Record<Keys, Type>

INFO

The Record<Keys, Type> utility type is available starting from TypeScript 2.1. For more information see in TypeScript Handbook, Release Note.

Overview

Record<Keys, Type> is a built in utility type in TypeScript that 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.

Syntax

type Record<K extends keyof any, T> = { [P in K]: T };

• Keys (K): This is the type of object's keys that the record type generates for us.
• Type (T): This is the type of the object's value generated by the record type.

Record<Keys, Type>;

Examples

Example #1

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' },
};

Example #2

function mapObject<K extends string, T, U>(obj: Record<K, T>, f: (x: T) => U): Record<K, U>;

const names = { foo: 'hello', bar: 'world', baz: 'bye' };
const lengths = mapObject(names, s => s.length); // { foo: number, bar: number, baz: number }

Example #3

function groupBy<T, K extends PropertyKey>(items: readonly T[], callbackFn: (item: T) => K): Record<K, T[]> {
  const group = {} as Record<K, T[]>;

  for (const item of items) {
    const key = callbackFn(item);

    if (!(key in group)) {
      group[key] = [];
    }

    group[key].push(item);
  }

  return group;
}

Example #4

type Team = 'frontEnd' | 'backEnd' | 'devops';

interface Employee {
  name: string;
  age: number;
  hobby: string[];
}

const team: Record<Team, Employee[]> = {
  frontEnd: [],
  backEnd: [],
  devops: [],
};

team.frontEnd.push({
  name: 'Ky',
  age: 30,
  hobby: ['hiking', 'football'],
});