A collection of essential TypeScript types
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Many of the types here should have been built-in. You can help by suggesting some of them to the TypeScript project.
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 first.
Help wanted with reviewing proposals and pull requests.
npm install type-fest
Requires TypeScript >=4.7
import type {Except} from 'type-fest';
type Foo = {
unicorn: string;
rainbow: boolean;
};
type FooWithoutRainbow = Except<Foo, 'rainbow'>;
//=> {unicorn: string}
Click the type names for complete docs.
Primitive
- Matches any primitive value.Class
- Matches aclass
.Constructor
- Matches aclass
constructor.AbstractClass
- Matches anabstract class
.AbstractConstructor
- Matches anabstract class
constructor.TypedArray
- Matches any typed array, likeUint8Array
orFloat64Array
.ObservableLike
- Matches a value that is like an Observable.
EmptyObject
- Represents a strictly empty plain object, the{}
value.IsEmptyObject
- Returns aboolean
for whether the type is strictly equal to an empty plain object, the{}
value.Except
- Create a type from an object type without certain keys. This is a stricter version ofOmit
.Writable
- Create a type that stripsreadonly
from all or some of an object's keys. The inverse ofReadonly<T>
.WritableDeep
- Create a deeply mutable version of anobject
/ReadonlyMap
/ReadonlySet
/ReadonlyArray
type. The inverse ofReadonlyDeep<T>
. UseWritable<T>
if you only need one level deep.Merge
- Merge two types into a new type. Keys of the second type overrides keys of the first type.MergeDeep
- Merge two objects or two arrays/tuples recursively into a new type.MergeExclusive
- Create a type that has mutually exclusive keys.OverrideProperties
- Override only existing properties of the given type. Similar toMerge
, but enforces that the original type has the properties you want to override.RequireAtLeastOne
- Create a type that requires at least one of the given keys.RequireExactlyOne
- Create a type that requires exactly a single key of the given keys and disallows more.RequireAllOrNone
- Create a type that requires all of the given keys or none of the given keys.RequiredDeep
- Create a deeply required version of another type. UseRequired<T>
if you only need one level deep.OmitIndexSignature
- Omit any index signatures from the given object type, leaving only explicitly defined properties.PickIndexSignature
- Pick only index signatures from the given object type, leaving out all explicitly defined properties.PartialDeep
- Create a deeply optional version of another type. UsePartial<T>
if you only need one level deep.PartialOnUndefinedDeep
- Create a deep version of another type where all keys acceptingundefined
type are set to optional.ReadonlyDeep
- Create a deeply immutable version of anobject
/Map
/Set
/Array
type. UseReadonly<T>
if you only need one level deep.LiteralUnion
- 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.Opaque
- Create an opaque type.UnwrapOpaque
- Revert an opaque type back to its original type.InvariantOf
- Create an invariant type, which is a type that does not accept supertypes and subtypes.SetOptional
- Create a type that makes the given keys optional.SetReadonly
- Create a type that makes the given keys readonly.SetRequired
- Create a type that makes the given keys required.SetNonNullable
- Create a type that makes the given keys non-nullable.ValueOf
- Create a union of the given object's values, and optionally specify which keys to get the values from.ConditionalKeys
- Extract keys from a shape where values extend the givenCondition
type.ConditionalPick
- LikePick
except it selects properties from a shape where the values extend the givenCondition
type.ConditionalPickDeep
- LikeConditionalPick
except that it selects the properties deeply.ConditionalExcept
- LikeOmit
except it removes properties from a shape where the values extend the givenCondition
type.UnionToIntersection
- Convert a union type to an intersection type.LiteralToPrimitive
- Convert a literal type to the primitive type it belongs to.LiteralToPrimitiveDeep
- LikeLiteralToPrimitive
except it converts literal types inside an object or array deeply.Stringified
- Create a type with the keys of the given type changed tostring
type.IterableElement
- Get the element type of anIterable
/AsyncIterable
. For example, an array or a generator.Entry
- Create a type that represents the type of an entry of a collection.Entries
