TypeScript: Handbook - Utility Types

TypeScript provides several utility types to facilitate common type transformations. These utilities are available globally.

`Partial<Type>`

Constructs a type with all properties of Type set to optional. This utility will return a type that represents all subsets of a given type.

Example

ts
interface Todo {
  title: string;
  description: string;
}

function updateTodo(todo: Todo, fieldsToUpdate: Partial<Todo>) {
  return { ...todo, ...fieldsToUpdate };
}

const todo1 = {
  title: "organize desk",
  description: "clear clutter",
};

const todo2 = updateTodo(todo1, {
  description: "throw out trash",
});

`Readonly<Type>`

Constructs a type with all properties of Type set to readonly, meaning the properties of the constructed type cannot be reassigned.

Example

ts
// @errors: 2540
interface Todo {
  title: string;
}

const todo: Readonly<Todo> = {
  title: "Delete inactive users",
};

todo.title = "Hello";

This utility is useful for representing assignment expressions that will fail at runtime (i.e. when attempting to reassign properties of a frozen object).

`Object.freeze`

ts
function freeze<Type>(obj: Type): Readonly<Type>;

`Record<Keys,Type>`

Constructs a type with a set of properties Keys of type Type. This utility can be used to map the properties of a type to another type.

Example

ts
interface PageInfo {
  title: string;
}

type Page = "home" | "about" | "contact";

const nav: Record<Page, PageInfo> = {
  about: { title: "about" },
  contact: { title: "contact" },
  home: { title: "home" },
};

nav.about;
// ^?

`Pick<Type, Keys>`

Constructs a type by picking the set of properties Keys from Type.

Example

ts
interface Todo {
  title: string;
  description: string;
  completed: boolean;
}

type TodoPreview = Pick<Todo, "title" | "completed">;

const todo: TodoPreview = {
  title: "Clean room",
  completed: false,
};

todo;
// ^?

`Omit<Type, Keys>`

Constructs a type by picking all properties from Type and then removing Keys.

Example

ts
interface Todo {
  title: string;
  description: string;
  completed: boolean;
}

type TodoPreview = Omit<Todo, "description">;

const todo: TodoPreview = {
  title: "Clean room",
  completed: false,
};

todo;
// ^?

`Exclude<Type, ExcludedUnion>`

Constructs a type by excluding from Type all union members that are assignable to ExcludedUnion.

Example

ts
type T0 = Exclude<"a" | "b" | "c", "a">;
//    ^?
type T1 = Exclude<"a" | "b" | "c", "a" | "b">;
//    ^?
type T2 = Exclude<string | number | (() => void), Function>;
//    ^?

`Extract<Type, Union>`

Constructs a type by extracting from Type all union members that are assignable to Union.

Example

ts
type T0 = Extract<"a" | "b" | "c", "a" | "f">;
//    ^?
type T1 = Extract<string | number | (() => void), Function>;
//    ^?

`NonNullable<Type>`

Constructs a type by excluding null and undefined from Type.

Example

ts
type T0 = NonNullable<string | number | undefined>;
//    ^?
type T1 = NonNullable<string[] | null | undefined>;
//    ^?

`Parameters<Type>`

Constructs a tuple type from the types used in the parameters of a function type Type.

Example

ts
// @errors: 2344
declare function f1(arg: { a: number; b: string }): void;

type T0 = Parameters<() => string>;
//    ^?
type T1 = Parameters<(s: string) => void>;
//    ^?
type T2 = Parameters<<T>(arg: T) => T>;
//    ^?
type T3 = Parameters<typeof f1>;
//    ^?
type T4 = Parameters<any>;
//    ^?
type T5 = Parameters<never>;
//    ^?
type T6 = Parameters<string>;
//    ^?
type T7 = Parameters<Function>;
//    ^?

`ConstructorParameters<Type>`

Constructs a tuple or array type from the types of a constructor function type. It produces a tuple type with all the parameter types (or the type never if Type is not a function).

