Utility Types

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

Awaited<Type>

Released: 4.5

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

Example

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

Partial<Type>

Released:
2.1

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",
});

Required<Type>

Released:
2.8

Constructs a type consisting of all properties of Type 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 };

Readonly<Type>

Released:
2.1

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";

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

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

Record<Keys, Type>

Released:
2.1

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

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

Pick<Type, Keys>

Released:
2.1

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

Omit<Type, Keys>

Released:
3.5

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

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;
 
type TodoInfo = Omit<Todo, "completed" | "createdAt">;
 
const todoInfo: TodoInfo = {
  title: "Pick up kids",
  description: "Kindergarten closes at 5pm",
};
 
todoInfo;

Exclude<UnionType, ExcludedMembers>

Released:
2.8

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

Example

type T0 = Exclude<"a" | "b" | "c", "a">;
type T1 = Exclude<"a" | "b" | "c", "a" | "b">;
type T2 = Exclude<string | number | (() => void), Function>;
 
type Shape =
  | { kind: "circle"; radius: number }
  | { kind: "square"; x: number }
  | { kind: "triangle"; x: number; y: number };
 
type T3 = Exclude<Shape, { kind: "circle" }>

Extract<Type, Union>

Released:
2.8

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

NonNullable<Type>

Released:
2.8

Constructs a type by excluding null and undefined from Type.

Example

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

Parameters<Type>

Released:
3.1

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 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>

Released:
3.1

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 T1 = ConstructorParameters<FunctionConstructor>;
type T2 = ConstructorParameters<RegExpConstructor>;
class C {
  constructor(a: number, b: string) {}
}
type T3 = ConstructorParameters<typeof C>;
type T4 = ConstructorParameters<any>;
 
type T5 = ConstructorParameters<Function>;

ReturnType<Type>

Released:
2.8

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

Example

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>

Released:
2.8

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 T1 = InstanceType<any>;
type T2 = InstanceType<never>;
type T3 = InstanceType<string>;
type T4 = InstanceType<Function>;

ThisParameterType<Type>

Released:
3.3

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);
}

OmitThisParameter<Type>

Released:
3.3

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());

ThisType<Type>

Released:
2.3

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);

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): void }. 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.

Intrinsic String Manipulation Types

Uppercase<StringType>

Lowercase<StringType>

Capitalize<StringType>

Uncapitalize<StringType>

To help with string manipulation around template string literals, TypeScript includes a set of types which can be used in string manipulation within the type system. You can find those in the Template Literal Types documentation.