type-fest
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A collection of essential TypeScript types
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<br>
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<b>A collection of essential TypeScript types</b>
<br>
<br>
<br>
<br>
<hr>
<div align="center">
<p>
<p>
<sup>
<a href="https://github.com/sponsors/sindresorhus">Sindre Sorhus' open source work is supported by the community</a>
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<sup>Special thanks to:</sup>
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<b>Your app, enterprise-ready.</b>
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<sub>Start selling to enterprise customers with just a few lines of code.</sub>
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<sup>Add Single Sign-On (and more) in minutes instead of months.</sup>
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<b>The better identity infrastructure for developers</b>
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<sup>Logto is an open-source Auth0 alternative designed for every app.</sup>
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[](https://giphy.com/gifs/illustration-rainbow-unicorn-26AHG5KGFxSkUWw1i)
[](https://www.npmjs.com/package/type-fest?activeTab=dependents)
[](https://www.npmjs.com/package/type-fest)
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/main/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.
**Help wanted with reviewing [proposals](https://github.com/sindresorhus/type-fest/issues) and [pull requests](https://github.com/sindresorhus/type-fest/pulls).**
## Install
```sh
npm install type-fest
```
*Requires TypeScript >=5.1*
*Works best with [`{strict: true}`](https://www.typescriptlang.org/tsconfig#strict) in your tsconfig.*
## Usage
```ts
import type {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/primitive.d.ts) - Matches any [primitive value](https://developer.mozilla.org/en-US/docs/Glossary/Primitive).
- [`Class`](source/basic.d.ts) - Matches a [`class`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Classes).
- [`Constructor`](source/basic.d.ts) - Matches a [`class` constructor](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Classes).
- [`AbstractClass`](source/basic.d.ts) - Matches an [`abstract class`](https://www.typescriptlang.org/docs/handbook/classes.html#abstract-classes).
- [`AbstractConstructor`](source/basic.d.ts) - Matches an [`abstract class`](https://www.typescriptlang.org/docs/handbook/release-notes/typescript-4-2.html#abstract-construct-signatures) constructor.
- [`TypedArray`](source/typed-array.d.ts) - Matches any [typed array](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/TypedArray), like `Uint8Array` or `Float64Array`.
- [`ObservableLike`](source/observable-like.d.ts) - Matches a value that is like an [Observable](https://github.com/tc39/proposal-observable).
### Utilities
- [`EmptyObject`](source/empty-object.d.ts) - Represents a strictly empty plain object, the `{}` value.
- [`NonEmptyObject`](source/non-empty-object.d.ts) - Represents an object with at least 1 non-optional key.
- [`UnknownRecord`](source/unknown-record.d.ts) - Represents an object with `unknown` value. You probably want this instead of `{}`.
- [`UnknownArray`](source/unknown-array.d.ts) - Represents an array with `unknown` value.
- [`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/utility-types.html#omittype-keys).
- [`Writable`](source/writable.d.ts) - Create a type that strips `readonly` from the given type. Inverse of `Readonly<T>`.
- [`WritableDeep`](source/writable-deep.d.ts) - Create a deeply mutable version of an `object`/`ReadonlyMap`/`ReadonlySet`/`ReadonlyArray` type. The inverse of `ReadonlyDeep<T>`. Use `Writable<T>` if you only need one level deep.
- [`Merge`](source/merge.d.ts) - Merge two types into a new type. Keys of the second type overrides keys of the first type.
- [`MergeDeep`](source/merge-deep.d.ts) - Merge two objects or two arrays/tuples recursively into a new type.
- [`MergeExclusive`](source/merge-exclusive.d.ts) - Create a type that has mutually exclusive keys.
- [`OverrideProperties`](source/override-properties.d.ts) - Override only existing properties of the given type. Similar to `Merge`, but enforces that the original type has the properties you want to override.
- [`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.
- [`RequireAllOrNone`](source/require-all-or-none.d.ts) - Create a type that requires all of the given keys or none of the given keys.
- [`RequireOneOrNone`](source/require-one-or-none.d.ts) - Create a type that requires exactly a single key of the given keys and disallows more, or none of the given keys.
