SDF5Xm2gJBzdfQPD3WPxE5PAlBxdAPLYNQAelgh4aOHDaPQEMowrIAC+3oJ+AMKMrlrAXFhSAFZ4LEhC9g4-0BmA4JBISXgiCkBQABpILrJ5MhUGhYcATGD6Bk4Hh-jNgABrPDkOBlXyQAAq9ngYmJpOAAHcEOCRjAXqwYODfoo6DhakUSph+Uh7GI4P0xER4Cj0OSQGwMP8tP1hgAlX7swwAHgRl2RvIANALSA08ABtAC6AD4VM1Wm0Kow0MMrYaHYJjGYLLJXZb3at1HYnC43Go-QHQDcvA6-JsmEJXARgCDgMYWAhjIYhDAU+YiMAAFIwex0ZmilMITCGF79TLAGRsAgJYAAZRwSEZGzEABFTOZUrJ5Yn+jwnWgeER6HB7AAKJrADpdXqS4ZqYultTG6azVfqHswPBbtauLY7fayQ7HIbAAAMwBuAEoYw9IBq2Ixs9h2eFMOQYPQObALQKJgggABeYhghCIpikkKRpOQRIknAsZUiIeCttECBEP8NSMCkjDDAARMGziuIYxHwYOjDCMBmDNnAuTxA6irdCOBB1Lh5Dqpqn66tISIykawBnOCtqqC0gbjqc9DgpGkxegOliyfJDrRkAA)

	```ts
	// Positions of employees in our company.
	type MemberPosition = 'intern' | 'developer' | 'tech-lead';

	// Interface describing properties of a single employee.
	interface Employee {
			firstName: string;
			lastName: string;
			yearsOfExperience: number;
	}

	// Create an object that has all possible `MemberPosition` values set as keys.
	// Those keys will store a collection of Employees of the same position.
	const team: Record<MemberPosition, Employee[]> = {
			intern: [],
			developer: [],
			'tech-lead': [],
	};

	// Our team has decided to help John with his dream of becoming Software Developer.
	team.intern.push({
		firstName: 'John',
		lastName: 'Doe',
		yearsOfExperience: 0
	});

	// `Record` forces you to initialize all of the property keys.
	// TypeScript Error: "tech-lead" property is missing
	const teamEmpty: Record<MemberPosition, null> = {
			intern: null,
			developer: null,
	};
	```

- [`Exclude<T, U>`](https://github.com/Microsoft/TypeScript/blob/2961bc3fc0ea1117d4e53bc8e97fa76119bc33e3/src/lib/es5.d.ts#L1436-L1439) - Exclude from `T` those types that are assignable to `U`.

			Example

	[Playground](https://typescript-play.js.org/?target=6#code/JYOwLgpgTgZghgYwgAgMrQG7QMIHsQzADmyA3gFDLIAOuUYAXMiAK4A2byAPsgM5hRQJHqwC2AI2gBucgF9y5MAE9qKAEoQAjiwj8AEnBAATNtGQBeZAAooWphu26wAGmS3e93bRC8IASgsAPmRDJRlyAHoI5ABRAA8ENhYjFFYOZGVVZBgoXFFkAAM0zh5+QRBhZhYJaAKAOkjogEkQZAQ4X2QAdwALCFbaemRgXmQtFjhOMFwq9K6ULuB0lk6U+HYwZAxJnQaYFhAEMGB8ZCIIMAAFOjAANR2IK0HGWISklIAedCgsKDwCYgAbQA5M9gQBdVzFQJ+JhiSRQMiUYYwayZCC4VHPCzmSzAspCYEBWxgFhQAZwKC+FpgJ43VwARgADH4ZFQSWSBjcZPJyPtDsdTvxKWBvr8rD1DCZoJ5HPopaYoK4EPhCEQmGKcKriLCtrhgEYkVQVT5Nr4fmZLLZtMBbFZgT0wGBqES6ghbHBIJqoBKFdBWQpjfh+DQbhY2tqiHVsbjLMVkAB+ZAAZiZaeQTHOVxu9ySjxNaujNwDVHNvzqbBGkBAdPoAfkQA)

	```ts
	interface ServerConfig {
		port: null | string | number;
	}

	type RequestHandler = (request: Request, response: Response) => void;

	// Exclude `null` type from `null | string | number`.
	// In case the port is equal to `null`, we will use default value.
	function getPortValue(port: Exclude<ServerConfig['port'], null>): number {
		if (typeof port === 'string') {
			return parseInt(port, 10);
		}

		return port;
	}

	function startServer(handler: RequestHandler, config: ServerConfig): void {
		const server = require('http').createServer(handler);

		const port = config.port === null ? 3000 : getPortValue(config.port);
		server.listen(port);
	}
	```

- [`Extract<T, U>`](https://github.com/Microsoft/TypeScript/blob/2961bc3fc0ea1117d4e53bc8e97fa76119bc33e3/src/lib/es5.d.ts#L1441-L1444) - Extract from `T` those types that are assignable to `U`.

			Example

	[Playground](https://typescript-play.js.org/?target=6#code/CYUwxgNghgTiAEAzArgOzAFwJYHtXzSwEdkQBJYACgEoAueVZAWwCMQYBuAKDDwGcM8MgBF4AXngBlAJ6scESgHIRi6ty5ZUGdoihgEABXZ888AN5d48ANoiAuvUat23K6ihMQ9ATE0BzV3goPy8GZjZOLgBfLi4Aejj4AEEICBwAdz54MAALKFQQ+BxEeAAHY1NgKAwoIKy0grr4DByEUpgccpgMaXgAaxBerCzi+B9-ZulygDouFHRsU1z8kKMYE1RhaqgAHkt4AHkWACt4EAAPbVRgLLWNgBp9gGlBs8uQa6yAUUuYPQwdgNpKM7nh7mMML4CgA+R5WABqUAgpDeVxuhxO1he0jsXGh8EoOBO9COx3BQPo2PBADckaR6IjkSA6PBqTgsMBzPsicdrEC7OJWXSQNwYvFEgAVTS9JLXODpeDpKBZFg4GCoWa8VACIJykAKiQWKy2YQOAioYikCg0OEMDyhRSy4DyxS24KhAAMjyi6gS8AAwjh5OD0iBFHAkJoEOksC1mnkMJq8gUQKDNttKPlnfrwYp3J5XfBHXqoKpfYkAOI4ansTxaeDADmoRSCCBYAbxhC6TDx6rwYHIRX5bScjA4bLJwoDmDwDkfbA9JMrVMVdM1TN69LgkTgwgkchUahqIA)

	```ts
	declare function uniqueId(): number;

	const ID = Symbol('ID');

	interface Person {
		[ID]: number;
		name: string;
		age: number;
	}

	// Allows changing the person data as long as the property key is of string type.
	function changePersonData<
		Obj extends Person,
		Key extends Extract<keyof Person, string>,
		Value extends Obj[Key]
	> (obj: Obj, key: Key, value: Value): void {
		obj[key] = value;
	}

	// Tiny Andrew was born.
	const andrew = {
		[ID]: uniqueId(),
		name: 'Andrew',
		age: 0,
	};

	// Cool, we're fine with that.
	changePersonData(andrew, 'name', 'Pony');

	// Goverment didn't like the fact that you wanted to change your identity.
	changePersonData(andrew, ID, uniqueId());
	```

- [`NonNullable<T>`](https://github.com/Microsoft/TypeScript/blob/2961bc3fc0ea1117d4e53bc8e97fa76119bc33e3/src/lib/es5.d.ts#L1446-L1449) - Exclude `null` and `undefined` from `T`.