- Create a type that represents the type of the entries of a collection.SetReturnType
- Create a function type with a return type of your choice and the same parameters as the given function type.Simplify
- 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.Get
- Get a deeply-nested property from an object using a key path, like Lodash's.get()
function.StringKeyOf
- Get keys of the given type as strings.Schema
- Create a deep version of another object type where property values are recursively replaced into a given value type.Exact
- Create a type that does not allow extra properties.OptionalKeysOf
- Extract all optional keys from the given type.HasOptionalKeys
- Create atrue
/false
type depending on whether the given type has any optional fields.RequiredKeysOf
- Extract all required keys from the given type.HasRequiredKeys
- Create atrue
/false
type depending on whether the given type has any required fields.ReadonlyKeysOf
- Extract all readonly keys from the given type.HasReadonlyKeys
- Create atrue
/false
type depending on whether the given type has any readonly fields.WritableKeysOf
- Extract all writable (non-readonly) keys from the given type.HasWritableKeys
- Create atrue
/false
type depending on whether the given type has any writable fields.Spread
- Mimic the type inferred by TypeScript when merging two objects or two arrays/tuples using the spread syntax.IsEqual
- Returns a boolean for whether the two given types are equal.TaggedUnion
- Create a union of types that share a common discriminant property.
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
:
type IfAny<T, TypeIfAny = true, TypeIfNotAny = false> = (
IsAny<T> extends true ? TypeIfAny : TypeIfNotAny
);
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
- Returns a boolean for whether the given type is a literal type.IsStringLiteral
- Returns a boolean for whether the given type is astring
literal type.IsNumericLiteral
- Returns a boolean for whether the given type is anumber
orbigint
literal type.IsBooleanLiteral
- Returns a boolean for whether the given type is atrue
orfalse
literal type.IsSymbolLiteral
- Returns a boolean for whether the given type is asymbol
literal type.IsAny
- Returns a boolean for whether the given type isany
. (Conditional version:IfAny
.)IsNever
- Returns a boolean for whether the given type isnever
. (Conditional version:IfNever
.)IsUnknown
- Returns a boolean for whether the given type isunknown
. (Conditional version:IfUnknown
.)
Jsonify
- Transform a type to one that is assignable to theJsonValue
type.Jsonifiable
- Matches a value that can be losslessly converted to JSON.JsonPrimitive
- Matches a JSON primitive.JsonObject
- Matches a JSON object.JsonArray
- Matches a JSON array.JsonValue
- Matches any valid JSON value.
Promisable
- Create a type that represents either the value or the value wrapped inPromiseLike
.AsyncReturnType
- Unwrap the return type of a function that returns aPromise
.Asyncify
- Create an async version of the given function type.
Trim
- Remove leading and trailing spaces from a string.Split
- Represents an array of strings split using a given character or character set.Replace
- Represents a string with some or all matches replaced by a replacement.
Includes
- Returns a boolean for whether the given array includes the given item.Join
- Join an array of strings and/or numbers using the given string as a delimiter.LastArrayElement
- Extracts the type of the last element of an array.FixedLengthArray
- Create a type that represents an array of the given type and length.MultidimensionalArray
- Create a type that represents a multidimensional array of the given type and dimensions.MultidimensionalReadonlyArray
- Create a type that represents a multidimensional readonly array of the given type and dimensions.ReadonlyTuple
- Create a type that represents a read-only tuple of the given type and length.TupleToUnion
- Convert a tuple/array into a union type of its elements.
PositiveInfinity
- Matches the hiddenInfinity
type.NegativeInfinity
- Matches the hidden-Infinity
type.Finite
- A finitenumber
.Integer
- Anumber
that is an integer.Float
- Anumber
that is not an integer.NegativeFloat
- A negative (-∞ < x < 0
)number
that is not an integer.Negative
- A negativenumber
/bigint
(-∞ < x < 0
)NonNegative
- A non-negativenumber
/bigint
(0 <= x < ∞
).NegativeInteger
- A negative (-∞ < x < 0
)number
that is an integer.NonNegativeInteger
- A non-negative (0 <= x < ∞
)number
that is an integer.