- [`SingleKeyObject`](source/single-key-object.d.ts) - Create a type that only accepts an object with a single key.
- [`RequiredDeep`](source/required-deep.d.ts) - Create a deeply required version of another type. Use [`Required<T>`](https://www.typescriptlang.org/docs/handbook/utility-types.html#requiredtype) if you only need one level deep.
- [`PickDeep`](source/pick-deep.d.ts) - Pick properties from a deeply-nested object. Use [`Pick<T>`](https://www.typescriptlang.org/docs/handbook/utility-types.html#picktype-keys) if you only need one level deep.
- [`OmitDeep`](source/omit-deep.d.ts) - Omit properties from a deeply-nested object. Use [`Omit<T>`](https://www.typescriptlang.org/docs/handbook/utility-types.html#omittype-keys) if you only need one level deep.
- [`OmitIndexSignature`](source/omit-index-signature.d.ts) - Omit any index signatures from the given object type, leaving only explicitly defined properties.
- [`PickIndexSignature`](source/pick-index-signature.d.ts) - Pick only index signatures from the given object type, leaving out all explicitly defined properties.
- [`PartialDeep`](source/partial-deep.d.ts) - Create a deeply optional version of another type. Use [`Partial<T>`](https://www.typescriptlang.org/docs/handbook/utility-types.html#partialtype) if you only need one level deep.
- [`PartialOnUndefinedDeep`](source/partial-on-undefined-deep.d.ts) - Create a deep version of another type where all keys accepting `undefined` type are set to optional.
- [`UndefinedOnPartialDeep`](source/undefined-on-partial-deep.d.ts) - Create a deep version of another type where all optional keys are set to also accept `undefined`.
- [`ReadonlyDeep`](source/readonly-deep.d.ts) - Create a deeply immutable version of an `object`/`Map`/`Set`/`Array` type. Use [`Readonly<T>`](https://www.typescriptlang.org/docs/handbook/utility-types.html#readonlytype) 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).
- [`Tagged`](source/tagged.d.ts) - Create a [tagged type](https://medium.com/@KevinBGreene/surviving-the-typescript-ecosystem-branding-and-type-tagging-6cf6e516523d) that can support [multiple tags](https://github.com/sindresorhus/type-fest/issues/665) and [per-tag metadata](https://medium.com/@ethanresnick/advanced-typescript-tagged-types-improved-with-type-level-metadata-5072fc125fcf). (This replaces the previous [`Opaque`](source/tagged.d.ts) type, which is now deprecated.)
- [`UnwrapTagged`](source/tagged.d.ts) - Get the untagged portion of a tagged type created with `Tagged`. (This replaces the previous [`UnwrapOpaque`](source/tagged.d.ts) type, which is now deprecated.)
- [`InvariantOf`](source/invariant-of.d.ts) - Create an [invariant type](https://basarat.gitbook.io/typescript/type-system/type-compatibility#footnote-invariance), which is a type that does not accept supertypes and subtypes.
- [`SetOptional`](source/set-optional.d.ts) - Create a type that makes the given keys optional.
- [`SetReadonly`](source/set-readonly.d.ts) - Create a type that makes the given keys readonly.
- [`SetRequired`](source/set-required.d.ts) - Create a type that makes the given keys required.
- [`SetNonNullable`](source/set-non-nullable.d.ts) - Create a type that makes the given keys non-nullable.
- [`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.
- [`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.
- [`ConditionalPickDeep`](source/conditional-pick-deep.d.ts) - Like `ConditionalPick` except that it selects the properties deeply.
- [`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.
- [`LiteralToPrimitive`](source/literal-to-primitive.d.ts) - Convert a [literal type](https://www.typescriptlang.org/docs/handbook/2/everyday-types.html#literal-types) to the [primitive type](source/primitive.d.ts) it belongs to.
- [`LiteralToPrimitiveDeep`](source/literal-to-primitive-deep.d.ts) - Like `LiteralToPrimitive` except it converts literal types inside an object or array deeply.
- [`Stringified`](source/stringified.d.ts) - Create a type with the keys of the given type changed to `string` type.