			Example

	Works with strictNullChecks set to true. (Read more here)

	[Playground](https://typescript-play.js.org/?target=6#code/C4TwDgpgBACg9gJ2AOQK4FsBGEFQLxQDOwCAlgHYDmUAPlORtrnQwDasDcAUFwPQBU-WAEMkUOADMowqAGNWwwoSgATCBIqlgpOOSjAAFsOBRSy1IQgr9cKJlSlW1mZYQA3HFH68u8xcoBlHA8EACEHJ08Aby4oKDBUTFZSWXjEFEYcAEIALihkXTR2YSSIAB54JDQsHAA+blj4xOTUsHSACkMzPKD3HHDHNQQAGjSkPMqMmoQASh7g-oihqBi4uNIpdraxPAI2VhmVxrX9AzMAOm2ppnwoAA4ABifuE4BfKAhWSyOTuK7CS7pao3AhXF5rV48E4ICDAVAIPT-cGQyG+XTEIgLMJLTx7CAAdygvRCA0iCHaMwarhJOIQjUBSHaACJHk8mYdeLwxtdcVAAOSsh58+lXdr7Dlcq7A3n3J4PEUdADMcspUE53OluAIUGVTx46oAKuAIAFZGQwCYAKIIBCILjUxaDHAMnla+iodjcIA)

	```ts
	type PortNumber = string | number | null;

	/** Part of a class definition that is used to build a server */
	class ServerBuilder {
			portNumber!: NonNullable<PortNumber>;

			port(this: ServerBuilder, port: PortNumber): ServerBuilder {
					if (port == null) {
							this.portNumber = 8000;
					} else {
							this.portNumber = port;
					}

					return this;
			}
	}

	const serverBuilder = new ServerBuilder();

	serverBuilder
			.port('8000')   // portNumber = '8000'
			.port(null)     // portNumber =  8000
			.port(3000);    // portNumber =  3000

	// TypeScript error
	serverBuilder.portNumber = null;
	```

- [`Parameters<T>`](https://github.com/Microsoft/TypeScript/blob/2961bc3fc0ea1117d4e53bc8e97fa76119bc33e3/src/lib/es5.d.ts#L1451-L1454) - Obtain the parameters of a function type in a tuple.

			Example

	[Playground](https://typescript-play.js.org/?target=6#code/GYVwdgxgLglg9mABAZwBYmMANgUwBQxgAOIUAXIgIZgCeA2gLoCUFAbnDACaIDeAUIkQB6IYgCypSlBxUATrMo1ECsJzgBbLEoipqAc0J7EMKMgDkiHLnU4wp46pwAPHMgB0fAL58+oSLARECEosLAA5ABUYG2QAHgAxJGdpVWREPDdMylk9ZApqemZEAF4APipacrw-CApEgBogkKwAYThwckQwEHUAIxxZJl4BYVEImiIZKF0oZRwiWVdbeygJmThgOYgcGFYcbhqApCJsyhtpWXcR1cnEePBoeDAABVPzgbTixFeFd8uEsClADcIxGiygIFkSEOT3SmTc2VydQeRx+ZxwF2QQ34gkEwDgsnSuFmMBKiAADEDjIhYk1Qm0OlSYABqZnYka4xA1DJZHJYkGc7yCbyeRA+CAIZCzNAYbA4CIAdxg2zJwVCkWirjwMswuEaACYmCCgA)

	```ts
	function shuffle(input: any[]): void {
		// Mutate array randomly changing its' elements indexes.
	}

	function callNTimes<Fn extends (...args: any[]) => any> (func: Fn, callCount: number) {
		// Type that represents the type of the received function parameters.
		type FunctionParameters = Parameters<Fn>;

		return function (...args: FunctionParameters) {
			for (let i = 0; i < callCount; i++) {
				func(...args);
			}
		}
	}

	const shuffleTwice = callNTimes(shuffle, 2);
	```

- [`ConstructorParameters<T>`](https://github.com/Microsoft/TypeScript/blob/2961bc3fc0ea1117d4e53bc8e97fa76119bc33e3/src/lib/es5.d.ts#L1456-L1459) - Obtain the parameters of a constructor function type in a tuple.

			Example

	[Playground](https://typescript-play.js.org/?target=6#code/MYGwhgzhAECCBOAXAlqApgWQPYBM0mgG8AoaaFRENALmgkXmQDsBzAblOmCycTV4D8teo1YdO3JiICuwRFngAKClWENmLAJRFOZRAAtkEAHQq00ALzlklNBzIBfYk+KhIMAJJTEYJsDQAwmDA+mgAPAAq0GgAHnxMODCKTGgA7tCKxllg8CwQtL4AngDaALraFgB80EWa1SRkAA6MAG5gfNAB4FABPDJyCrQR9tDNyG0dwMGhtBhgjWEiGgA00F70vv4RhY3hEZXVVinpc42KmuJkkv3y8Bly8EPaDWTkhiZd7r3e8LK3llwGCMXGQWGhEOsfH5zJlsrl8p0+gw-goAAo5MAAW3BaHgEEilU0tEhmzQ212BJ0ry4SOg+kg+gBBiMximIGA0nAfAQLGk2N4EAAEgzYcYcnkLsRdDTvNEYkYUKwSdCme9WdM0MYwYhFPSIPpJdTkAAzDKxBUaZX+aAAQgsVmkCTQxuYaBw2ng4Ok8CYcotSu8pMur09iG9vuObxZnx6SN+AyUWTF8MN0CcZE4Ywm5jZHK5aB5fP4iCFIqT4oRRTKRLo6lYVNeAHpG50wOzOe1zHr9NLQ+HoABybsD4HOKXXRA1JCoKhBELmI5pNaB6Fz0KKBAodDYPAgSUTmqYsAALx4m5nC6nW9nGq14KtaEUA9gR9PvuNCjQ9BgACNvcwNBtAcLiAA)

	```ts
	class ArticleModel {
		title: string;
		content?: string;

		constructor(title: string) {
			this.title = title;
		}
	}

	class InstanceCache<T extends (new (...args: any[]) => any)> {
		private ClassConstructor: T;
		private cache: Map<string, InstanceType<T>> = new Map();

		constructor (ctr: T) {
			this.ClassConstructor = ctr;
		}

		getInstance (...args: ConstructorParameters<T>): InstanceType<T> {
			const hash = this.calculateArgumentsHash(...args);

			const existingInstance = this.cache.get(hash);
			if (existingInstance !== undefined) {
				return existingInstance;
			}

			return new this.ClassConstructor(...args);
		}

		private calculateArgumentsHash(...args: any[]): string {
			// Calculate hash.
			return 'hash';
		}
	}

	const articleCache = new InstanceCache(ArticleModel);
	const amazonArticle = articleCache.getInstance('Amazon forests burining!');
	```

- [`ReturnType<T>`](https://github.com/Microsoft/TypeScript/blob/2961bc3fc0ea1117d4e53bc8e97fa76119bc33e3/src/lib/es5.d.ts#L1461-L1464) – Obtain the return type of a function type.

			Example

	[Playground](https://typescript-play.js.org/?target=6#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-YhAA+qegAAYLKQLQj3ZsEsdccmnGcLor2Dn8xGedHGpEIBzEzspfsfMHDNAANTQACMVaIljV5GQkRA5DYmIpVKQAgAJARO9le33BDXIyi0YuLW2nJFGLqkOvxFB0YPdBSaLZ0IwNzyPkO8-xkGgsLh8Al427a3hWAhXwwHA8EHT5PmgAB1bAQBAANJ24adKWpft72RaBUTgRBUCAj89HAM8xCTaBjggABRQx0DuHJv25P9dCkWRZVIAAiBjoFImpmjlFBgA0NpsjadByDacgIDAEAIAAQmYpjoGYgAZSBsmGPw6DtZiiFA8CoJguDmAQmoZ2QvtUKQLdoAYmBTwgdEiCAA)

	```ts
	/** Provides every element of the iterable `iter` into the `callback` function and stores the results in an array. */
	function mapIter<
			Elem,
			Func extends (elem: Elem) => any,
			Ret extends ReturnType<Func>
	>(iter: Iterable<Elem>, callback: Func): Ret[] {
			const mapped: Ret[] = [];

			for (const elem of iter) {
					mapped.push(callback(elem));
			}

			return mapped;
	}

	const setObject: Set<string> = new Set();
	const mapObject: Map<number, string> = new Map();

	mapIter(setObject, (value: string) => value.indexOf('Foo')); // number[]

	mapIter(mapObject, ([key, value]: [number, string]) => {
			return key % 2 === 0 ? value : 'Odd';
	}); // string[]
	```

- [`InstanceType<T>`](https://github.com/Microsoft/TypeScript/blob/2961bc3fc0ea1117d4e53bc8e97fa76119bc33e3/src/lib/es5.d.ts#L1466-L1469) – Obtain the instance type of a constructor function type.

