Merge

Creates a type that deeply merges two object types.

type Result = Merge<Target, Source>;

Usage

Merge<T, S>

Use Merge when you need the type of the result of deeply merging two objects — for example, the type of a configuration after applying overrides with merge.

import type { Merge } from 'es-toolkit/types';

type Defaults = { host: string; port: number };
type Overrides = { debug: boolean };

type Config = Merge<Defaults, Overrides>;
// => { host: string; port: number; debug: boolean }

Nested objects are merged as well. TypeScript's built-in merge type T & S does not merge nested objects, but the Merge type does.

import type { Merge } from 'es-toolkit/types';

type Target = { server: { host: string; port: number } };
type Source = { server: { tls: boolean } };

type Result = Merge<Target, Source>;
// => { server: { host: string; port: number; tls: boolean } }

When keys overlap, the value type of the second object is used. With TypeScript's built-in merge type T & S, the value of an overlapping key can show up as never, but with the Merge type the second object's value type is used.

import type { Merge } from 'es-toolkit/types';

type Target = { id: string; value: string };
type Source = { value: number };

type Result = Merge<Target, Source>;
// => { id: string; value: number }

type Broken = Target & Source;
// => { id: string; value: never } (string & number becomes never)

Type Parameters

• T: Type of the target object.
• S: Type of the source object merged into T.