Utility Types

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

Awaited<Type>

Released:

This type is meant to model operations like await in async functions, or the .then() method on Promise s - specifically, the way that they recursively unwrap Promise s.

Example

ts




    
type A = Awaited<Promise<string>>;
    
type A = string
 
type B = Awaited<Promise<Promise<number>>>;
    
type B = number
 
type C = Awaited<boolean | Promise<number>>;
    
type C = number | booleanTry

Partial<Type>

Released:

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",
});Try

Required<Type>

Released:

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

Example

ts
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

Readonly<Type>

Released:

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

Example

ts




    
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

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

Record<Keys, Type>

Released:

Constructs an object type whose property keys are Keys and whose property values are Type . This utility can be used to map the properties of a type to another type.

Example

ts
type CatName = "miffy" | "boris" | "mordred";
 
interface CatInfo {
  age: number;
  breed: string;
}
 
const cats: Record<CatName, CatInfo> = {
  miffy: { age: 10, breed: "Persian" },
  boris: { age: 5, breed: "Maine Coon" },
  mordred: { age: 16, breed: "British Shorthair" },
};
 
cats.boris;
 
const cats: Record<CatName, CatInfo>Try

Pick<Type, Keys>

Released:

Constructs a type by picking the set of properties Keys (string literal or union of string literals) 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;
 
const todo: TodoPreviewTry

Omit<Type, Keys>

Released:

Constructs a type by picking all properties from Type and then removing Keys (string literal or union of string literals). The opposite of Pick .

Example

ts




    
interface Todo {
  title: string;
  description: string;
  completed: boolean;
  createdAt: number;
}
 
type TodoPreview = Omit<Todo, "description">;
 
const todo: TodoPreview = {
  title: "Clean room",
  completed: false,
  createdAt: 1615544252770,
};
 
todo;
 
const todo: TodoPreview
 
type TodoInfo = Omit<Todo, "completed" | "createdAt">;
 
const todoInfo: TodoInfo = {
  title: "Pick up kids",
  description: "Kindergarten closes at 5pm",
};
 
todoInfo;
   
const todoInfo: TodoInfoTry

Exclude<UnionType, ExcludedMembers>

Released:

Constructs a type by excluding from UnionType all union members that are assignable to ExcludedMembers .

Example

ts
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 | number
 
type Shape =
  | { kind: "circle"; radius: number }
  | { kind: "square"; x: number }
  | { kind: "triangle"; x: number; y: number };
 
type T3 = Exclude<Shape, { kind: "circle" }>
     
type T3 = {
    kind: "square";
    x: number;
} | {
    kind: "triangle";
    x: number;
    y: number;
}Try

Extract<Type, Union>

Released:

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 T0 = "a"
type T1 = Extract<string | number | (() => void), Function>;
     
type T1 = () => void
 
type Shape =
  | { kind: "circle"; radius: number }
  | { kind: "square"; x: number }
  | { kind: "triangle"; x: number; y: number };
 
type T2 = Extract<Shape, { kind: "circle" }>
     
type T2 = {
    kind: "circle";
    radius: number;
}Try

NonNullable<Type>

Released:

Constructs a type by excluding null and undefined from Type .

Example

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

Parameters<Type>

Released:

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

For overloaded functions, this will be the parameters of the last signature; see Inferring Within Conditional Types .

Example

ts
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 = never
Try

ts
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]
class C {
  constructor(a: number, b: string) {}
}
type T3 = ConstructorParameters<typeof C>;
     
type T3 = [a: number, b: string]
type T4 = ConstructorParameters<any>;
     
type T4 = unknown[]
 
type T5 = ConstructorParameters<Function>;
Type 'Function' does not satisfy the constraint 'abstract new (...args: any) => any'.
  Type 'Function' provides no match for the signature 'new (...args: any): any'.2344Type 'Function' does not satisfy the constraint 'abstract new (...args: any) => any'.
  Type 'Function' provides no match for the signature 'new (...args: any): any'.
     
type T5 = neverTry

ts
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 = any
Try

ts
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 'abstract new (...args: any) => any'.2344Type 'string' does not satisfy the constraint 'abstract new (...args: any) => any'.
     
type T3 = any
type T4 = InstanceType<Function>;
Type 'Function' does not satisfy the constraint 'abstract new (...args: any) => any'.
  Type 'Function' provides no match for the signature 'new (...args: any): any'.2344Type 'Function' does not satisfy the constraint 'abstract new (...args: any) => any'.
  Type 'Function' provides no match for the signature 'new (...args: any): any'.
     
type T4 = anyTry

ts
function createStreetLight<C extends string>(
  colors: C[],
  defaultColor?: NoInfer<C>,
) {
  // ...
}
createStreetLight(["red", "yellow", "green"], "red");  // OK
createStreetLight(["red", "yellow", "green"], "blue");  // Error

ts




    
function toHex(this: Number) {
  return this.toString(16);
}
 
function numberToString(n: ThisParameterType<typeof toHex>) {
  return toHex.apply(n);
}Try

ts
function toHex(this: Number) {
  return this.toString(16);
}
 
const fiveToHex: OmitThisParameter<typeof toHex> = toHex.bind(5);
 
console.log(fiveToHex());Try

ts
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