			Example

	[Playground](https://typescript-play.js.org/?target=6#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-YhAA+qegAAYLKQLQj3ZsEsdccmnGcLor2Dn8xGedHGpEIBzEzspfsfMHDNAANTQACMVaIljV5GQkRA5DYmIpVKQAgAJARO9le33BDXIyi0YuLW2nJFGLqkOvxFB0YPdBSaLZ0IwNzyPkO8-xkGgsLh8Al427a3hWAhXwwHA8EHT5PmgAB1bAQBAANJ24adKWpft72RaBUTgRBUCAj89HAM8xCTaBjggABRQx0DuHJv25P9dCkWRZVIAAiBjoFImpmjlFBgA0NpsjadByDacgIDAEAIAAQmYpjoGYgAZSBsmGPw6DtZiiFA8CoJguDmAQmoZ2QvtUKQLdoAYmBTwgdEiCAA)

	```ts
	class IdleService {
			doNothing (): void {}
	}

	class News {
			title: string;
			content: string;

			constructor(title: string, content: string) {
					this.title = title;
					this.content = content;
			}
	}

	const instanceCounter: Map<Function, number> = new Map();

	interface Constructor {
			new(...args: any[]): any;
	}

	// Keep track how many instances of `Constr` constructor have been created.
	function getInstance<
			Constr extends Constructor,
			Args extends ConstructorParameters<Constr>
	>(constructor: Constr, ...args: Args): InstanceType<Constr> {
			let count = instanceCounter.get(constructor) || 0;

			const instance = new constructor(...args);

			instanceCounter.set(constructor, count + 1);

			console.log(`Created ${count + 1} instances of ${Constr.name} class`);

			return instance;
	}

	const idleService = getInstance(IdleService);
	// Will log: `Created 1 instances of IdleService class`
	const newsEntry = getInstance(News, 'New ECMAScript proposals!', 'Last month...');
	// Will log: `Created 1 instances of News class`
	```

- [`Omit<T, K>`](https://github.com/microsoft/TypeScript/blob/71af02f7459dc812e85ac31365bfe23daf14b4e4/src/lib/es5.d.ts#L1446) – Constructs a type by picking all properties from T and then removing K.

			Example

	[Playground](https://typescript-play.js.org/?target=6#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-SgfXlHqVUE4xiAA)

	```ts
	interface Animal {
			imageUrl: string;
			species: string;
			images: string[];
			paragraphs: string[];
	}

	// Creates new type with all properties of the `Animal` interface
	// except 'images' and 'paragraphs' properties. We can use this
	// type to render small hover tooltip for a wiki entry list.
	type AnimalShortInfo = Omit<Animal, 'images' | 'paragraphs'>;

	function renderAnimalHoverInfo (animals: AnimalShortInfo[]): HTMLElement {
			const container =  document.createElement('div');
			// Internal implementation.
			return container;
	}
	```

You can find some examples in the [TypeScript docs](https://www.typescriptlang.org/docs/handbook/advanced-types.html#predefined-conditional-types).

## Maintainers

- [Sindre Sorhus](https://github.com/sindresorhus)
- [Jarek Radosz](https://github.com/CvX)
- [Dimitri Benin](https://github.com/BendingBender)
- [Pelle Wessman](https://github.com/voxpelli)

## License

(MIT OR CC0-1.0)

---

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		Tidelift helps make open source sustainable for maintainers while giving companies
assurances about security, maintenance, and licensing for their dependencies.


code:
import {PromiseValue} from './promise-value';

type AsyncFunction = (...args: any[]) => Promise<unknown>;

/**
Unwrap the return type of a function that returns a `Promise`.

There has been [discussion](https://github.com/microsoft/TypeScript/pull/35998) about implementing this type in TypeScript.

@example
```ts
import {AsyncReturnType} from 'type-fest';
import {asyncFunction} from 'api';

// This type resolves to the unwrapped return type of `asyncFunction`.
type Value = AsyncReturnType<typeof asyncFunction>;

async function doSomething(value: Value) {}

asyncFunction().then(value => doSomething(value));
```
*/
export type AsyncReturnType<Target extends AsyncFunction> = PromiseValue<ReturnType<Target>>;


code:
import {PromiseValue} from './promise-value';
import {SetReturnType} from './set-return-type';

/**
Create an async version of the given function type, by boxing the return type in `Promise` while keeping the same parameter types.

Use-case: You have two functions, one synchronous and one asynchronous that do the same thing. Instead of having to duplicate the type definition, you can use `Asyncify` to reuse the synchronous type.

@example
```
import {Asyncify} from 'type-fest';

// Synchronous function.
function getFooSync(someArg: SomeType): Foo {
	// …
}

type AsyncifiedFooGetter = Asyncify<typeof getFooSync>;
//=> type AsyncifiedFooGetter = (someArg: SomeType) => Promise<Foo>;

// Same as `getFooSync` but asynchronous.
const getFooAsync: AsyncifiedFooGetter = (someArg) => {
	// TypeScript now knows that `someArg` is `SomeType` automatically.
	// It also knows that this function must return `Promise<Foo>`.
	// If you have `@typescript-eslint/promise-function-async` linter rule enabled, it will even report that "Functions that return promises must be async.".

	// …
}
```
*/
export type Asyncify<Fn extends (...args: any[]) => any> = SetReturnType<Fn, Promise<PromiseValue<ReturnType<Fn>>>>;


code:
/// <reference lib="esnext"/>

// TODO: This can just be `export type Primitive = not object` when the `not` keyword is out.
/**
Matches any [primitive value](https://developer.mozilla.org/en-US/docs/Glossary/Primitive).
*/
export type Primitive =
	| null
	| undefined
	| string
	| number
	| boolean
	| symbol
	| bigint;

// TODO: Remove the `= unknown` sometime  in the future when most users are on TS 3.5 as it's now the default
/**
Matches a [`class` constructor](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Classes).
*/
export type Class<T = unknown, Arguments extends any[] = any[]> = new(...arguments_: Arguments) => T;

/**
Matches any [typed array](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/TypedArray), like `Uint8Array` or `Float64Array`.
*/
export type TypedArray =
	| Int8Array
	| Uint8Array
	| Uint8ClampedArray
	| Int16Array
	| Uint16Array
	| Int32Array
	| Uint32Array
	| Float32Array
	| Float64Array
	| BigInt64Array
	| BigUint64Array;

/**
Matches a JSON object.

This type can be useful to enforce some input to be JSON-compatible or as a super-type to be extended from. Don't use this as a direct return type as the user would have to double-cast it: `jsonObject as unknown as CustomResponse`. Instead, you could extend your CustomResponse type from it to ensure your type only uses JSON-compatible types: `interface CustomResponse extends JsonObject { … }`.
*/
export type JsonObject = {[Key in string]?: JsonValue};

/**
Matches a JSON array.
*/
export interface JsonArray extends Array<JsonValue> {}

/**
Matches any valid JSON value.
*/
export type JsonValue = string | number | boolean | null | JsonObject | JsonArray;

declare global {
	interface SymbolConstructor {
		readonly observable: symbol;
	}
}

/**
Matches a value that is like an [Observable](https://github.com/tc39/proposal-observable).
*/
export interface ObservableLike {
	subscribe(observer: (value: unknown) => void): void;
	[Symbol.observable](): ObservableLike;
}


code:
import {Except} from './except';
import {ConditionalKeys} from './conditional-keys';

/**
Exclude keys from a shape that matches the given `Condition`.

This is useful when you want to create a new type with a specific set of keys from a shape. For example, you might want to exclude all the primitive properties from a class and form a new shape containing everything but the primitive properties.

@example
```
import {Primitive, ConditionalExcept} from 'type-fest';

class Awesome {
	name: string;
	successes: number;
	failures: bigint;

	run() {}
}

type ExceptPrimitivesFromAwesome = ConditionalExcept<Awesome, Primitive>;
//=> {run: () => void}
```

@example
```
import {ConditionalExcept} from 'type-fest';

interface Example {
	a: string;
	b: string | number;
	c: () => void;
	d: {};
}

type NonStringKeysOnly = ConditionalExcept<Example, string>;
//=> {b: string | number; c: () => void; d: {}}
```
*/
export type ConditionalExcept<Base, Condition> = Except<
	Base,
	ConditionalKeys<Base, Condition>
>;


code:
/**
Extract the keys from a type where the value type of the key extends the given `Condition`.

Internally this is used for the `ConditionalPick` and `ConditionalExcept` types.

@example
```
import {ConditionalKeys} from 'type-fest';

interface Example {
	a: string;
	b: string | number;
	c?: string;
	d: {};
}

type StringKeysOnly = ConditionalKeys<Example, string>;
//=> 'a'
```

To support partial types, make sure your `Condition` is a union of undefined (for example, `string | undefined`) as demonstrated below.

@example
```
type StringKeysAndUndefined = ConditionalKeys<Example, string | undefined>;
//=> 'a' | 'c'
```
*/
export type ConditionalKeys<Base, Condition> = NonNullable<
	// Wrap in `NonNullable` to strip away the `undefined` type from the produced union.
	{
		// Map through all the keys of the given base type.
		[Key in keyof Base]:
			// Pick only keys with types extending the given `Condition` type.
			Base[Key] extends Condition
				// Retain this key since the condition passes.
				? Key
				// Discard this key since the condition fails.
				: never;

	// Convert the produced object into a union type of the keys which passed the conditional test.
	}[keyof Base]
>;


code:
import {ConditionalKeys} from './conditional-keys';

/**
Pick keys from the shape that matches the given `Condition`.

