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A library for deep (recursive) merging of Javascript objects


Version published
Weekly downloads
increased by0.94%

Weekly downloads




Merges the enumerable properties of two or more objects deeply.

UMD bundle is 723B minified+gzipped

Getting Started

Example Usage

const x = { foo: { bar: 3 }, array: [{ does: 'work', too: [ 1, 2, 3 ] }] } const y = { foo: { baz: 4 }, quux: 5, array: [{ does: 'work', too: [ 4, 5, 6 ] }, { really: 'yes' }] } const output = { foo: { bar: 3, baz: 4 }, array: [{ does: 'work', too: [ 1, 2, 3 ] }, { does: 'work', too: [ 4, 5, 6 ] }, { really: 'yes' }], quux: 5 } merge(x, y) // => output


With npm do:

npm install deepmerge

deepmerge can be used directly in the browser without the use of package managers/bundlers as well: UMD version from


deepmerge exposes a CommonJS entry point:

const merge = require('deepmerge')

The ESM entry point was dropped due to a Webpack bug.


merge(x, y, [options])

Merge two objects x and y deeply, returning a new merged object with the elements from both x and y.

If an element at the same key is present for both x and y, the value from y will appear in the result.

Merging creates a new object, so that neither x or y is modified.

Note: By default, arrays are merged by concatenating them.

merge.all(arrayOfObjects, [options])

Merges any number of objects into a single result object.

const foobar = { foo: { bar: 3 } } const foobaz = { foo: { baz: 4 } } const bar = { bar: 'yay!' } merge.all([ foobar, foobaz, bar ]) // => { foo: { bar: 3, baz: 4 }, bar: 'yay!' }



There are multiple ways to merge two arrays, below are a few examples but you can also create your own custom function.

Your arrayMerge function will be called with three arguments: a target array, the source array, and an options object with these properties:

  • isMergeableObject(value)
  • cloneUnlessOtherwiseSpecified(value, options)
arrayMerge example: overwrite target array

Overwrites the existing array values completely rather than concatenating them:

const overwriteMerge = (destinationArray, sourceArray, options) => sourceArray merge( [1, 2, 3], [3, 2, 1], { arrayMerge: overwriteMerge } ) // => [3, 2, 1]
arrayMerge example: combine arrays

Combines objects at the same index in the two arrays.

This was the default array merging algorithm pre-version-2.0.0.

const combineMerge = (target, source, options) => { const destination = target.slice() source.forEach((item, index) => { if (typeof destination[index] === 'undefined') { destination[index] = options.cloneUnlessOtherwiseSpecified(item, options) } else if (options.isMergeableObject(item)) { destination[index] = merge(target[index], item, options) } else if (target.indexOf(item) === -1) { destination.push(item) } }) return destination } merge( [{ a: true }], [{ b: true }, 'ah yup'], { arrayMerge: combineMerge } ) // => [{ a: true, b: true }, 'ah yup']


By default, deepmerge clones every property from almost every kind of object.

You may not want this, if your objects are of special types, and you want to copy the whole object instead of just copying its properties.

You can accomplish this by passing in a function for the isMergeableObject option.

If you only want to clone properties of plain objects, and ignore all "special" kinds of instantiated objects, you probably want to drop in is-plain-object.

const isPlainObject = require('is-plain-object') function SuperSpecial() { this.special = 'oh yeah man totally' } const instantiatedSpecialObject = new SuperSpecial() const target = { someProperty: { cool: 'oh for sure' } } const source = { someProperty: instantiatedSpecialObject } const defaultOutput = merge(target, source) // => 'oh for sure' defaultOutput.someProperty.special // => 'oh yeah man totally' defaultOutput.someProperty instanceof SuperSpecial // => false const customMergeOutput = merge(target, source, { isMergeableObject: isPlainObject }) // => undefined customMergeOutput.someProperty.special // => 'oh yeah man totally' customMergeOutput.someProperty instanceof SuperSpecial // => true


Specifies a function which can be used to override the default merge behavior for a property, based on the property name.

The customMerge function will be passed the key for each property, and should return the function which should be used to merge the values for that property.

It may also return undefined, in which case the default merge behaviour will be used.

const alex = { name: { first: 'Alex', last: 'Alexson' }, pets: ['Cat', 'Parrot'] } const tony = { name: { first: 'Tony', last: 'Tonison' }, pets: ['Dog'] } const mergeNames = (nameA, nameB) => `${nameA.first} and ${nameB.first}` const options = { customMerge: (key) => { if (key === 'name') { return mergeNames } } } const result = merge(alex, tony, options) // => 'Alex and Tony' result.pets // => ['Cat', 'Parrot', 'Dog']



Defaults to true.

If clone is false then child objects will be copied directly instead of being cloned. This was the default behavior before version 2.x.


With npm do:

npm test





What is deepmerge?

A library for deep (recursive) merging of Javascript objects

Is deepmerge popular?

The npm package deepmerge receives a total of 22,686,640 weekly downloads. As such, deepmerge popularity was classified as popular.

Is deepmerge well maintained?

We found that deepmerge demonstrated a not healthy version release cadence and project activity because the last version was released a year ago.It has 1 open source maintainer collaborating on the project.

Last updated on 28 Oct 2019

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