A collection of essential TypeScript types

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[![](https://img.shields.io/badge/unicorn-approved-ff69b4.svg)](https://giphy.com/gifs/illustration-rainbow-unicorn-26AHG5KGFxSkUWw1i) [![npm dependents](https://badgen.net/npm/dependents/type-fest)](https://www.npmjs.com/package/type-fest?activeTab=dependents) [![npm downloads](https://badgen.net/npm/dt/type-fest)](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; //=> {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`. - [`WritableDeep`](source/writable-deep.d.ts) - Create a deeply mutable version of an `object`/`ReadonlyMap`/`ReadonlySet`/`ReadonlyArray` type. The inverse of `ReadonlyDeep`. Use `Writable` 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`](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`](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`](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`](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`](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 = ( IsAny extends true ? TypeIfAny : TypeIfNotAny ); ``` #### Usage ```ts import type {IsAny, IfAny} from 'type-fest'; type ShouldBeTrue = IsAny 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` vs `Record`) 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.

Example

[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}; ```

### 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. - [`Partial`](https://www.typescriptlang.org/docs/handbook/utility-types.html#partialtype) - 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: Key, value: NodeConfig[Key]) { this.configuration[key] = value; } config(config: Partial) { 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`` allows us to provide only a part of the // NodeConfig interface. new NodeAppBuilder().config({appName: 'ToDoApp'}); ```

- [`Required`](https://www.typescriptlang.org/docs/handbook/utility-types.html#requiredtype) - 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) { // 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`](https://www.typescriptlang.org/docs/handbook/utility-types.html#readonlytype) - 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; 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`](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`.

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; // 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`](https://www.typescriptlang.org/docs/handbook/utility-types.html#recordkeys-type) - 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 = { 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 = { intern: null, developer: null, }; ```

- [`Exclude`](https://www.typescriptlang.org/docs/handbook/utility-types.html#excludetype-excludedunion) - Exclude from `T` those types that are assignable to `U`.

Example

[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): 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`](https://www.typescriptlang.org/docs/handbook/utility-types.html#extracttype-union) - 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, 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()); ```

- [`NonNullable`](https://www.typescriptlang.org/docs/handbook/utility-types.html#nonnullabletype) - Exclude `null` and `undefined` from `T`.

Example

Works with strictNullChecks set to true. [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; 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`](https://www.typescriptlang.org/docs/handbook/utility-types.html#parameterstype) - 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 any> (func: Fn, callCount: number) { // Type that represents the type of the received function parameters. type FunctionParameters = Parameters; return function (...arguments_: FunctionParameters) { for (let i = 0; i < callCount; i++) { func(...arguments_); } } } const shuffleTwice = callNTimes(shuffle, 2); ```

- [`ConstructorParameters`](https://www.typescriptlang.org/docs/handbook/utility-types.html#constructorparameterstype) - 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 any)> { private ClassConstructor: T; private cache: Map> = new Map(); constructor (ctr: T) { this.ClassConstructor = ctr; } getInstance (...arguments_: ConstructorParameters): InstanceType { 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!'); ```

- [`ReturnType`](https://www.typescriptlang.org/docs/handbook/utility-types.html#returntypetype) - 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 >(iter: Iterable, callback: Func): Ret[] { const mapped: Ret[] = []; for (const elem of iter) { mapped.push(callback(elem)); } return mapped; } const setObject: Set = new Set(); const mapObject: Map = new Map(); mapIter(setObject, (value: string) => value.indexOf('Foo')); // number[] mapIter(mapObject, ([key, value]: [number, string]) => { return key % 2 === 0 ? value : 'Odd'; }); // string[] ```

- [`InstanceType`](https://www.typescriptlang.org/docs/handbook/utility-types.html#instancetypetype) - 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 = new Map(); interface Constructor { new(...arguments_: any[]): any; } // Keep track how many instances of `Constr` constructor have been created. function getInstance< Constr extends Constructor, Arguments extends ConstructorParameters >(constructor: Constr, ...arguments_: Arguments): InstanceType { let count = instanceCounter.get(constructor) || 0; const instance = new constructor(...arguments_); 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`](https://www.typescriptlang.org/docs/handbook/utility-types.html#omittype-keys) - 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; function renderAnimalHoverInfo (animals: AnimalShortInfo[]): HTMLElement { const container = document.createElement('div'); // Internal implementation. return container; } ```

- [`Uppercase`](https://www.typescriptlang.org/docs/handbook/utility-types.html#uppercasestringtype) - Transforms every character in a string into uppercase.

Example
```ts type T = Uppercase<'hello'>; // 'HELLO' type T2 = Uppercase<'foo' | 'bar'>; // 'FOO' | 'BAR' type T3 = Uppercase<`aB${S}`>; type T4 = T3<'xYz'>; // 'ABXYZ' type T5 = Uppercase; // string type T6 = Uppercase; // any type T7 = Uppercase; // never type T8 = Uppercase<42>; // Error, type 'number' does not satisfy the constraint 'string' ```
- [`Lowercase`](https://www.typescriptlang.org/docs/handbook/utility-types.html#lowercasestringtype) - Transforms every character in a string into lowercase.

Example
```ts type T = Lowercase<'HELLO'>; // 'hello' type T2 = Lowercase<'FOO' | 'BAR'>; // 'foo' | 'bar' type T3 = Lowercase<`aB${S}`>; type T4 = T3<'xYz'>; // 'abxyz' type T5 = Lowercase; // string type T6 = Lowercase; // any type T7 = Lowercase; // never type T8 = Lowercase<42>; // Error, type 'number' does not satisfy the constraint 'string' ```
- [`Capitalize`](https://www.typescriptlang.org/docs/handbook/utility-types.html#capitalizestringtype) - Transforms the first character in a string into uppercase.

Example
```ts type T = Capitalize<'hello'>; // 'Hello' type T2 = Capitalize<'foo' | 'bar'>; // 'Foo' | 'Bar' type T3 = Capitalize<`aB${S}`>; type T4 = T3<'xYz'>; // 'ABxYz' type T5 = Capitalize; // string type T6 = Capitalize; // any type T7 = Capitalize; // never type T8 = Capitalize<42>; // Error, type 'number' does not satisfy the constraint 'string' ```
- [`Uncapitalize`](https://www.typescriptlang.org/docs/handbook/utility-types.html#uncapitalizestringtype) - Transforms the first character in a string into lowercase.

Example
```ts type T = Uncapitalize<'Hello'>; // 'hello' type T2 = Uncapitalize<'Foo' | 'Bar'>; // 'foo' | 'bar' type T3 = Uncapitalize<`AB${S}`>; type T4 = T3<'xYz'>; // 'aBxYz' type T5 = Uncapitalize; // string type T6 = Uncapitalize; // any type T7 = Uncapitalize; // never type T8 = Uncapitalize<42>; // Error, type 'number' does not satisfy the constraint 'string' ```
You can find some examples in the [TypeScript docs](https://www.typescriptlang.org/docs/handbook/utility-types.html). ## Maintainers - [Sindre Sorhus](https://github.com/sindresorhus) - [Haozheng Li](https://github.com/Emiyaaaaa) - [Jarek Radosz](https://github.com/CvX) - [Dimitri Benin](https://github.com/BendingBender) - [Pelle Wessman](https://github.com/voxpelli) - [Sébastien Mischler](https://github.com/skarab42) ## License SPDX-License-Identifier: (MIT OR CC0-1.0)