This is useful when you want to create a new type from a specific subset of an existing type. For example, you might want to pick all the primitive properties from a class and form a new automatically derived type.

@example
```
import {Primitive, ConditionalPick} from 'type-fest';

class Awesome {
	name: string;
	successes: number;
	failures: bigint;

	run() {}
}

type PickPrimitivesFromAwesome = ConditionalPick<Awesome, Primitive>;
//=> {name: string; successes: number; failures: bigint}
```

@example
```
import {ConditionalPick} from 'type-fest';

interface Example {
	a: string;
	b: string | number;
	c: () => void;
	d: {};
}

type StringKeysOnly = ConditionalPick<Example, string>;
//=> {a: string}
```
*/
export type ConditionalPick<Base, Condition> = Pick<
	Base,
	ConditionalKeys<Base, Condition>
>;


code:
import {ArrayEntry, MapEntry, ObjectEntry, SetEntry} from './entry';

type ArrayEntries<BaseType extends readonly unknown[]> = Array<ArrayEntry<BaseType>>;
type MapEntries<BaseType> = Array<MapEntry<BaseType>>;
type ObjectEntries<BaseType> = Array<ObjectEntry<BaseType>>;
type SetEntries<BaseType extends Set<unknown>> = Array<SetEntry<BaseType>>;

/**
Many collections have an `entries` method which returns an array of a given object's own enumerable string-keyed property [key, value] pairs. The `Entries` type will return the type of that collection's entries.

For example the {@link https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/entries|`Object`}, {@link https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Map/entries|`Map`}, {@link https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/entries|`Array`}, and {@link https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Set/entries|`Set`} collections all have this method. Note that `WeakMap` and `WeakSet` do not have this method since their entries are not enumerable.

@see `Entry` if you want to just access the type of a single entry.

@example
```
import {Entries} from 'type-fest';

interface Example {
	someKey: number;
}

const manipulatesEntries = (examples: Entries<Example>) => examples.map(example => [
	// Does some arbitrary processing on the key (with type information available)
	example[0].toUpperCase(),

	// Does some arbitrary processing on the value (with type information available)
	example[1].toFixed()
]);

const example: Example = {someKey: 1};
const entries = Object.entries(example) as Entries<Example>;
const output = manipulatesEntries(entries);

// Objects
const objectExample = {a: 1};
const objectEntries: Entries<typeof objectExample> = [['a', 1]];

// Arrays
const arrayExample = ['a', 1];
const arrayEntries: Entries<typeof arrayExample> = [[0, 'a'], [1, 1]];

// Maps
const mapExample = new Map([['a', 1]]);
const mapEntries: Entries<typeof map> = [['a', 1]];

// Sets
const setExample = new Set(['a', 1]);
const setEntries: Entries<typeof setExample> = [['a', 'a'], [1, 1]];
```
*/
export type Entries<BaseType> =
	BaseType extends Map<unknown, unknown> ? MapEntries<BaseType>
		: BaseType extends Set<unknown> ? SetEntries<BaseType>
		: BaseType extends unknown[] ? ArrayEntries<BaseType>
		: BaseType extends object ? ObjectEntries<BaseType>
		: never;


code:
type MapKey<BaseType> = BaseType extends Map<infer KeyType, unknown> ? KeyType : never;
type MapValue<BaseType> = BaseType extends Map<unknown, infer ValueType> ? ValueType : never;

export type ArrayEntry<BaseType extends readonly unknown[]> = [number, BaseType[number]];
export type MapEntry<BaseType> = [MapKey<BaseType>, MapValue<BaseType>];
export type ObjectEntry<BaseType> = [keyof BaseType, BaseType[keyof BaseType]];
export type SetEntry<BaseType> = BaseType extends Set<infer ItemType> ? [ItemType, ItemType] : never;

/**
Many collections have an `entries` method which returns an array of a given object's own enumerable string-keyed property [key, value] pairs. The `Entry` type will return the type of that collection's entry.

For example the {@link https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/entries|`Object`}, {@link https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Map/entries|`Map`}, {@link https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/entries|`Array`}, and {@link https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Set/entries|`Set`} collections all have this method. Note that `WeakMap` and `WeakSet` do not have this method since their entries are not enumerable.

@see `Entries` if you want to just access the type of the array of entries (which is the return of the `.entries()` method).

@example
```
import {Entry} from 'type-fest';

interface Example {
	someKey: number;
}

const manipulatesEntry = (example: Entry<Example>) => [
	// Does some arbitrary processing on the key (with type information available)
	example[0].toUpperCase(),

	// Does some arbitrary processing on the value (with type information available)
	example[1].toFixed(),
];

const example: Example = {someKey: 1};
const entry = Object.entries(example)[0] as Entry<Example>;
const output = manipulatesEntry(entry);

// Objects
const objectExample = {a: 1};
const objectEntry: Entry<typeof objectExample> = ['a', 1];

// Arrays
const arrayExample = ['a', 1];
const arrayEntryString: Entry<typeof arrayExample> = [0, 'a'];
const arrayEntryNumber: Entry<typeof arrayExample> = [1, 1];

// Maps
const mapExample = new Map([['a', 1]]);
const mapEntry: Entry<typeof map> = ['a', 1];

// Sets
const setExample = new Set(['a', 1]);
const setEntryString: Entry<typeof setExample> = ['a', 'a'];
const setEntryNumber: Entry<typeof setExample> = [1, 1];
```
*/
export type Entry<BaseType> =
	BaseType extends Map<unknown, unknown> ? MapEntry<BaseType>
		: BaseType extends Set<unknown> ? SetEntry<BaseType>
		: BaseType extends unknown[] ? ArrayEntry<BaseType>
		: BaseType extends object ? ObjectEntry<BaseType>
		: never;


code:
/**
Create a type from an object type without certain keys.

This type is a stricter version of [`Omit`](https://www.typescriptlang.org/docs/handbook/release-notes/typescript-3-5.html#the-omit-helper-type). The `Omit` type does not restrict the omitted keys to be keys present on the given type, while `Except` does. The benefits of a stricter type are avoiding typos and allowing the compiler to pick up on rename refactors automatically.

Please upvote [this issue](https://github.com/microsoft/TypeScript/issues/30825) if you want to have the stricter version as a built-in in TypeScript.

@example
```
import {Except} from 'type-fest';

type Foo = {
	a: number;
	b: string;
	c: boolean;
};

type FooWithoutA = Except<Foo, 'a' | 'c'>;
//=> {b: string};
```
*/
export type Except<ObjectType, KeysType extends keyof ObjectType> = Pick<ObjectType, Exclude<keyof ObjectType, KeysType>>;


code:
/**
Methods to exclude.
*/
type ArrayLengthMutationKeys = 'splice' | 'push' | 'pop' | 'shift' | 'unshift';

/**
Create a type that represents an array of the given type and length. The array's length and the `Array` prototype methods that manipulate its length are excluded in the resulting type.

Please participate in [this issue](https://github.com/microsoft/TypeScript/issues/26223) if you want to have a similiar type built into TypeScript.

Use-cases:
- Declaring fixed-length tuples or arrays with a large number of items.
- Creating a range union (for example, `0 | 1 | 2 | 3 | 4` from the keys of such a type) without having to resort to recursive types.
- Creating an array of coordinates with a static length, for example, length of 3 for a 3D vector.

@example
```
import {FixedLengthArray} from 'type-fest';

type FencingTeam = FixedLengthArray<string, 3>;

const guestFencingTeam: FencingTeam = ['Josh', 'Michael', 'Robert'];

const homeFencingTeam: FencingTeam = ['George', 'John'];
//=> error TS2322: Type string[] is not assignable to type 'FencingTeam'

guestFencingTeam.push('Sam');
//=> error TS2339: Property 'push' does not exist on type 'FencingTeam'
```
*/
export type FixedLengthArray<Element, Length extends number, ArrayPrototype = [Element, ...Element[]]> = Pick<
	ArrayPrototype,
	Exclude<keyof ArrayPrototype, ArrayLengthMutationKeys>
> & {
	[index: number]: Element;
	[Symbol.iterator]: () => IterableIterator<Element>;
	readonly length: Length;
};


code:
/**
Get the element type of an `Iterable`/`AsyncIterable`. For example, an array or a generator.

This can be useful, for example, if you want to get the type that is yielded in a generator function. Often the return type of those functions are not specified.

This type works with both `Iterable`s and `AsyncIterable`s, so it can be use with synchronous and asynchronous generators.