- [`IterableElement`](source/iterable-element.d.ts) - Get the element type of an `Iterable`/`AsyncIterable`. For example, `Array`, `Set`, `Map`, generator, stream, etc.
- [`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.
- [`SetParameterType`](source/set-parameter-type.d.ts) - Create a function that replaces some parameters with the given parameters.
- [`Simplify`](source/simplify.d.ts) - Useful to flatten the type output to improve type hints shown in editors. And also to transform an interface into a type to aide with assignability.
- [`SimplifyDeep`](source/simplify-deep.d.ts) - Deeply simplifies an object type.
- [`Get`](source/get.d.ts) - Get a deeply-nested property from an object using a key path, like [Lodash's `.get()`](https://lodash.com/docs/latest#get) function.
- [`StringKeyOf`](source/string-key-of.d.ts) - Get keys of the given type as strings.
- [`Schema`](source/schema.d.ts) - Create a deep version of another object type where property values are recursively replaced into a given value type.
- [`Exact`](source/exact.d.ts) - Create a type that does not allow extra properties.
- [`OptionalKeysOf`](source/optional-keys-of.d.ts) - Extract all optional keys from the given type.
- [`KeysOfUnion`](source/keys-of-union.d.ts) - Create a union of all keys from a given type, even those exclusive to specific union members.
- [`HasOptionalKeys`](source/has-optional-keys.d.ts) - Create a `true`/`false` type depending on whether the given type has any optional fields.
- [`RequiredKeysOf`](source/required-keys-of.d.ts) - Extract all required keys from the given type.
- [`HasRequiredKeys`](source/has-required-keys.d.ts) - Create a `true`/`false` type depending on whether the given type has any required fields.
- [`ReadonlyKeysOf`](source/readonly-keys-of.d.ts) - Extract all readonly keys from the given type.
- [`HasReadonlyKeys`](source/has-readonly-keys.d.ts) - Create a `true`/`false` type depending on whether the given type has any readonly fields.
- [`WritableKeysOf`](source/writable-keys-of.d.ts) - Extract all writable (non-readonly) keys from the given type.
- [`HasWritableKeys`](source/has-writable-keys.d.ts) - Create a `true`/`false` type depending on whether the given type has any writable fields.
- [`Spread`](source/spread.d.ts) - Mimic the type inferred by TypeScript when merging two objects or two arrays/tuples using the spread syntax.
- [`IsEqual`](source/is-equal.d.ts) - Returns a boolean for whether the two given types are equal.
- [`TaggedUnion`](source/tagged-union.d.ts) - Create a union of types that share a common discriminant property.
- [`IntRange`](source/int-range.d.ts) - Generate a union of numbers.
- [`ArrayIndices`](source/array-indices.d.ts) - Provides valid indices for a constant array or tuple.
- [`ArrayValues`](source/array-values.d.ts) - Provides all values for a constant array or tuple.
- [`ArraySplice`](source/array-splice.d.ts) - Creates a new array type by adding or removing elements at a specified index range in the original array.
- [`ArrayTail`](source/array-tail.d.ts) - Extracts the type of an array or tuple minus the first element.
- [`SetFieldType`](source/set-field-type.d.ts) - Create a type that changes the type of the given keys.
- [`Paths`](source/paths.d.ts) - Generate a union of all possible paths to properties in the given object.
- [`SharedUnionFieldsDeep`](source/shared-union-fields-deep.d.ts) - Create a type with shared fields from a union of object types, deeply traversing nested structures.
- [`DistributedOmit`](source/distributed-omit.d.ts) - Omits keys from a type, distributing the operation over a union.
- [`DistributedPick`](source/distributed-pick.d.ts) - Picks keys from a type, distributing the operation over a union.
- [`And`](source/and.d.ts) - Returns a boolean for whether two given types are both true.
- [`Or`](source/or.d.ts) - Returns a boolean for whether either of two given types are true.
- [`NonEmptyTuple`](source/non-empty-tuple.d.ts) - Matches any non-empty tuple.
- [`FindGlobalType`](source/find-global-type.d.ts) - Tries to find the type of a global with the given name.
