Lodash _.assignIn() Method

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

Definition

Like _.assign, but iterates over own and inherited enumerable source properties. Alias: _.extend. Part of Lodash Object since 4.0.0 (stable — not deprecated).

Pass the object and path or keys shown below. Check whether the helper mutates or returns a new object.

Syntax

Call _.assignIn. Start with this line.

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

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

Library declarations
Lodash
_.assignIn(object, ...sources)

Parameters

ParameterDescription
objectRequired. Destination (mutated and returned)
...sourcesOptional. Own + inherited enumerable string keys

Return Value

Return ValueDescription
*The mutated object

Lodash Examples

Example 1 — Official: Own + Inherited Keys

Prototype properties are copied along with own instance keys.

Lodash
function Foo() {
  this.a = 1;
}

function Bar() {
  this.c = 3;
}

Foo.prototype.b = 2;
Bar.prototype.d = 4;

_.assignIn({ 'a': 0 }, new Foo, new Bar);
// → { 'a': 1, 'b': 2, 'c': 3, 'd': 4 }
Try It Yourself

How It Works

  1. Own keys. a from Foo and c from Bar are copied.
  2. Inherited. b and d come from each prototype.
  3. Overwrite. Destination a: 0 becomes a: 1.
  4. Result. All four string keys land on the destination.

Example 2 — assignIn vs assign

Same inputs; only assignIn keeps prototype keys.

Lodash
function Foo() { this.a = 1; }
Foo.prototype.b = 2;

_.assign({}, new Foo());
// → { a: 1 }

_.assignIn({}, new Foo());
// → { a: 1, b: 2 }
Try It Yourself

How It Works

  1. assign. Own keys only → just a.
  2. assignIn. Also walks the prototype → includes b.
  3. Same shallow rules. Nested values are still by reference.
  4. Choose. Use assignIn when prototype data matters.

Example 3 — Alias _.extend

_.extend is the same function as _.assignIn.

Lodash
function Foo() { this.a = 1; }
Foo.prototype.b = 2;

_.extend({}, new Foo());
// → { a: 1, b: 2 }

_.extend === _.assignIn;
// → true
Try It Yourself

How It Works

  1. Alias. Docs list _.extend → assignIn.
  2. Same call. Behavior matches _.assignIn exactly.
  3. Identity. In the full build, they are the same function.
  4. Style. Prefer assignIn in new code; extend is fine in older codebases.

Example 4 — Non-Mutating Pattern

Pass {} as the destination to leave sources untouched.

Lodash
function User() {
  this.name = 'Ada';
}
User.prototype.role = 'admin';

var user = new User();
var copy = _.assignIn({}, user);
// → { name: 'Ada', role: 'admin' }

// user is unchanged
Try It Yourself

How It Works

  1. Empty target. {} receives the copies.
  2. Own + proto. name and role both land on copy.
  3. Flattened. Inherited role becomes an own key on the result.
  4. Safe. Original instance is not mutated.

Example 5 — Later Sources Win

Conflicts resolve left to right, including inherited keys.

Lodash
function A() { this.x = 1; }
A.prototype.y = 2;

function B() { this.x = 9; }
B.prototype.z = 3;

_.assignIn({}, new A(), new B());
// → { x: 9, y: 2, z: 3 }
Try It Yourself

How It Works

  1. First source. Adds x: 1 and inherited y: 2.
  2. Second source. Overwrites x to 9 and adds z.
  3. Order. Rightmost source wins for each key.
  4. Same as assign. Only the key set (own vs inherited) differs.
Compatibility

Browser & Runtime Support

_.assignIn is a Lodash Object helper (available since 4.0.0, stable in 4.x; alias _.extend). It runs anywhere Lodash runs — modern browsers (via CDN or bundle) and Node.js / Bun / Deno with npm install lodash.

✓ Stable · Lodash 4.0.0+

_.assignIn()

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

100% With Lodash 4.x
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
_.assignIn Lodash 4.0.0+

Bottom line: Safe for production once Lodash is on the page or in your bundle. Prefer import assignIn from "lodash/assignIn" for smaller builds. Use _.assign when you only need own keys.

Frequently Asked Questions

_.assignIn(object, ...sources) copies own and inherited enumerable string-keyed properties from each source onto object, left to right. Later sources overwrite earlier keys. It returns the mutated object.
Yes. _.extend is the documented alias of _.assignIn. Same function, two names.
_.assign copies own keys only. _.assignIn also copies enumerable inherited keys from the prototype chain.
Yes. Use _.assignIn({}, a, b) when you want a new object.
No. It is shallow — nested objects and arrays are copied by reference. Use _.merge for recursive plain-object merges.
Use import assignIn from "lodash/assignIn" or require("lodash/assignIn"). Full build: import _ from "lodash" then _.assignIn or _.extend.

Next: _.assignInWith()

Like _.assignIn, but with a customizer to decide each assigned value.

_.assignInWith() 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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