Introduction

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

Partial<T>

Constructs a type with all properties of T 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<T>

Constructs a type with all properties of T 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"; // Error: cannot reassign a readonly property

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<T>(obj: T): Readonly<T>;

Record<K,T>

Constructs a type with a set of properties K of type T. 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 x: Record<Page, PageInfo> = {
  about: { title: "about" },
  contact: { title: "contact" },
  home: { title: "home" }
};

Pick<T,K>

Constructs a type by picking the set of properties K from T.

Example

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

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

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

Omit<T,K>

Constructs a type by picking all properties from T and then removing K.

Example

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

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

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

Exclude<T,U>

Constructs a type by excluding from T all properties that are assignable to U.

Example

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

Extract<T,U>

Constructs a type by extracting from T all properties that are assignable to U.

Example

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

NonNullable<T>

Constructs a type by excluding null and undefined from T.

Example

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

Parameters<T>

Constructs a tuple type of the types of the parameters of a function type T.

Example

ts
declare function f1(arg: { a: number; b: string }): void;
type T0 = Parameters<() => string>; // []
type T1 = Parameters<(s: string) => void>; // [string]
type T2 = Parameters<<T>(arg: T) => T>; // [unknown]
type T3 = Parameters<typeof f1>; // [{ a: number, b: string }]
type T4 = Parameters<any>; // unknown[]
type T5 = Parameters<never>; // never
type T6 = Parameters<string>; // Error
type T7 = Parameters<Function>; // Error

ConstructorParameters<T>

The ConstructorParameters<T> type lets us extract all parameter types of a constructor function type. It produces a tuple type with all the parameter types (or the type never if T is not a function).

Example

ts
type T0 = ConstructorParameters<ErrorConstructor>; // [(string | undefined)?]
type T1 = ConstructorParameters<FunctionConstructor>; // string[]
type T2 = ConstructorParameters<RegExpConstructor>; // [string | RegExp, (string | undefined)?]

ReturnType<T>

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

Example

ts
declare function f1(): { a: number; b: string };
type T0 = ReturnType<() => string>; // string
type T1 = ReturnType<(s: string) => void>; // void
type T2 = ReturnType<<T>() => T>; // {}
type T3 = ReturnType<<T extends U, U extends number[]>() => T>; // number[]
type T4 = ReturnType<typeof f1>; // { a: number, b: string }
type T5 = ReturnType<any>; // any
type T6 = ReturnType<never>; // never
type T7 = ReturnType<string>; // Error
type T8 = ReturnType<Function>; // Error

InstanceType<T>

Constructs a type consisting of the instance type of a constructor function type T.

Example

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

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

Required<T>

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

Example

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

const obj: Props = { a: 5 }; // OK

const obj2: Required<Props> = { a: 5 }; // Error: property 'b' missing

ThisParameterType

Extracts the type of the this parameter of 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

Removes the this parameter from a function type.

Example

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

// The return type of `bind` is already using `OmitThisParameter`, this is just for demonstration.
const fiveToHex: OmitThisParameter<typeof toHex> = toHex.bind(5);

console.log(fiveToHex());

ThisType<T>

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
// Compile with --noImplicitThis

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.