Here is an example of `IterableElement` in action with a generator function:

@example
```
function * iAmGenerator() {
	yield 1;
	yield 2;
}

type MeNumber = IterableElement<ReturnType<typeof iAmGenerator>>
```

And here is an example with an async generator:

@example
```
async function * iAmGeneratorAsync() {
	yield 'hi';
	yield true;
}

type MeStringOrBoolean = IterableElement<ReturnType<typeof iAmGeneratorAsync>>
```

Many types in JavaScript/TypeScript are iterables. This type works on all types that implement those interfaces. For example, `Array`, `Set`, `Map`, `stream.Readable`, etc.

An example with an array of strings:

@example
```
type MeString = IterableElement<string[]>
```
*/
export type IterableElement<TargetIterable> =
	TargetIterable extends Iterable<infer ElementType> ?
	ElementType :
	TargetIterable extends AsyncIterable<infer ElementType> ?
	ElementType :
	never;


code:
import {Primitive} from './basic';

/**
Allows creating a union type by combining primitive types and literal types without sacrificing auto-completion in IDEs for the literal type part of the union.

Currently, when a union type of a primitive type is combined with literal types, TypeScript loses all information about the combined literals. Thus, when such type is used in an IDE with autocompletion, no suggestions are made for the declared literals.

This type is a workaround for [Microsoft/TypeScript#29729](https://github.com/Microsoft/TypeScript/issues/29729). It will be removed as soon as it's not needed anymore.

@example
```
import {LiteralUnion} from 'type-fest';

// Before

type Pet = 'dog' | 'cat' | string;

const pet: Pet = '';
// Start typing in your TypeScript-enabled IDE.
// You **will not** get auto-completion for `dog` and `cat` literals.

// After

type Pet2 = LiteralUnion<'dog' | 'cat', string>;

const pet: Pet2 = '';
// You **will** get auto-completion for `dog` and `cat` literals.
```
 */
export type LiteralUnion<
	LiteralType,
	BaseType extends Primitive
> = LiteralType | (BaseType & {_?: never});


code:
// Helper type. Not useful on its own.
type Without<FirstType, SecondType> = {[KeyType in Exclude<keyof FirstType, keyof SecondType>]?: never};

/**
Create a type that has mutually exclusive keys.

This type was inspired by [this comment](https://github.com/Microsoft/TypeScript/issues/14094#issuecomment-373782604).

This type works with a helper type, called `Without`. `Without<FirstType, SecondType>` produces a type that has only keys from `FirstType` which are not present on `SecondType` and sets the value type for these keys to `never`. This helper type is then used in `MergeExclusive` to remove keys from either `FirstType` or `SecondType`.

@example
```
import {MergeExclusive} from 'type-fest';

interface ExclusiveVariation1 {
	exclusive1: boolean;
}

interface ExclusiveVariation2 {
	exclusive2: string;
}

type ExclusiveOptions = MergeExclusive<ExclusiveVariation1, ExclusiveVariation2>;

let exclusiveOptions: ExclusiveOptions;

exclusiveOptions = {exclusive1: true};
//=> Works
exclusiveOptions = {exclusive2: 'hi'};
//=> Works
exclusiveOptions = {exclusive1: true, exclusive2: 'hi'};
//=> Error
```
*/
export type MergeExclusive<FirstType, SecondType> =
	(FirstType | SecondType) extends object ?
		(Without<FirstType, SecondType> & SecondType) | (Without<SecondType, FirstType> & FirstType) :
		FirstType | SecondType;


code:
import {Except} from './except';

/**
Merge two types into a new type. Keys of the second type overrides keys of the first type.

@example
```
import {Merge} from 'type-fest';

type Foo = {
	a: number;
	b: string;
};

type Bar = {
	b: number;
};

const ab: Merge<Foo, Bar> = {a: 1, b: 2};
```
*/
export type Merge<FirstType, SecondType> = Except<FirstType, Extract<keyof FirstType, keyof SecondType>> & SecondType;


code:
/**
Convert an object with `readonly` keys into a mutable object. Inverse of `Readonly<T>`.

This can be used to [store and mutate options within a class](https://github.com/sindresorhus/pageres/blob/4a5d05fca19a5fbd2f53842cbf3eb7b1b63bddd2/source/index.ts#L72), [edit `readonly` objects within tests](https://stackoverflow.com/questions/50703834), and [construct a `readonly` object within a function](https://github.com/Microsoft/TypeScript/issues/24509).

@example
```
import {Mutable} from 'type-fest';

type Foo = {
	readonly a: number;
	readonly b: string;
};

const mutableFoo: Mutable<Foo> = {a: 1, b: '2'};
mutableFoo.a = 3;
```
*/
export type Mutable<ObjectType> = {
	// For each `Key` in the keys of `ObjectType`, make a mapped type by removing the `readonly` modifier from the key.
	-readonly [KeyType in keyof ObjectType]: ObjectType[KeyType];
};


code:
/**
Create an opaque type, which hides its internal details from the public, and can only be created by being used explicitly.

The generic type parameter can be anything. It doesn't have to be an object.

[Read more about opaque types.](https://codemix.com/opaque-types-in-javascript/)

There have been several discussions about adding this feature to TypeScript via the `opaque type` operator, similar to how Flow does it. Unfortunately, nothing has (yet) moved forward:
	- [Microsoft/TypeScript#15408](https://github.com/Microsoft/TypeScript/issues/15408)
	- [Microsoft/TypeScript#15807](https://github.com/Microsoft/TypeScript/issues/15807)

@example
```
import {Opaque} from 'type-fest';

type AccountNumber = Opaque<number, 'AccountNumber'>;
type AccountBalance = Opaque<number, 'AccountBalance'>;

// The Token parameter allows the compiler to differentiate between types, whereas "unknown" will not. For example, consider the following structures:
type ThingOne = Opaque<string>;
type ThingTwo = Opaque<string>;

// To the compiler, these types are allowed to be cast to each other as they have the same underlying type. They are both `string & { __opaque__: unknown }`.
// To avoid this behaviour, you would instead pass the "Token" parameter, like so.
type NewThingOne = Opaque<string, 'ThingOne'>;
type NewThingTwo = Opaque<string, 'ThingTwo'>;

// Now they're completely separate types, so the following will fail to compile.
function createNewThingOne (): NewThingOne {
	// As you can see, casting from a string is still allowed. However, you may not cast NewThingOne to NewThingTwo, and vice versa.
	return 'new thing one' as NewThingOne;
}

// This will fail to compile, as they are fundamentally different types.
const thingTwo = createNewThingOne() as NewThingTwo;

// Here's another example of opaque typing.
function createAccountNumber(): AccountNumber {
	return 2 as AccountNumber;
}

function getMoneyForAccount(accountNumber: AccountNumber): AccountBalance {
	return 4 as AccountBalance;
}

// This will compile successfully.
getMoneyForAccount(createAccountNumber());

// But this won't, because it has to be explicitly passed as an `AccountNumber` type.
getMoneyForAccount(2);

// You can use opaque values like they aren't opaque too.
const accountNumber = createAccountNumber();

// This will not compile successfully.
const newAccountNumber = accountNumber + 2;

// As a side note, you can (and should) use recursive types for your opaque types to make them stronger and hopefully easier to type.
type Person = {
	id: Opaque<number, Person>;
	name: string;
};
```
*/
export type Opaque<Type, Token = unknown> = Type & {readonly __opaque__: Token};


code:
import {LiteralUnion} from './literal-union';

declare namespace PackageJson {
	/**
	A person who has been involved in creating or maintaining the package.
	*/
	export type Person =
		| string
		| {
			name: string;
			url?: string;
			email?: string;
		};

	export type BugsLocation =
		| string
		| {
			/**
			The URL to the package's issue tracker.
			*/
			url?: string;

			/**
			The email address to which issues should be reported.
			*/
			email?: string;
		};

	export interface DirectoryLocations {
		[directoryType: string]: unknown;

		/**
		Location for executable scripts. Sugar to generate entries in the `bin` property by walking the folder.
		*/
		bin?: string;

		/**
		Location for Markdown files.
		*/
		doc?: string;

		/**
		Location for example scripts.
		*/
		example?: string;

		/**
		Location for the bulk of the library.
		*/
		lib?: string;

		/**
		Location for man pages. Sugar to generate a `man` array by walking the folder.
		*/
		man?: string;

		/**
		Location for test files.
		*/
		test?: string;
	}

	export type Scripts = {
		/**
		Run **before** the package is published (Also run on local `npm install` without any arguments).
		*/
		prepublish?: string;