- [`FindGlobalInstanceType`](source/find-global-type.d.ts) - Tries to find one or more types from their globally-defined constructors.
### Type Guard
#### `IsType` vs. `IfType`
For every `IsT` type (e.g. `IsAny`), there is an associated `IfT` type that can help simplify conditional types. While the `IsT` types return a `boolean`, the `IfT` types act like an `If`/`Else` - they resolve to the given `TypeIfT` or `TypeIfNotT` depending on whether `IsX` is `true` or not. By default, `IfT` returns a `boolean`:
```ts
type IfAny<T, TypeIfAny = true, TypeIfNotAny = false> = (
IsAny<T> extends true ? TypeIfAny : TypeIfNotAny
);
```
#### Usage
```ts
import type {IsAny, IfAny} from 'type-fest';
type ShouldBeTrue = IsAny<any> extends true ? true : false;
//=> true
type ShouldBeFalse = IfAny<'not any'>;
//=> false
type ShouldBeNever = IfAny<'not any', 'not never', 'never'>;
//=> 'never'
```
- [`IsLiteral`](source/is-literal.d.ts) - Returns a boolean for whether the given type is a [literal type](https://www.typescriptlang.org/docs/handbook/2/everyday-types.html#literal-types).
- [`IsStringLiteral`](source/is-literal.d.ts) - Returns a boolean for whether the given type is a `string` [literal type](https://www.typescriptlang.org/docs/handbook/2/everyday-types.html#literal-types).
- [`IsNumericLiteral`](source/is-literal.d.ts) - Returns a boolean for whether the given type is a `number` or `bigint` [literal type](https://www.typescriptlang.org/docs/handbook/2/everyday-types.html#literal-types).
- [`IsBooleanLiteral`](source/is-literal.d.ts) - Returns a boolean for whether the given type is a `true` or `false` [literal type](https://www.typescriptlang.org/docs/handbook/2/everyday-types.html#literal-types).
- [`IsSymbolLiteral`](source/is-literal.d.ts) - Returns a boolean for whether the given type is a `symbol` [literal type](https://www.typescriptlang.org/docs/handbook/2/everyday-types.html#literal-types).
- [`IsAny`](source/is-any.d.ts) - Returns a boolean for whether the given type is `any`. (Conditional version: [`IfAny`](source/if-any.d.ts))
- [`IsNever`](source/is-never.d.ts) - Returns a boolean for whether the given type is `never`. (Conditional version: [`IfNever`](source/if-never.d.ts))
- [`IsUnknown`](source/is-unknown.d.ts) - Returns a boolean for whether the given type is `unknown`. (Conditional version: [`IfUnknown`](source/if-unknown.d.ts))
- [`IsEmptyObject`](source/empty-object.d.ts) - Returns a boolean for whether the type is strictly equal to an empty plain object, the `{}` value. (Conditional version: [`IfEmptyObject`](source/if-empty-object.d.ts))
- [`IsNull`](source/is-null.d.ts) - Returns a boolean for whether the given type is `null`. (Conditional version: [`IfNull`](source/if-null.d.ts))
### JSON
- [`Jsonify`](source/jsonify.d.ts) - Transform a type to one that is assignable to the `JsonValue` type.
- [`Jsonifiable`](source/jsonifiable.d.ts) - Matches a value that can be losslessly converted to JSON.
- [`JsonPrimitive`](source/basic.d.ts) - Matches a JSON primitive.
- [`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.
### Structured clone
- [`StructuredCloneable`](source/structured-cloneable.d.ts) - Matches a value that can be losslessly cloned using `structuredClone`.
### Async
- [`Promisable`](source/promisable.d.ts) - Create a type that represents either the value or the value wrapped in `PromiseLike`.
- [`AsyncReturnType`](source/async-return-type.d.ts) - Unwrap the return type of a function that returns a `Promise`.
- [`Asyncify`](source/asyncify.d.ts) - Create an async version of the given function type.
### String
- [`Trim`](source/trim.d.ts) - Remove leading and trailing spaces from a string.
