es-toolkit/docs/reference/object/merge.md

merge

Merges the properties of the source object into the target object.

This function performs a deep merge, meaning nested objects and arrays are merged recursively.

• If a property in the source object is an array or an object and the corresponding property in the target object is also an array or object, they will be merged.
• If a property in the source object is undefined, it will not overwrite a defined property in the target object.

Unlike toMerged, this function mutates the target object.

Signature

function merge<T, S>(target: T, source: S): T & S;

Parameters

• target (T): The target object into which the source object properties will be merged. This object is modified in place.
• source (S): The source object whose properties will be merged into the target object.

Returns

(T & S): The updated target object with properties from the source object merged in.

Examples

const target = { a: 1, b: { x: 1, y: 2 } };
const source = { b: { y: 3, z: 4 }, c: 5 };
const result = merge(target, source);
console.log(result);
// Output: { a: 1, b: { x: 1, y: 3, z: 4 }, c: 5 }

const target = { a: [1, 2], b: { x: 1 } };
const source = { a: [3], b: { y: 2 } };
const result = merge(target, source);
console.log(result);
// Output: { a: [3, 2], b: { x: 1, y: 2 } }

const target = { a: null };
const source = { a: [1, 2, 3] };
const result = merge(target, source);
console.log(result);
// Output: { a: [1, 2, 3] }

Demo

::: sandpack

import { merge } from 'es-toolkit';

const target = { a: 1, b: { x: 1, y: 2 } };
const source = { b: { y: 3, z: 4 }, c: 5 };
const result = merge(target, source);
console.log(result);

:::

Performance Comparison

	Bundle Size	Performance
es-toolkit	271 bytes (97.8% smaller)	1,952,436 times (3.65× faster)
es-toolkit/compat	4,381 bytes (64.9% smaller)	706,558 times (1.32× faster)
lodash-es	12,483 bytes	533,484 times