		/**
		Run both **before** the package is packed and published, and on local `npm install` without any arguments. This is run **after** `prepublish`, but **before** `prepublishOnly`.
		*/
		prepare?: string;

		/**
		Run **before** the package is prepared and packed, **only** on `npm publish`.
		*/
		prepublishOnly?: string;

		/**
		Run **before** a tarball is packed (on `npm pack`, `npm publish`, and when installing git dependencies).
		*/
		prepack?: string;

		/**
		Run **after** the tarball has been generated and moved to its final destination.
		*/
		postpack?: string;

		/**
		Run **after** the package is published.
		*/
		publish?: string;

		/**
		Run **after** the package is published.
		*/
		postpublish?: string;

		/**
		Run **before** the package is installed.
		*/
		preinstall?: string;

		/**
		Run **after** the package is installed.
		*/
		install?: string;

		/**
		Run **after** the package is installed and after `install`.
		*/
		postinstall?: string;

		/**
		Run **before** the package is uninstalled and before `uninstall`.
		*/
		preuninstall?: string;

		/**
		Run **before** the package is uninstalled.
		*/
		uninstall?: string;

		/**
		Run **after** the package is uninstalled.
		*/
		postuninstall?: string;

		/**
		Run **before** bump the package version and before `version`.
		*/
		preversion?: string;

		/**
		Run **before** bump the package version.
		*/
		version?: string;

		/**
		Run **after** bump the package version.
		*/
		postversion?: string;

		/**
		Run with the `npm test` command, before `test`.
		*/
		pretest?: string;

		/**
		Run with the `npm test` command.
		*/
		test?: string;

		/**
		Run with the `npm test` command, after `test`.
		*/
		posttest?: string;

		/**
		Run with the `npm stop` command, before `stop`.
		*/
		prestop?: string;

		/**
		Run with the `npm stop` command.
		*/
		stop?: string;

		/**
		Run with the `npm stop` command, after `stop`.
		*/
		poststop?: string;

		/**
		Run with the `npm start` command, before `start`.
		*/
		prestart?: string;

		/**
		Run with the `npm start` command.
		*/
		start?: string;

		/**
		Run with the `npm start` command, after `start`.
		*/
		poststart?: string;

		/**
		Run with the `npm restart` command, before `restart`. Note: `npm restart` will run the `stop` and `start` scripts if no `restart` script is provided.
		*/
		prerestart?: string;

		/**
		Run with the `npm restart` command. Note: `npm restart` will run the `stop` and `start` scripts if no `restart` script is provided.
		*/
		restart?: string;

		/**
		Run with the `npm restart` command, after `restart`. Note: `npm restart` will run the `stop` and `start` scripts if no `restart` script is provided.
		*/
		postrestart?: string;
	} & Record<string, string>;

	/**
	Dependencies of the package. The version range is a string which has one or more space-separated descriptors. Dependencies can also be identified with a tarball or Git URL.
	*/
	export type Dependency = Record<string, string>;

	/**
	Conditions which provide a way to resolve a package entry point based on the environment.
	*/
	export type ExportCondition = LiteralUnion<
		| 'import'
		| 'require'
		| 'node'
		| 'deno'
		| 'browser'
		| 'electron'
		| 'react-native'
		| 'default',
		string
	>;

	/**
	Entry points of a module, optionally with conditions and subpath exports.
	*/
	export type Exports =
	| string
	| {[key in ExportCondition]: Exports}
	| {[key: string]: Exports}; // eslint-disable-line @typescript-eslint/consistent-indexed-object-style

	export interface NonStandardEntryPoints {
		/**
		An ECMAScript module ID that is the primary entry point to the program.
		*/
		module?: string;

		/**
		A module ID with untranspiled code that is the primary entry point to the program.
		*/
		esnext?:
		| string
		| {
			[moduleName: string]: string | undefined;
			main?: string;
			browser?: string;
		};

		/**
		A hint to JavaScript bundlers or component tools when packaging modules for client side use.
		*/
		browser?:
		| string
		| Record<string, string | false>;

		/**
		Denote which files in your project are "pure" and therefore safe for Webpack to prune if unused.

		[Read more.](https://webpack.js.org/guides/tree-shaking/)
		*/
		sideEffects?: boolean | string[];
	}

	export interface TypeScriptConfiguration {
		/**
		Location of the bundled TypeScript declaration file.
		*/
		types?: string;

		/**
		Location of the bundled TypeScript declaration file. Alias of `types`.
		*/
		typings?: string;
	}

	/**
	An alternative configuration for Yarn workspaces.
	*/
	export interface WorkspaceConfig {
		/**
		An array of workspace pattern strings which contain the workspace packages.
		*/
		packages?: WorkspacePattern[];

		/**
		Designed to solve the problem of packages which break when their `node_modules` are moved to the root workspace directory - a process known as hoisting. For these packages, both within your workspace, and also some that have been installed via `node_modules`, it is important to have a mechanism for preventing the default Yarn workspace behavior. By adding workspace pattern strings here, Yarn will resume non-workspace behavior for any package which matches the defined patterns.

		[Read more](https://classic.yarnpkg.com/blog/2018/02/15/nohoist/)
		*/
		nohoist?: WorkspacePattern[];
	}

	/**
	A workspace pattern points to a directory or group of directories which contain packages that should be included in the workspace installation process.

	The patterns are handled with [minimatch](https://github.com/isaacs/minimatch).

	@example
	`docs` → Include the docs directory and install its dependencies.
	`packages/*` → Include all nested directories within the packages directory, like `packages/cli` and `packages/core`.
	*/
	type WorkspacePattern = string;

	export interface YarnConfiguration {
		/**
		Used to configure [Yarn workspaces](https://classic.yarnpkg.com/docs/workspaces/).

		Workspaces allow you to manage multiple packages within the same repository in such a way that you only need to run `yarn install` once to install all of them in a single pass.

		Please note that the top-level `private` property of `package.json` **must** be set to `true` in order to use workspaces.
		*/
		workspaces?: WorkspacePattern[] | WorkspaceConfig;

		/**
		If your package only allows one version of a given dependency, and you’d like to enforce the same behavior as `yarn install --flat` on the command-line, set this to `true`.

		Note that if your `package.json` contains `"flat": true` and other packages depend on yours (e.g. you are building a library rather than an app), those other packages will also need `"flat": true` in their `package.json` or be installed with `yarn install --flat` on the command-line.
		*/
		flat?: boolean;

		/**
		Selective version resolutions. Allows the definition of custom package versions inside dependencies without manual edits in the `yarn.lock` file.
		*/
		resolutions?: Dependency;
	}

	export interface JSPMConfiguration {
		/**
		JSPM configuration.
		*/
		jspm?: PackageJson;
	}

	/**
	Type for [npm's `package.json` file](https://docs.npmjs.com/creating-a-package-json-file). Containing standard npm properties.
	*/
	export interface PackageJsonStandard {
		/**
		The name of the package.
		*/
		name?: string;

		/**
		Package version, parseable by [`node-semver`](https://github.com/npm/node-semver).
		*/
		version?: string;

		/**
		Package description, listed in `npm search`.
		*/
		description?: string;

		/**
		Keywords associated with package, listed in `npm search`.
		*/
		keywords?: string[];

		/**
		The URL to the package's homepage.
		*/
		homepage?: LiteralUnion<'.', string>;

		/**
		The URL to the package's issue tracker and/or the email address to which issues should be reported.
		*/
		bugs?: BugsLocation;

		/**
		The license for the package.
		*/
		license?: string;

		/**
		The licenses for the package.
		*/
		licenses?: Array<{
			type?: string;
			url?: string;
		}>;

		author?: Person;

		/**
		A list of people who contributed to the package.
		*/
		contributors?: Person[];

		/**
		A list of people who maintain the package.
		*/
		maintainers?: Person[];

		/**
		The files included in the package.
		*/
		files?: string[];

		/**
		Resolution algorithm for importing ".js" files from the package's scope.

		[Read more.](https://nodejs.org/api/esm.html#esm_package_json_type_field)
		*/
		type?: 'module' | 'commonjs';

		/**
		The module ID that is the primary entry point to the program.
		*/
		main?: string;

		/**
		Standard entry points of the package, with enhanced support for ECMAScript Modules.

		[Read more.](https://nodejs.org/api/esm.html#esm_package_entry_points)
		*/
		exports?: Exports;

		/**
		The executable files that should be installed into the `PATH`.
		*/
		bin?:
		| string
		| Record<string, string>;

		/**
		Filenames to put in place for the `man` program to find.
		*/
		man?: string | string[];

		/**
		Indicates the structure of the package.
		*/
		directories?: DirectoryLocations;

		/**
		Location for the code repository.
		*/
		repository?:
		| string
		| {
			type: string;
			url: string;

			/**
			Relative path to package.json if it is placed in non-root directory (for example if it is part of a monorepo).

