Lodash _.assignInWith() Method

Beginner

5 min read

Updated: Nov 2026

5 examples

Object

Decorative image with the word ASSIGNINWITH

Definition

Like _.assignIn, but accepts a customizer to produce each assigned value. If the customizer returns undefined, assignment is handled normally. Alias: _.extendWith. 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 _.assignInWith. Start with this line.

Lodash

_.assignInWith(object, ...sources, customizer)

Call	What it does
`_.assignInWith(object, ...sources, customizer)`	Primary signature.
`_.assignInWith(object)`	Work with an object’s keys, values, or paths.

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

Library declarations

Lodash

_.assignInWith(object, ...sources, customizer)

Parameters

Parameter	Description
object	Required. Destination (mutated and returned)
...sources	Optional. Own + inherited enumerable string keys
customizer	Required. Last argument — produces assigned values

Return Value

Return Value	Description
*	The value returned by _.assignInWith().

Lodash Examples

Example 1 — Official: Defaults-Style Customizer

Keep existing values; only fill keys that are still undefined.

Lodash

function customizer(objValue, srcValue) {
  return _.isUndefined(objValue) ? srcValue : objValue;
}

var defaults = _.partialRight(_.assignInWith, customizer);

defaults({ 'a': 1 }, { 'b': 2 }, { 'a': 3 });
// → { 'a': 1, 'b': 2 }

Try It Yourself

How It Works

Customizer. Prefer the destination value when it is defined.
partialRight. Binds the customizer as the last argument.
Sources. { b: 2 } fills a missing key; { a: 3 } cannot overwrite a: 1.
Related. Similar idea to _.defaults for own keys.

Example 2 — Direct `assignInWith` Call

Same customizer without partialRight.

Lodash

function customizer(objValue, srcValue) {
  return _.isUndefined(objValue) ? srcValue : objValue;
}

_.assignInWith({ a: 1 }, { b: 2 }, { a: 3 }, customizer);
// → { a: 1, b: 2 }

Try It Yourself

How It Works

Last arg. The customizer must be the final argument.
Sources in the middle. Any number of sources can appear before it.
Same rule. Keep defined destination values; take sources for missing keys.
Mutates. The first object is updated in place.

Example 3 — Without Customizer (`assignIn`)

Plain assignIn overwrites; the customizer can prevent that.

Lodash

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

_.assignInWith({ a: 1 }, { a: 3, b: 2 }, function (objValue, srcValue) {
  return _.isUndefined(objValue) ? srcValue : objValue;
});
// → { a: 1, b: 2 }

Try It Yourself

How It Works

assignIn. Later source wins → a becomes 3.
assignInWith. Customizer keeps a: 1.
New keys. b is still filled because it was undefined.
Use. Reach for With when overwrite rules are not “last wins”.

Example 4 — Alias `_.extendWith`

_.extendWith is the same function as _.assignInWith.

Lodash

function customizer(objValue, srcValue) {
  return _.isUndefined(objValue) ? srcValue : objValue;
}

_.extendWith({ a: 1 }, { b: 2 }, { a: 3 }, customizer);
// → { a: 1, b: 2 }

_.extendWith === _.assignInWith;
// → true

Try It Yourself

How It Works

Alias. Docs list _.extendWith → assignInWith.
Same call. Behavior matches exactly.
Identity. In the full build they are the same function.
Style. Prefer assignInWith in new code.

Example 5 — Inherited Source Keys

Like assignIn, prototype keys are included — then customized.

Lodash

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

function customizer(objValue, srcValue) {
  return _.isUndefined(objValue) ? srcValue : objValue;
}

_.assignInWith({ a: 9 }, new Foo(), customizer);
// → { a: 9, b: 2 }

_.assignWith({ a: 9 }, new Foo(), customizer);
// → { a: 9 }   // own keys only — no b

Try It Yourself

How It Works

Own key. Source a: 1 does not overwrite destination a: 9.
Inherited. Prototype b: 2 is still a candidate key.
Fill. b was undefined on the destination → assigned.
assignWith. Skips inherited b entirely.

Compatibility

Browser & Runtime Support

_.assignInWith is a Lodash Object helper (available since 4.0.0, stable in 4.x; alias _.extendWith). 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+

_.assignInWith()

Available in [email protected] full builds and the lodash/assignInWith 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

_.assignInWith Lodash 4.0.0+

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

Frequently Asked Questions

What does _.assignInWith() do?

_.assignInWith(object, ...sources, customizer) is like _.assignIn, but each assigned value can be produced by your customizer. If the customizer returns undefined, Lodash assigns the source value as usual.

Is _.assignInWith the same as _.extendWith?

Yes. _.extendWith is the documented alias of _.assignInWith.

Which arguments does the customizer receive?

Five arguments: (objValue, srcValue, key, object, source).

How is it different from _.assignWith?

Same customizer idea. _.assignInWith walks own and inherited enumerable string keys; _.assignWith walks own keys only.

Does it mutate the destination?

Yes. Use _.assignInWith({}, ...sources, customizer) when you want a new object.

How do I import only assignInWith?

Use import assignInWith from "lodash/assignInWith" or require("lodash/assignInWith"). Full build: import _ from "lodash" then _.assignInWith or _.extendWith.

Next: _.assignWith()

Like _.assign, but with a customizer for each assigned own-key value.

_.assignWith() tutorial →

About the author

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)

10 people found this page helpful

Lodash _.assignIn() Method Lodash _.assignWith() Method — Custom Own-Key Assign

Lodash _.assignInWith() Method

Definition

Syntax

Parameters

Return Value

Lodash Examples

Example 1 — Official: Defaults-Style Customizer

How It Works

Example 2 — Direct assignInWith Call

How It Works

Example 3 — Without Customizer (assignIn)

How It Works

Example 4 — Alias _.extendWith

How It Works

Example 5 — Inherited Source Keys

How It Works

Browser & Runtime Support

_.assignInWith()

Frequently Asked Questions

Related Topics

_.assignIn()

_.assignWith()

_.assign()

_.mergeWith()

_.defaults()

_.merge()

Next: _.assignWith()

About the author

Example 2 — Direct `assignInWith` Call

Example 3 — Without Customizer (`assignIn`)

Example 4 — Alias `_.extendWith`