Merge type

Combines two object types, the properties of the second replacing those of the first (like an object spread { ...a, ...b }), where an intersection would give never for conflicting properties.

import type { Merge } from 'utils';

Signature

export type Merge<A, B> = Simplify<Omit<A, keyof B> & B>;

Parameters

Name	Description
A	The base type.
B	The overriding type.

Example

type Row = Merge<Sample, { timestamp: Date }>; // `timestamp` becomes a Date

Tests

Line coverage: 0 %.

• ✅ lets the second type win

Source

src/types/merge.ts (on GitHub)

Uses

• Simplify: Flattens intersections and mapped types into a plain object type, so that editor tooltips show the properties instead of A & Omit<B, 'c'>.

Files it needs

src/types/merge.ts imports, directly or not:

• src/types/simplify.ts

Source code

import type { Simplify } from './simplify';

/**
 * Combines two object types, the properties of the second replacing those of the first (like an object
 * spread `{ ...a, ...b }`), where an intersection would give `never` for conflicting properties.
 *
 * @template A - The base type.
 * @template B - The overriding type.
 * @example
 * type Row = Merge<Sample, { timestamp: Date }>; // `timestamp` becomes a Date
 */
export type Merge<A, B> = Simplify<Omit<A, keyof B> & B>;

Test file

import type { Merge } from './merge';

describe('Merge', () => {
  it('lets the second type win', () => {
    expectTypeOf<Merge<{ a: number; b: number }, { b: string }>>().toEqualTypeOf<{ a: number; b: string }>();
  });
});

Keywords: merge, types, type