Extract<Type, Union>

INFO

The Extract<Type, Union> utility type is available starting from TypeScript version 2.8. For more information see in TypeScript Handbook, Release Note.

Overview

The Extract<Type, Union> is a built in utility type in TypeScript that constructs a type by extracting from Type all union members that are assignable to Union.

Syntax

type Extract<T, U> = T extends U ? T : never;

• Type (T): This represents a type from which you want to extract union members.
• Union (U): This represents the members or types that you want to extract from the Type.

Examples

Example #1

type User = 'admin' | 'editor' | 'viewer';

type PrivilegedUser = Extract<User, 'admin' | 'editor'>; // 'admin' | 'editor'

Example #2

type OnlyFunction = Extract<string | number | symbol | (() => void), Function>; // () => void

Example #3

type Primitive = string | number | boolean | bigint | symbol | null | undefined;

type NonPrimitive<T> = Extract<T, Primitive>;

type NonPrimitiveTypes = NonPrimitive<string | number | object>; // object

Example #4

declare function uniqueId(): number;

const ID = Symbol('ID');

interface Product {
    [ID]: number;
    name: string;
    price: number;
}

type ProductUpdatableKeys = Extract<keyof Product, string>;

function updateProduct <
    Obj extends Product,
    Key extends ProductUpdatableKeys,
    Value extends Obj[Key]
> (obj: Obj, key: Key, value: Value) {
    obj[key] = value;
}

const product: Product = {
    [ID]: uniqueId(),
    name: 'box',
    price: 5000
}

updateProduct(product, 'name', 'paper'); // ✅
updateProduct(product ID, uniqueId()); // ❌