CamelCase
- Convert a string literal to camel-case (fooBar
).CamelCasedProperties
- Convert object properties to camel-case (fooBar
).CamelCasedPropertiesDeep
- Convert object properties to camel-case recursively (fooBar
).KebabCase
- Convert a string literal to kebab-case (foo-bar
).KebabCasedProperties
- Convert a object properties to kebab-case recursively (foo-bar
).KebabCasedPropertiesDeep
- Convert object properties to kebab-case (foo-bar
).PascalCase
- Converts a string literal to pascal-case (FooBar
)PascalCasedProperties
- Converts object properties to pascal-case (FooBar
)PascalCasedPropertiesDeep
- Converts object properties to pascal-case (FooBar
)SnakeCase
- Convert a string literal to snake-case (foo_bar
).SnakeCasedProperties
- Convert object properties to snake-case (foo_bar
).SnakeCasedPropertiesDeep
- Convert object properties to snake-case recursively (foo_bar
).ScreamingSnakeCase
- Convert a string literal to screaming-snake-case (FOO_BAR
).DelimiterCase
- Convert a string literal to a custom string delimiter casing.DelimiterCasedProperties
- Convert object properties to a custom string delimiter casing.DelimiterCasedPropertiesDeep
- Convert object properties to a custom string delimiter casing recursively.
GlobalThis
- Declare locally scoped properties onglobalThis
.PackageJson
- Type for npm'spackage.json
file. It also includes support for TypeScript Declaration Files.TsConfigJson
- Type for TypeScript'stsconfig.json
file.
If we decline a type addition, we will make sure to document the better solution here.
Diff
andSpread
- 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
- You only save a few characters (Dictionary<number>
vsRecord<string, number>
) fromRecord
, which is more flexible and well-known. Also, you shouldn't use an object as a dictionary. We haveMap
in JavaScript now.ExtractProperties
andExtractMethods
- The types violate the single responsibility principle. Instead, refine your types into more granular type hierarchies.Url2Json
- 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
- The type only saves a couple of characters, not everyone knows what "nullish" means, and I'm also trying to get away fromnull
.TitleCase
- It's not solving a common need and is a better fit for a separate package.ExtendOr
andExtendAnd
- The benefits don't outweigh having to learn what they mean.PackageJsonExtras
- There are too many possible configurations that can be put intopackage.json
. If you would like to extendPackageJson
to support an additional configuration in your project, please see the Extending existing types section below.
If you know one of our types by a different name, add it here for discovery.
PartialBy
- SeeSetOptional
RecordDeep
- SeeSchema
Mutable
- SeeWritable
Prettify
- SeeSimplify
-
PackageJson
- There are a lot of tools that place extra configurations inside thepackage.json
file. You can extendPackageJson
to support these additional configurations.Example
import type {PackageJson as BasePackageJson} from 'type-fest'; import type {Linter} from 'eslint'; type PackageJson = BasePackageJson & {eslintConfig?: Linter.Config};
- typed-query-selector - Enhances
document.querySelector
anddocument.querySelectorAll
with a template literal type that matches element types returned from an HTML element query selector. Linter.Config
- Definitions for the ESLint configuration schema.
There are many advanced types most users don't know about.
