Utility Types

Introduction

• Record<K,T>
• Omit<T,K>
• Extract<T,U>
• InstanceType<T>
• ThisType<T>

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

Readonly<T>

Constructs a type with all properties of T 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'; // 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 ).

Object.freeze

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

interface PageInfo {
    title: string;
}
type Page = 'home' | 'about' | 'contact';
const x: Record<Page, PageInfo> = {
    about: { title: 'about' },
    contact: { title: 'contact' },
};

Pick<T,K>

Example

Omit<T,K>

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

Example

interface Todo {
    title: string;
    description: string;
    completed: boolean;
}
type TodoPreview = Omit<Todo, 'description'>;
const todo: TodoPreview = {
    completed: false,
};

Exclude<T,U>

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

Example

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

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

ReturnType<T>

Example

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 T6 = ReturnType<never>;  // any
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

class C {
    x = 0;
    y = 0;
}
type T0 = InstanceType<typeof C>;  // C
type T1 = InstanceType<any>;  // any
type T2 = InstanceType<never>;  // any
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

interface Props {
    a?: number;
    b?: string;
};
const obj: Props = { a: 5 }; // OK
const obj2: Required<Props> = { a: 5 }; // Error: property 'b' missing

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

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.