- [`Split`](source/split.d.ts) - Represents an array of strings split using a given character or character set.
- [`Replace`](source/replace.d.ts) - Represents a string with some or all matches replaced by a replacement.
- [`StringSlice`](source/string-slice.d.ts) - Returns a string slice of a given range, just like `String#slice()`.
- [`StringRepeat`](source/string-repeat.d.ts) - Returns a new string which contains the specified number of copies of a given string, just like `String#repeat()`.
### Array
- [`Arrayable`](source/arrayable.d.ts) - Create a type that represents either the value or an array of the value.
- [`Includes`](source/includes.d.ts) - Returns a boolean for whether the given array includes the given item.
- [`Join`](source/join.d.ts) - Join an array of strings and/or numbers using the given string as a delimiter.
- [`ArraySlice`](source/array-slice.d.ts) - Returns an array slice of a given range, just like `Array#slice()`.
- [`LastArrayElement`](source/last-array-element.d.ts) - Extracts the type of the last element of an array.
- [`FixedLengthArray`](source/fixed-length-array.d.ts) - Create a type that represents an array of the given type and length.
- [`MultidimensionalArray`](source/multidimensional-array.d.ts) - Create a type that represents a multidimensional array of the given type and dimensions.
- [`MultidimensionalReadonlyArray`](source/multidimensional-readonly-array.d.ts) - Create a type that represents a multidimensional readonly array of the given type and dimensions.
- [`ReadonlyTuple`](source/readonly-tuple.d.ts) - Create a type that represents a read-only tuple of the given type and length.
- [`TupleToUnion`](source/tuple-to-union.d.ts) - Convert a tuple/array into a union type of its elements.
- [`UnionToTuple`](source/union-to-tuple.d.ts) - Convert a union type into an unordered tuple type of its elements.
### Numeric
- [`PositiveInfinity`](source/numeric.d.ts) - Matches the hidden `Infinity` type.
- [`NegativeInfinity`](source/numeric.d.ts) - Matches the hidden `-Infinity` type.
- [`Finite`](source/numeric.d.ts) - A finite `number`.
- [`Integer`](source/numeric.d.ts) - A `number` that is an integer.
- [`Float`](source/numeric.d.ts) - A `number` that is not an integer.
- [`NegativeFloat`](source/numeric.d.ts) - A negative (`-∞ < x < 0`) `number` that is not an integer.
- [`Negative`](source/numeric.d.ts) - A negative `number`/`bigint` (`-∞ < x < 0`)
- [`NonNegative`](source/numeric.d.ts) - A non-negative `number`/`bigint` (`0 <= x < ∞`).
- [`NegativeInteger`](source/numeric.d.ts) - A negative (`-∞ < x < 0`) `number` that is an integer.
- [`NonNegativeInteger`](source/numeric.d.ts) - A non-negative (`0 <= x < ∞`) `number` that is an integer.
- [`IsNegative`](source/numeric.d.ts) - Returns a boolean for whether the given number is a negative number.
- [`IsFloat`](source/is-float.d.ts) - Returns a boolean for whether the given number is a float, like `1.5` or `-1.5`.
- [`IsInteger`](source/is-integer.d.ts) - Returns a boolean for whether the given number is a integer, like `-5`, `1.0` or `100`.
- [`GreaterThan`](source/greater-than.d.ts) - Returns a boolean for whether a given number is greater than another number.
- [`GreaterThanOrEqual`](source/greater-than-or-equal.d.ts) - Returns a boolean for whether a given number is greater than or equal to another number.
- [`LessThan`](source/less-than.d.ts) - Returns a boolean for whether a given number is less than another number.
- [`LessThanOrEqual`](source/less-than-or-equal.d.ts) - Returns a boolean for whether a given number is less than or equal to another number.
- [`Sum`](source/sum.d.ts) - Returns the sum of two numbers.
- [`Subtract`](source/subtract.d.ts) - Returns the difference between two numbers.
### Change case
- [`CamelCase`](source/camel-case.d.ts) - Convert a string literal to camel-case (`fooBar`).
- [`CamelCasedProperties`](source/camel-cased-properties.d.ts) - Convert object properties to camel-case (`fooBar`).
