Lodash _.merge() Method

Beginner
5 min read
Updated: Nov 2026
5 examples
Object
Decorative image with the word MERGE

Definition

Recursively merges own enumerable string-keyed properties of sources into the destination object. Part of Lodash Object since 0.5.0 (stable — not deprecated).

Warning: _.merge mutates the first argument and returns it. Pass _.merge({}, …sources) when you need a new object.

Syntax

Call _.merge. Start with this line.

Lodash
_.merge(object, [sources])
CallWhat it does
_.merge(object, [sources])Primary signature.
_.merge(object)Work with an object’s keys, values, or paths.

Prefer import merge from "lodash/merge" when you want a smaller bundle.

Library declarations
Lodash
_.merge(object, [sources])

Parameters

This method has no parameters beyond those shown in the signature.

Return Value

Return ValueDescription
*The mutated destination

Lodash Examples

Example 1 — Official: Nested Array Objects

Plain objects inside arrays are merged index by index.

Lodash
var object = { a: [{ b: 2 }, { d: 4 }] };
var other  = { a: [{ c: 3 }, { e: 5 }] };

_.merge(object, other);
// → { a: [{ b: 2, c: 3 }, { d: 4, e: 5 }] }
Try It Yourself

How It Works

  1. Key a. Both sides have an array — merge by index.
  2. Index 0. {b:2} + {c:3} → {b:2, c:3}.
  3. Index 1. {d:4} + {e:5} → {d:4, e:5}.
  4. Mutate. object itself is updated.

Example 2 — Destination Is Mutated

The first argument and the return value are the same object.

Lodash
var dest = { a: 1 };
var result = _.merge(dest, { b: 2 });

console.log(result);
// → { a: 1, b: 2 }

console.log(dest === result);
// → true
Try It Yourself

How It Works

  1. Write. Properties from the source land on dest.
  2. Return. Same reference as dest.
  3. Identity. dest === result is true.
  4. Care. Do not merge into shared defaults by accident.

Example 3 — Fresh Object Pattern

Start with {} so sources stay untouched.

Lodash
var a = { a: { x: 1 } };
var b = { a: { y: 2 }, b: 3 };

_.merge({}, a, b);
// → { a: { x: 1, y: 2 }, b: 3 }

// a and b are unchanged
Try It Yourself

How It Works

  1. Empty dest. Writes go into a new {}.
  2. Deep. Nested a.x and a.y combine.
  3. Sources. a and b are not mutated.
  4. Habit. Prefer this pattern in pure helpers.

Example 4 — Arrays Merge by Index

Not concatenation — matching indexes overwrite, extras remain.

Lodash
_.merge({ items: [1, 2, 3] }, { items: [10] });
// → { items: [10, 2, 3] }
Try It Yourself

How It Works

  1. Index 0. 1 is replaced by 10.
  2. Index 1–2. Source has no values → 2 and 3 stay.
  3. Not concat. Result length stays 3, not 4.
  4. Custom. Use _.mergeWith to concatenate.

Example 5 — merge vs assign

Deep merge keeps nested keys; shallow assign replaces the nest.

Lodash
var nested = { a: { x: 1 } };

_.assign({}, nested, { a: { y: 2 } });
// → { a: { y: 2 } }

_.merge({}, nested, { a: { y: 2 } });
// → { a: { x: 1, y: 2 } }
Try It Yourself

How It Works

  1. assign. Replaces whole a → loses x.
  2. merge. Walks into a → keeps x, adds y.
  3. Pick. Shallow overwrite → assign; nested combine → merge.
  4. Both. Here we use {} so sources stay clean.
Compatibility

Browser & Runtime Support

_.merge is a Lodash Object helper since 0.5.0. It runs anywhere Lodash runs — modern browsers (CDN or bundle) and Node.js / Bun / Deno with npm install lodash.

✓ Stable · Lodash 0.5.0+

_.merge()

Available in [email protected] full builds and the lodash/merge modular import. Works in evergreen browsers and current Node LTS when Lodash is installed.

100% With Lodash 0.5.0+
Google Chrome Supported · Desktop & Mobile
Full support
Mozilla Firefox Supported · Desktop & Mobile
Full support
Apple Safari Supported · macOS & iOS
Full support
Microsoft Edge Supported · Chromium
Full support
Internet Explorer No native support · Use a polyfill
Polyfill
Opera Supported · Modern versions
Full support
Samsung Internet Supported · Android
Full support
Bun Supported · JavaScript runtime
Supported
Deno Supported · JavaScript runtime
Supported
Node.js Supported · Server runtime
Supported
Android WebView Supported · Modern WebView
Full support
_.merge Lodash 0.5.0+

Bottom line: Safe for production once Lodash is loaded. Prefer import merge from "lodash/merge" for smaller bundles. Remember it mutates the destination — use {} as dest when you need a fresh object.

Frequently Asked Questions

_.merge(object, …sources) recursively merges own enumerable string-keyed properties of sources into object. Official: merging nested array objects combines properties at each index.
Yes. The destination is updated in place and also returned. Use _.merge({}, a, b) when you want a fresh object.
_.assign is shallow: nested objects are replaced whole. _.merge walks into plain objects and arrays and merges them recursively.
By index, not concatenated. _.merge({items:[1,2,3]}, {items:[10]}) → {items:[10,2,3]}. For concat rules, use _.mergeWith.
undefined in a source is skipped; the destination value is kept. null overwrites like any other value.
Use import merge from "lodash/merge" or require("lodash/merge") for tree-shaking. Full build: import _ from "lodash" then _.merge.

Next: _.mergeWith()

Deep merge with a customizer for arrays and special cases.

_.mergeWith() tutorial →

About the author

Mari Selvan M P
Mari Selvan M P 🔗

Developer, cloud engineer, and technical writer

  • Experience 12 years building web and cloud systems
  • Focus Full Stack Development, AWS, and Developer Education

I write practical tutorials so students and working developers can learn by doing—from databases and APIs to deployment on AWS.

AWS Certified Developer – Associate (Credly)

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