			[Read more.](https://github.com/npm/rfcs/blob/latest/implemented/0010-monorepo-subdirectory-declaration.md)
			*/
			directory?: string;
		};

		/**
		Script commands that are run at various times in the lifecycle of the package. The key is the lifecycle event, and the value is the command to run at that point.
		*/
		scripts?: Scripts;

		/**
		Is used to set configuration parameters used in package scripts that persist across upgrades.
		*/
		config?: Record<string, unknown>;

		/**
		The dependencies of the package.
		*/
		dependencies?: Dependency;

		/**
		Additional tooling dependencies that are not required for the package to work. Usually test, build, or documentation tooling.
		*/
		devDependencies?: Dependency;

		/**
		Dependencies that are skipped if they fail to install.
		*/
		optionalDependencies?: Dependency;

		/**
		Dependencies that will usually be required by the package user directly or via another dependency.
		*/
		peerDependencies?: Dependency;

		/**
		Indicate peer dependencies that are optional.
		*/
		peerDependenciesMeta?: Record<string, {optional: true}>;

		/**
		Package names that are bundled when the package is published.
		*/
		bundledDependencies?: string[];

		/**
		Alias of `bundledDependencies`.
		*/
		bundleDependencies?: string[];

		/**
		Engines that this package runs on.
		*/
		engines?: {
			[EngineName in 'npm' | 'node' | string]: string;
		};

		/**
		@deprecated
		*/
		engineStrict?: boolean;

		/**
		Operating systems the module runs on.
		*/
		os?: Array<LiteralUnion<
		| 'aix'
		| 'darwin'
		| 'freebsd'
		| 'linux'
		| 'openbsd'
		| 'sunos'
		| 'win32'
		| '!aix'
		| '!darwin'
		| '!freebsd'
		| '!linux'
		| '!openbsd'
		| '!sunos'
		| '!win32',
		string
		>>;

		/**
		CPU architectures the module runs on.
		*/
		cpu?: Array<LiteralUnion<
		| 'arm'
		| 'arm64'
		| 'ia32'
		| 'mips'
		| 'mipsel'
		| 'ppc'
		| 'ppc64'
		| 's390'
		| 's390x'
		| 'x32'
		| 'x64'
		| '!arm'
		| '!arm64'
		| '!ia32'
		| '!mips'
		| '!mipsel'
		| '!ppc'
		| '!ppc64'
		| '!s390'
		| '!s390x'
		| '!x32'
		| '!x64',
		string
		>>;

		/**
		If set to `true`, a warning will be shown if package is installed locally. Useful if the package is primarily a command-line application that should be installed globally.

		@deprecated
		*/
		preferGlobal?: boolean;

		/**
		If set to `true`, then npm will refuse to publish it.
		*/
		private?: boolean;

		/**
		A set of config values that will be used at publish-time. It's especially handy to set the tag, registry or access, to ensure that a given package is not tagged with 'latest', published to the global public registry or that a scoped module is private by default.
		*/
		publishConfig?: Record<string, unknown>;

		/**
		Describes and notifies consumers of a package's monetary support information.

		[Read more.](https://github.com/npm/rfcs/blob/latest/accepted/0017-add-funding-support.md)
		*/
		funding?: string | {
			/**
			The type of funding.
			*/
			type?: LiteralUnion<
			| 'github'
			| 'opencollective'
			| 'patreon'
			| 'individual'
			| 'foundation'
			| 'corporation',
			string
			>;

			/**
			The URL to the funding page.
			*/
			url: string;
		};
	}
}

/**
Type for [npm's `package.json` file](https://docs.npmjs.com/creating-a-package-json-file). Also includes types for fields used by other popular projects, like TypeScript and Yarn.
*/
export type PackageJson =
PackageJson.PackageJsonStandard &
PackageJson.NonStandardEntryPoints &
PackageJson.TypeScriptConfiguration &
PackageJson.YarnConfiguration &
PackageJson.JSPMConfiguration;


code:
import {Primitive} from './basic';

/**
Create a type from another type with all keys and nested keys set to optional.

Use-cases:
- Merging a default settings/config object with another object, the second object would be a deep partial of the default object.
- Mocking and testing complex entities, where populating an entire object with its keys would be redundant in terms of the mock or test.

@example
```
import {PartialDeep} from 'type-fest';

const settings: Settings = {
	textEditor: {
		fontSize: 14;
		fontColor: '#000000';
		fontWeight: 400;
	}
	autocomplete: false;
	autosave: true;
};

const applySavedSettings = (savedSettings: PartialDeep<Settings>) => {
	return {...settings, ...savedSettings};
}

settings = applySavedSettings({textEditor: {fontWeight: 500}});
```
*/
export type PartialDeep<T> = T extends Primitive
	? Partial<T>
	: T extends Map<infer KeyType, infer ValueType>
	? PartialMapDeep<KeyType, ValueType>
	: T extends Set<infer ItemType>
	? PartialSetDeep<ItemType>
	: T extends ReadonlyMap<infer KeyType, infer ValueType>
	? PartialReadonlyMapDeep<KeyType, ValueType>
	: T extends ReadonlySet<infer ItemType>
	? PartialReadonlySetDeep<ItemType>
	: T extends ((...arguments: any[]) => unknown)
	? T | undefined
	: T extends object
	? PartialObjectDeep<T>
	: unknown;

/**
Same as `PartialDeep`, but accepts only `Map`s and  as inputs. Internal helper for `PartialDeep`.
*/
interface PartialMapDeep<KeyType, ValueType> extends Map<PartialDeep<KeyType>, PartialDeep<ValueType>> {}

/**
Same as `PartialDeep`, but accepts only `Set`s as inputs. Internal helper for `PartialDeep`.
*/
interface PartialSetDeep<T> extends Set<PartialDeep<T>> {}

/**
Same as `PartialDeep`, but accepts only `ReadonlyMap`s as inputs. Internal helper for `PartialDeep`.
*/
interface PartialReadonlyMapDeep<KeyType, ValueType> extends ReadonlyMap<PartialDeep<KeyType>, PartialDeep<ValueType>> {}

/**
Same as `PartialDeep`, but accepts only `ReadonlySet`s as inputs. Internal helper for `PartialDeep`.
*/
interface PartialReadonlySetDeep<T> extends ReadonlySet<PartialDeep<T>> {}

/**
Same as `PartialDeep`, but accepts only `object`s as inputs. Internal helper for `PartialDeep`.
*/
type PartialObjectDeep<ObjectType extends object> = {
	[KeyType in keyof ObjectType]?: PartialDeep<ObjectType[KeyType]>
};


code:
/**
Create a type that represents either the value or the value wrapped in `PromiseLike`.

Use-cases:
- A function accepts a callback that may either return a value synchronously or may return a promised value.
- This type could be the return type of `Promise#then()`, `Promise#catch()`, and `Promise#finally()` callbacks.

Please upvote [this issue](https://github.com/microsoft/TypeScript/issues/31394) if you want to have this type as a built-in in TypeScript.

@example
```
import {Promisable} from 'type-fest';

async function logger(getLogEntry: () => Promisable<string>): Promise<void> {
    const entry = await getLogEntry();
    console.log(entry);
}

logger(() => 'foo');
logger(() => Promise.resolve('bar'));
```
*/
export type Promisable<T> = T | PromiseLike<T>;


code:
/**
Returns the type that is wrapped inside a `Promise` type.
If the type is a nested Promise, it is unwrapped recursively until a non-Promise type is obtained.
If the type is not a `Promise`, the type itself is returned.

@example
```
import {PromiseValue} from 'type-fest';

type AsyncData = Promise<string>;
let asyncData: PromiseValue<AsyncData> = Promise.resolve('ABC');

type Data = PromiseValue<AsyncData>;
let data: Data = await asyncData;

// Here's an example that shows how this type reacts to non-Promise types.
type SyncData = PromiseValue<string>;
let syncData: SyncData = getSyncData();

// Here's an example that shows how this type reacts to recursive Promise types.
type RecursiveAsyncData = Promise<Promise<string> >;
let recursiveAsyncData: PromiseValue<RecursiveAsyncData> = Promise.resolve(Promise.resolve('ABC'));
```
*/
export type PromiseValue<PromiseType, Otherwise = PromiseType> = PromiseType extends Promise<infer Value>
	? { 0: PromiseValue<Value>; 1: Value }[PromiseType extends Promise<unknown> ? 0 : 1]
	: Otherwise;


code:
import {Primitive} from './basic';

/**
Convert `object`s, `Map`s, `Set`s, and `Array`s and all of their keys/elements into immutable structures recursively.