- [`CamelCasedPropertiesDeep`](source/camel-cased-properties-deep.d.ts) - Convert object properties to camel-case recursively (`fooBar`).
- [`KebabCase`](source/kebab-case.d.ts) - Convert a string literal to kebab-case (`foo-bar`).
- [`KebabCasedProperties`](source/kebab-cased-properties.d.ts) - Convert a object properties to kebab-case recursively (`foo-bar`).
- [`KebabCasedPropertiesDeep`](source/kebab-cased-properties-deep.d.ts) - Convert object properties to kebab-case (`foo-bar`).
- [`PascalCase`](source/pascal-case.d.ts) - Converts a string literal to pascal-case (`FooBar`)
- [`PascalCasedProperties`](source/pascal-cased-properties.d.ts) - Converts object properties to pascal-case (`FooBar`)
- [`PascalCasedPropertiesDeep`](source/pascal-cased-properties-deep.d.ts) - Converts object properties to pascal-case (`FooBar`)
- [`SnakeCase`](source/snake-case.d.ts) - Convert a string literal to snake-case (`foo_bar`).
- [`SnakeCasedProperties`](source/snake-cased-properties.d.ts) - Convert object properties to snake-case (`foo_bar`).
- [`SnakeCasedPropertiesDeep`](source/snake-cased-properties-deep.d.ts) - Convert object properties to snake-case recursively (`foo_bar`).
- [`ScreamingSnakeCase`](source/screaming-snake-case.d.ts) - Convert a string literal to screaming-snake-case (`FOO_BAR`).
- [`DelimiterCase`](source/delimiter-case.d.ts) - Convert a string literal to a custom string delimiter casing.
- [`DelimiterCasedProperties`](source/delimiter-cased-properties.d.ts) - Convert object properties to a custom string delimiter casing.
- [`DelimiterCasedPropertiesDeep`](source/delimiter-cased-properties-deep.d.ts) - Convert object properties to a custom string delimiter casing recursively.
### Miscellaneous
- [`GlobalThis`](source/global-this.d.ts) - Declare locally scoped properties on `globalThis`.
- [`PackageJson`](source/package-json.d.ts) - Type for [npm's `package.json` file](https://docs.npmjs.com/creating-a-package-json-file). It also includes support for [TypeScript Declaration Files](https://www.typescriptlang.org/docs/handbook/declaration-files/publishing.html).
- [`TsConfigJson`](source/tsconfig-json.d.ts) - Type for [TypeScript's `tsconfig.json` file](https://www.typescriptlang.org/docs/handbook/tsconfig-json.html).
## 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 pull request 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://www.typescriptlang.org/docs/handbook/utility-types.html#recordkeys-type), which is more flexible and well-known. Also, you shouldn't use an object as a dictionary. We have `Map` in JavaScript now.
- [`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.
- [`Url2Json`](https://github.com/sindresorhus/type-fest/pull/262) - Inferring search parameters from a URL string is a cute idea, but not very useful in practice, since search parameters are usually dynamic and defined separately.
- [`Nullish`](https://github.com/sindresorhus/type-fest/pull/318) - The type only saves a couple of characters, not everyone knows what "nullish" means, and I'm also trying to [get away from `null`](https://github.com/sindresorhus/meta/discussions/7).
- [`TitleCase`](https://github.com/sindresorhus/type-fest/pull/303) - It's not solving a common need and is a better fit for a separate package.
- [`ExtendOr` and `ExtendAnd`](https://github.com/sindresorhus/type-fest/pull/247) - The benefits don't outweigh having to learn what they mean.
- [`PackageJsonExtras`](https://github.com/sindresorhus/type-fest/issues/371) - There are too many possible configurations that can be put into `package.json`. If you would like to extend `PackageJson` to support an additional configuration in your project, please see the *Extending existing types* section below.