-
Partial<T>
- Make all properties inT
optional.Example
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>
- Make all properties inT
required.Example
interface ContactForm { email?: string; message?: string; } function submitContactForm(formData: Required<ContactForm>) { // Send the form data to the server. } submitContactForm({ email: '[email protected]', message: 'Hi! Could you tell me more about…', }); // TypeScript error: missing property 'message' submitContactForm({ email: '[email protected]', });
-
Readonly<T>
- Make all properties inT
readonly.Example
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>
- FromT
, pick a set of properties whose keys are in the unionK
.Example
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>
- Construct a type with a set of propertiesK
of typeT
.Example
// 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>
- Exclude fromT
those types that are assignable toU
.Example
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>
- Extract fromT
those types that are assignable toU
.Example
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>
- Excludenull
andundefined
fromT
.Example
Works withstrictNullChecks
set totrue
.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>
- Obtain the parameters of a function type in a tuple.Example
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);
-
ConstructorParameters<T>
- Obtain the parameters of a constructor function type in a tuple.Example
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 burining!');
-
ReturnType<T>
- Obtain the return type of a function type.Example
/** 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>
- Obtain the instance type of a constructor function type.Example
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(...arguments_: any[]): any; } // Keep track how many instances of `Constr` constructor have been created. function getInstance< Constr extends Constructor, Arguments extends ConstructorParameters<Constr> >(constructor: Constr, ...arguments_: Arguments): InstanceType<Constr> { let count = instanceCounter.get(constructor) || 0; const instance = new constructor(...arguments_); instanceCounter.set(constructor, count + 1); console.log(`Created ${count + 1} instances of ${Constr.name} class`); return instance; } const idleService = getInstance(IdleService); // Will log: `Created 1 instances of IdleService class` const newsEntry = getInstance(News, 'New ECMAScript proposals!', 'Last month...'); // Will log: `Created 1 instances of News class`
-
Omit<T, K>
- Constructs a type by picking all properties from T and then removing K.Example
interface Animal { imageUrl: string; species: string; images: string[]; paragraphs: string[]; } // Creates new type with all properties of the `Animal` interface // except 'images' and 'paragraphs' properties. We can use this // type to render small hover tooltip for a wiki entry list. type AnimalShortInfo = Omit<Animal, 'images' | 'paragraphs'>; function renderAnimalHoverInfo (animals: AnimalShortInfo[]): HTMLElement { const container = document.createElement('div'); // Internal implementation. return container; }
-
Uppercase<S extends string>
- Transforms every character in a string into uppercase.Example
type T = Uppercase<'hello'>; // 'HELLO' type T2 = Uppercase<'foo' | 'bar'>; // 'FOO' | 'BAR' type T3<S extends string> = Uppercase<`aB${S}`>; type T4 = T3<'xYz'>; // 'ABXYZ' type T5 = Uppercase<string>; // string type T6 = Uppercase<any>; // any type T7 = Uppercase<never>; // never type T8 = Uppercase<42>; // Error, type 'number' does not satisfy the constraint 'string'
-
Lowercase<S extends string>
- Transforms every character in a string into lowercase.Example
type T = Lowercase<'HELLO'>; // 'hello' type T2 = Lowercase<'FOO' | 'BAR'>; // 'foo' | 'bar' type T3<S extends string> = Lowercase<`aB${S}`>; type T4 = T3<'xYz'>; // 'abxyz' type T5 = Lowercase<string>; // string type T6 = Lowercase<any>; // any type T7 = Lowercase<never>; // never type T8 = Lowercase<42>; // Error, type 'number' does not satisfy the constraint 'string'
-
Capitalize<S extends string>
- Transforms the first character in a string into uppercase.Example
type T = Capitalize<'hello'>; // 'Hello' type T2 = Capitalize<'foo' | 'bar'>; // 'Foo' | 'Bar' type T3<S extends string> = Capitalize<`aB${S}`>; type T4 = T3<'xYz'>; // 'ABxYz' type T5 = Capitalize<string>; // string type T6 = Capitalize<any>; // any type T7 = Capitalize<never>; // never type T8 = Capitalize<42>; // Error, type 'number' does not satisfy the constraint 'string'
-
Uncapitalize<S extends string>
- Transforms the first character in a string into lowercase.Example
type T = Uncapitalize<'Hello'>; // 'hello' type T2 = Uncapitalize<'Foo' | 'Bar'>; // 'foo' | 'bar' type T3<S extends string> = Uncapitalize<`AB${S}`>; type T4 = T3<'xYz'>; // 'aBxYz' type T5 = Uncapitalize<string>; // string type T6 = Uncapitalize<any>; // any type T7 = Uncapitalize<never>; // never type T8 = Uncapitalize<42>; // Error, type 'number' does not satisfy the constraint 'string'
You can find some examples in the TypeScript docs.
SPDX-License-Identifier: (MIT OR CC0-1.0)