This is useful when a deeply nested structure needs to be exposed as completely immutable, for example, an imported JSON module or when receiving an API response that is passed around.

Please upvote [this issue](https://github.com/microsoft/TypeScript/issues/13923) if you want to have this type as a built-in in TypeScript.

@example
```
// data.json
{
	"foo": ["bar"]
}

// main.ts
import {ReadonlyDeep} from 'type-fest';
import dataJson = require('./data.json');

const data: ReadonlyDeep<typeof dataJson> = dataJson;

export default data;

// test.ts
import data from './main';

data.foo.push('bar');
//=> error TS2339: Property 'push' does not exist on type 'readonly string[]'
```
*/
export type ReadonlyDeep<T> = T extends Primitive | ((...arguments: any[]) => unknown)
	? T
	: T extends ReadonlyMap<infer KeyType, infer ValueType>
	? ReadonlyMapDeep<KeyType, ValueType>
	: T extends ReadonlySet<infer ItemType>
	? ReadonlySetDeep<ItemType>
	: T extends object
	? ReadonlyObjectDeep<T>
	: unknown;

/**
Same as `ReadonlyDeep`, but accepts only `ReadonlyMap`s as inputs. Internal helper for `ReadonlyDeep`.
*/
interface ReadonlyMapDeep<KeyType, ValueType>
	extends ReadonlyMap<ReadonlyDeep<KeyType>, ReadonlyDeep<ValueType>> {}

/**
Same as `ReadonlyDeep`, but accepts only `ReadonlySet`s as inputs. Internal helper for `ReadonlyDeep`.
*/
interface ReadonlySetDeep<ItemType>
	extends ReadonlySet<ReadonlyDeep<ItemType>> {}

/**
Same as `ReadonlyDeep`, but accepts only `object`s as inputs. Internal helper for `ReadonlyDeep`.
*/
type ReadonlyObjectDeep<ObjectType extends object> = {
	readonly [KeyType in keyof ObjectType]: ReadonlyDeep<ObjectType[KeyType]>
};


code:
import {Except} from './except';

/**
Create a type that requires at least one of the given keys. The remaining keys are kept as is.

@example
```
import {RequireAtLeastOne} from 'type-fest';

type Responder = {
	text?: () => string;
	json?: () => string;

	secure?: boolean;
};

const responder: RequireAtLeastOne<Responder, 'text' | 'json'> = {
	json: () => '{"message": "ok"}',
	secure: true
};
```
*/
export type RequireAtLeastOne<
	ObjectType,
	KeysType extends keyof ObjectType = keyof ObjectType
> = {
	// For each `Key` in `KeysType` make a mapped type:
	[Key in KeysType]-?: Required<Pick<ObjectType, Key>> & // 1. Make `Key`'s type required
		// 2. Make all other keys in `KeysType` optional
		Partial<Pick<ObjectType, Exclude<KeysType, Key>>>;
}[KeysType] &
	// 3. Add the remaining keys not in `KeysType`
	Except<ObjectType, KeysType>;


code:
// TODO: Remove this when we target TypeScript >=3.5.
type _Omit<T, K extends keyof any> = Pick<T, Exclude<keyof T, K>>;

/**
Create a type that requires exactly one of the given keys and disallows more. The remaining keys are kept as is.

Use-cases:
- Creating interfaces for components that only need one of the keys to display properly.
- Declaring generic keys in a single place for a single use-case that gets narrowed down via `RequireExactlyOne`.

The caveat with `RequireExactlyOne` is that TypeScript doesn't always know at compile time every key that will exist at runtime. Therefore `RequireExactlyOne` can't do anything to prevent extra keys it doesn't know about.

@example
```
import {RequireExactlyOne} from 'type-fest';

type Responder = {
	text: () => string;
	json: () => string;
	secure: boolean;
};

const responder: RequireExactlyOne<Responder, 'text' | 'json'> = {
	// Adding a `text` key here would cause a compile error.

	json: () => '{"message": "ok"}',
	secure: true
};
```
*/
export type RequireExactlyOne<ObjectType, KeysType extends keyof ObjectType = keyof ObjectType> =
	{[Key in KeysType]: (
		Required<Pick<ObjectType, Key>> &
		Partial<Record<Exclude<KeysType, Key>, never>>
	)}[KeysType] & _Omit<ObjectType, KeysType>;


code:
import {Except} from './except';

/**
Create a type that makes the given keys optional. The remaining keys are kept as is. The sister of the `SetRequired` type.

Use-case: You want to define a single model where the only thing that changes is whether or not some of the keys are optional.

@example
```
import {SetOptional} from 'type-fest';

type Foo = {
	a: number;
	b?: string;
	c: boolean;
}

type SomeOptional = SetOptional<Foo, 'b' | 'c'>;
// type SomeOptional = {
// 	a: number;
// 	b?: string; // Was already optional and still is.
// 	c?: boolean; // Is now optional.
// }
```
*/
export type SetOptional<BaseType, Keys extends keyof BaseType = keyof BaseType> =
	// Pick just the keys that are not optional from the base type.
	Except<BaseType, Keys> &
	// Pick the keys that should be optional from the base type and make them optional.
	Partial<Pick<BaseType, Keys>> extends
	// If `InferredType` extends the previous, then for each key, use the inferred type key.
	infer InferredType
		? {[KeyType in keyof InferredType]: InferredType[KeyType]}
		: never;


code:
import {Except} from './except';

/**
Create a type that makes the given keys required. The remaining keys are kept as is. The sister of the `SetOptional` type.

Use-case: You want to define a single model where the only thing that changes is whether or not some of the keys are required.

@example
```
import {SetRequired} from 'type-fest';

type Foo = {
	a?: number;
	b: string;
	c?: boolean;
}

type SomeRequired = SetRequired<Foo, 'b' | 'c'>;
// type SomeRequired = {
// 	a?: number;
// 	b: string; // Was already required and still is.
// 	c: boolean; // Is now required.
// }
```
*/
export type SetRequired<BaseType, Keys extends keyof BaseType = keyof BaseType> =
	// Pick just the keys that are not required from the base type.
	Except<BaseType, Keys> &
	// Pick the keys that should be required from the base type and make them required.
	Required<Pick<BaseType, Keys>> extends
	// If `InferredType` extends the previous, then for each key, use the inferred type key.
	infer InferredType
		? {[KeyType in keyof InferredType]: InferredType[KeyType]}
		: never;


code:
type IsAny<T> = 0 extends (1 & T) ? true : false; // https://stackoverflow.com/a/49928360/3406963
type IsNever<T> = [T] extends [never] ? true : false;
type IsUnknown<T> = IsNever<T> extends false ? T extends unknown ? unknown extends T ? IsAny<T> extends false ? true : false : false : false : false;

/**
Create a function type with a return type of your choice and the same parameters as the given function type.

Use-case: You want to define a wrapped function that returns something different while receiving the same parameters. For example, you might want to wrap a function that can throw an error into one that will return `undefined` instead.

@example
```
import {SetReturnType} from 'type-fest';

type MyFunctionThatCanThrow = (foo: SomeType, bar: unknown) => SomeOtherType;

type MyWrappedFunction = SetReturnType<MyFunctionThatCanThrow, SomeOtherType | undefined>;
//=> type MyWrappedFunction = (foo: SomeType, bar: unknown) => SomeOtherType | undefined;
```
*/
export type SetReturnType<Fn extends (...args: any[]) => any, TypeToReturn> =
	// Just using `Parameters<Fn>` isn't ideal because it doesn't handle the `this` fake parameter.
	Fn extends (this: infer ThisArg, ...args: infer Arguments) => any ? (
		// If a function did not specify the `this` fake parameter, it will be inferred to `unknown`.
		// We want to detect this situation just to display a friendlier type upon hovering on an IntelliSense-powered IDE.
		IsUnknown<ThisArg> extends true ? (...args: Arguments) => TypeToReturn : (this: ThisArg, ...args: Arguments) => TypeToReturn
	) : (
		// This part should be unreachable, but we make it meaningful just in case…
		(...args: Parameters<Fn>) => TypeToReturn
	);


code:
/**
Create a type with the keys of the given type changed to `string` type.

Use-case: Changing interface values to strings in order to use them in a form model.

@example
```
import {Stringified} from 'type-fest';

type Car {
	model: string;
	speed: number;
}

const carForm: Stringified<Car> = {
	model: 'Foo',
	speed: '101'
};
```
*/
export type Stringified<ObjectType> = {[KeyType in keyof ObjectType]: string};