## Alternative type names
*If you know one of our types by a different name, add it here for discovery.*
- `Prettify`- See [`Simplify`](source/simplify.d.ts)
- `Expand`- See [`Simplify`](source/simplify.d.ts)
- `PartialBy` - See [`SetOptional`](source/set-optional.d.ts)
- `RecordDeep`- See [`Schema`](source/schema.d.ts)
- `Mutable`- See [`Writable`](source/writable.d.ts)
- `RequireOnlyOne`, `OneOf` - See [`RequireExactlyOne`](source/require-exactly-one.d.ts)
- `AtMostOne` - See [`RequireOneOrNone`](source/require-one-or-none.d.ts)
- `AllKeys` - See [`KeysOfUnion`](source/keys-of-union.d.ts)
- `Branded` - See [`Tagged`](source/tagged.d.ts)
- `Opaque` - See [`Tagged`](source/tagged.d.ts)
- `SetElement` - See [`IterableElement`](source/iterable-element.d.ts)
- `SetEntry` - See [`IterableElement`](source/iterable-element.d.ts)
- `SetValues` - See [`IterableElement`](source/iterable-element.d.ts)
## Tips
### Extending existing types
- [`PackageJson`](source/package-json.d.ts) - There are a lot of tools that place extra configurations inside the `package.json` file. You can extend `PackageJson` to support these additional configurations.
<details>
<summary>
Example
</summary>
[Playground](https://www.typescriptlang.org/play?#code/JYWwDg9gTgLgBDAnmApnA3gBQIYGMDW2A5igFIDOEAdnNuXAEJ0o4HFmVUC+cAZlBBBwA5ElQBaXinIxhAbgCwAKFCRYCZGnQAZYFRgooPfoJHSANntmKlysWlaESFanAC8jZo-YuaAMgwLKwBhal5gIgB+AC44XX1DADpQqnCiLhsgA)
```ts
import type {PackageJson as BasePackageJson} from 'type-fest';
import type {Linter} from 'eslint';
type PackageJson = BasePackageJson & {eslintConfig?: Linter.Config};
```
</details>
### Related
- [typed-query-selector](https://github.com/g-plane/typed-query-selector) - Enhances `document.querySelector` and `document.querySelectorAll` with a template literal type that matches element types returned from an HTML element query selector.
- [`Linter.Config`](https://github.com/DefinitelyTyped/DefinitelyTyped/blob/master/types/eslint/index.d.ts) - Definitions for the [ESLint configuration schema](https://eslint.org/docs/user-guide/configuring/language-options).
### Built-in types
There are many advanced types most users don't know about.
- [`Awaited<T>`](https://www.typescriptlang.org/docs/handbook/utility-types.html#awaitedtype) - Extract the type of a value that a `Promise` resolves to.
<details>
<summary>
Example
</summary>
[Playground](https://www.typescriptlang.org/play/?#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-C1QcxIKhJc0mWNWhngwK0YJQEJpdj8Wy5mEIU4rQFURXuZWq+5PF4raPJuPte0eHQ+fxkXHpWG6GCQKBOApuITIQGNCMM2xRGgqIPIeWwKJQOqmOACadafr+rToGiFDSj-RNEfFUo6EbgaDwJB0vGz9wnhqImpRb2Es8QBlLhZwDYjuBkGQrz+kMyC6OEfjnBAACONCXGAm5aCAEDKsqHTpPIs4fMgXjQNE2aFhkxx4d+gbBqoQjWJKChKKIxbwqWZqGI2VpqtQECPNo0BJpaSA4tCZEhhAYYRu23HMbxn7IDSUJAA)
```ts
interface User {
id: number;
name: string;
age: number;
}
class UserApiService {
async fetchUser(userId: number): Promise<User> {
// Fetch the user data from the database.
// The actual implementation might look like this:
// const response = await fetch('/api/user/${userId}');
// const data = response.json();
// return data;
return {
id: 1,
name: 'John Doe',
age: 30
};
}
}
type FetchedUser = Awaited<ReturnType<UserApiService['fetchUser']>>;
async function handleUserData(apiService: UserApiService, userId: number) {
try {
const user: FetchedUser = await apiService.fetchUser(userId);
// After fetching user data, you can perform various actions such as updating the user interface,
// caching the data for future use, or making additional API requests as needed.