Example

ts
// @errors: 2344
// @strict: false
type T0 = ConstructorParameters<ErrorConstructor>;
//    ^?
type T1 = ConstructorParameters<FunctionConstructor>;
//    ^?
type T2 = ConstructorParameters<RegExpConstructor>;
//    ^?
type T3 = ConstructorParameters<any>;
//    ^?

type T4 = ConstructorParameters<Function>;
//    ^?

`ReturnType<Type>`

Constructs a type consisting of the return type of function Type.

Example

ts
// @errors: 2344 2344
declare function f1(): { a: number; b: string };

type T0 = ReturnType<() => string>;
//    ^?
type T1 = ReturnType<(s: string) => void>;
//    ^?
type T2 = ReturnType<<T>() => T>;
//    ^?
type T3 = ReturnType<<T extends U, U extends number[]>() => T>;
//    ^?
type T4 = ReturnType<typeof f1>;
//    ^?
type T5 = ReturnType<any>;
//    ^?
type T6 = ReturnType<never>;
//    ^?
type T7 = ReturnType<string>;
//    ^?
type T8 = ReturnType<Function>;
//    ^?

`InstanceType<Type>`

Constructs a type consisting of the instance type of a constructor function in Type.

Example

ts
// @errors: 2344 2344
// @strict: false
class C {
  x = 0;
  y = 0;
}

type T0 = InstanceType<typeof C>;
//    ^?
type T1 = InstanceType<any>;
//    ^?
type T2 = InstanceType<never>;
//    ^?
type T3 = InstanceType<string>;
//    ^?
type T4 = InstanceType<Function>;
//    ^?

`Required<Type>`

Constructs a type consisting of all properties of T set to required. The opposite of Partial.

Example

ts
// @errors: 2741
interface Props {
  a?: number;
  b?: string;
}

const obj: Props = { a: 5 };

const obj2: Required<Props> = { a: 5 };

`ThisParameterType<Type>`

Extracts the type of the this parameter for a function type, or unknown if the function type has no this parameter.

Example

ts
function toHex(this: Number) {
  return this.toString(16);
}

function numberToString(n: ThisParameterType<typeof toHex>) {
  return toHex.apply(n);
}

`OmitThisParameter<Type>`

Removes the this parameter from Type. If Type has no explicitly declared this parameter, the result is simply Type. Otherwise, a new function type with no this parameter is created from Type. Generics are erased and only the last overload signature is propagated into the new function type.

Example

ts
function toHex(this: Number) {
  return this.toString(16);
}

const fiveToHex: OmitThisParameter<typeof toHex> = toHex.bind(5);

console.log(fiveToHex());

`ThisType<Type>`

This utility does not return a transformed type. Instead, it serves as a marker for a contextual this type. Note that the --noImplicitThis flag must be enabled to use this utility.

Example

ts
// @noImplicitThis: false
type ObjectDescriptor<D, M> = {
  data?: D;
  methods?: M & ThisType<D & M>; // Type of 'this' in methods is D & M
};

function makeObject<D, M>(desc: ObjectDescriptor<D, M>): D & M {
  let data: object = desc.data || {};
  let methods: object = desc.methods || {};
  return { ...data, ...methods } as D & M;
}

let obj = makeObject({
  data: { x: 0, y: 0 },
  methods: {
    moveBy(dx: number, dy: number) {
      this.x += dx; // Strongly typed this
      this.y += dy; // Strongly typed this
    },
  },
});

obj.x = 10;
obj.y = 20;
obj.moveBy(5, 5);

In the example above, the methods object in the argument to makeObject has a contextual type that includes ThisType<D & M> and therefore the type of this in methods within the methods object is { x: number, y: number } & { moveBy(dx: number, dy: number): number }. Notice how the type of the methods property simultaneously is an inference target and a source for the this type in methods.

The ThisType<T> marker interface is simply an empty interface declared in lib.d.ts. Beyond being recognized in the contextual type of an object literal, the interface acts like any empty interface.