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
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",
});Try

Readonly<Type>

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

Example
interface Todo {
  title: string;
}

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

todo.title = "Hello";
Cannot assign to 'title' because it is a read-only property.2540Cannot assign to 'title' because it is a read-only property.Try

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
tsfunction 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
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;
// ^ = const nav: RecordTry

Pick<Type, Keys>

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

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

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

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

todo;
// ^ = const todo: PickTry

Omit<Type, Keys>

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

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

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

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

todo;
// ^ = const todo: PickTry

Exclude<Type, ExcludedUnion>

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

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

Extract<Type, Union>

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

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

NonNullable<Type>

Constructs a type by excluding null and undefined from Type.

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

Parameters<Type>

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

Example
declare function f1(arg: { a: number; b: string }): void;

type T0 = Parameters<() => string>;
//    ^ = type T0 = []
type T1 = Parameters<(s: string) => void>;
//    ^ = type T1 = [s: string]
type T2 = Parameters<<T>(arg: T) => T>;
//    ^ = type T2 = [arg: unknown]
type T3 = Parameters<typeof f1>;
//    ^ = type T3 = [arg: {
//        a: number;
//        b: string;
//    }]
type T4 = Parameters<any>;
//    ^ = type T4 = unknown[]
type T5 = Parameters<never>;
//    ^ = type T5 = never
type T6 = Parameters<string>;
Type 'string' does not satisfy the constraint '(...args: any) => any'.2344Type 'string' does not satisfy the constraint '(...args: any) => any'.//    ^ = type T6 = never
type T7 = Parameters<Function>;
Type 'Function' does not satisfy the constraint '(...args: any) => any'.
  Type 'Function' provides no match for the signature '(...args: any): any'.2344Type 'Function' does not satisfy the constraint '(...args: any) => any'.
  Type 'Function' provides no match for the signature '(...args: any): any'.//    ^ = type T7 = neverTry

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
type T0 = ConstructorParameters<ErrorConstructor>;
//    ^ = type T0 = [message?: string]
type T1 = ConstructorParameters<FunctionConstructor>;
//    ^ = type T1 = string[]
type T2 = ConstructorParameters<RegExpConstructor>;
//    ^ = type T2 = [pattern: string | RegExp, flags?: string]
type T3 = ConstructorParameters<any>;
//    ^ = type T3 = unknown[]

type T4 = ConstructorParameters<Function>;
Type 'Function' does not satisfy the constraint 'new (...args: any) => any'.
  Type 'Function' provides no match for the signature 'new (...args: any): any'.2344Type 'Function' does not satisfy the constraint 'new (...args: any) => any'.
  Type 'Function' provides no match for the signature 'new (...args: any): any'.//    ^ = type T4 = neverTry

ReturnType<Type>

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

Example
declare function f1(): { a: number; b: string };

type T0 = ReturnType<() => string>;
//    ^ = type T0 = string
type T1 = ReturnType<(s: string) => void>;
//    ^ = type T1 = void
type T2 = ReturnType<<T>() => T>;
//    ^ = type T2 = unknown
type T3 = ReturnType<<T extends U, U extends number[]>() => T>;
//    ^ = type T3 = number[]
type T4 = ReturnType<typeof f1>;
//    ^ = type T4 = {
//        a: number;
//        b: string;
//    }
type T5 = ReturnType<any>;
//    ^ = type T5 = any
type T6 = ReturnType<never>;
//    ^ = type T6 = never
type T7 = ReturnType<string>;
Type 'string' does not satisfy the constraint '(...args: any) => any'.2344Type 'string' does not satisfy the constraint '(...args: any) => any'.//    ^ = type T7 = any
type T8 = ReturnType<Function>;
Type 'Function' does not satisfy the constraint '(...args: any) => any'.
  Type 'Function' provides no match for the signature '(...args: any): any'.2344Type 'Function' does not satisfy the constraint '(...args: any) => any'.
  Type 'Function' provides no match for the signature '(...args: any): any'.//    ^ = type T8 = anyTry

InstanceType<Type>

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

Example
class C {
  x = 0;
  y = 0;
}

type T0 = InstanceType<typeof C>;
//    ^ = type T0 = C
type T1 = InstanceType<any>;
//    ^ = type T1 = any
type T2 = InstanceType<never>;
//    ^ = type T2 = never
type T3 = InstanceType<string>;
Type 'string' does not satisfy the constraint 'new (...args: any) => any'.2344Type 'string' does not satisfy the constraint 'new (...args: any) => any'.//    ^ = type T3 = any
type T4 = InstanceType<Function>;
Type 'Function' does not satisfy the constraint 'new (...args: any) => any'.
  Type 'Function' provides no match for the signature 'new (...args: any): any'.2344Type 'Function' does not satisfy the constraint 'new (...args: any) => any'.
  Type 'Function' provides no match for the signature 'new (...args: any): any'.//    ^ = type T4 = anyTry

Required<Type>

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

Example
interface Props {
  a?: number;
  b?: string;
}

const obj: Props = { a: 5 };

const obj2: Required<Props> = { a: 5 };
Property 'b' is missing in type '{ a: number; }' but required in type 'Required<Props>'.2741Property 'b' is missing in type '{ a: number; }' but required in type 'Required<Props>'.Try

ThisParameterType<Type>

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

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

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

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

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

console.log(fiveToHex());Try

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
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);Try

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.