} catch (error) {
// Error handling
}
}
const userApiService = new UserApiService();
handleUserData(userApiService, 1);
```
- [`Partial<T>`](https://www.typescriptlang.org/docs/handbook/utility-types.html#partialtype) - Make all properties in `T` optional.
<details>
<summary>
Example
</summary>
[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'});
```
</details>
- [`Required<T>`](https://www.typescriptlang.org/docs/handbook/utility-types.html#requiredtype) - Make all properties in `T` required.
<details>
<summary>
Example
</summary>
[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',
});
```
</details>
- [`Readonly<T>`](https://www.typescriptlang.org/docs/handbook/utility-types.html#readonlytype) - Make all properties in `T` readonly.
<details>
<summary>
Example
</summary>
[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;
```
</details>
- [`Pick<T, K>`](https://www.typescriptlang.org/docs/handbook/utility-types.html#picktype-keys) - From `T`, pick a set of properties whose keys are in the union `K`.
<details>
<summary>
Example
</summary>
[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'
}
]);
```
</details>
- [`Record<K, T>`](https://www.typescriptlang.org/docs/handbook/utility-types.html#recordkeys-type) - Construct a type with a set of properties `K` of type `T`.
<details>
<summary>
Example
</summary>
[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,
};
```
</details>
- [`Exclude<T, U>`](https://www.typescriptlang.org/docs/handbook/utility-types.html#excludetype-excludedunion) - Exclude from `T` those types that are assignable to `U`.
<details>
<summary>
Example
</summary>
[Playground](https://typescript-play.js.org/?target=6#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)
```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);
}
```
</details>
- [`Extract<T, U>`](https://www.typescriptlang.org/docs/handbook/utility-types.html#extracttype-union) - Extract from `T` those types that are assignable to `U`.
<details>
<summary>
Example
</summary>
[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');
// Government didn't like the fact that you wanted to change your identity.
changePersonData(andrew, ID, uniqueId());
```
</details>
- [`NonNullable<T>`](https://www.typescriptlang.org/docs/handbook/utility-types.html#nonnullabletype) - Exclude `null` and `undefined` from `T`.
<details>
<summary>
Example
</summary>
Works with <a href="https://www.typescriptlang.org/tsconfig#strictNullChecks"><code>strictNullChecks</code></a> set to <code>true</code>.
[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;
```
</details>
- [`Parameters<T>`](https://www.typescriptlang.org/docs/handbook/utility-types.html#parameterstype) - Obtain the parameters of a function type in a tuple.
<details>
<summary>
Example
</summary>
[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 (...arguments_: any[]) => any> (func: Fn, callCount: number) {
// Type that represents the type of the received function parameters.
type FunctionParameters = Parameters<Fn>;
return function (...arguments_: FunctionParameters) {
for (let i = 0; i < callCount; i++) {
func(...arguments_);
}
}
}
const shuffleTwice = callNTimes(shuffle, 2);
```
</details>
- [`ConstructorParameters<T>`](https://www.typescriptlang.org/docs/handbook/utility-types.html#constructorparameterstype) - Obtain the parameters of a constructor function type in a tuple.
<details>
<summary>
Example
</summary>
[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 (...arguments_: any[]) => any)> {
private ClassConstructor: T;
private cache: Map<string, InstanceType<T>> = new Map();
constructor (ctr: T) {
this.ClassConstructor = ctr;
}
getInstance (...arguments_: ConstructorParameters<T>): InstanceType<T> {
const hash = this.calculateArgumentsHash(...arguments_);
const existingInstance = this.cache.get(hash);
if (existingInstance !== undefined) {
return existingInstance;
}
return new this.ClassConstructor(...arguments_);
}
private calculateArgumentsHash(...arguments_: any[]): string {
// Calculate hash.
return 'hash';
}
}
const articleCache = new InstanceCache(ArticleModel);
const amazonArticle = articleCache.getInstance('Amazon forests burning!');
```
</details>
- [`ReturnType<T>`](https://www.typescriptlang.org/docs/handbook/utility-types.html#returntypetype) - Obtain the return type of a function type.
<details>
<summary>
Example
</summary>
[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