Lodash _.assignWith() Method

Beginner

5 min read

Updated: Nov 2026

5 examples

Object

Decorative image with the word ASSIGNWITH

Definition

Like _.assign, but accepts a customizer to produce each assigned value. If the customizer returns undefined, assignment is handled normally. Own enumerable string keys only. Since Lodash 4.0.0.

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

Syntax

Call _.assignWith. Start with this line.

Lodash

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

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

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

Library declarations

Lodash

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

Parameters

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

Return Value

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

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(_.assignWith, 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.

Example 2 — Direct `assignWith` Call

Same customizer without partialRight.

Lodash

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

_.assignWith({ 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 (`assign`)

Plain assign overwrites; the customizer can prevent that.

Lodash

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

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

Try It Yourself

How It Works

assign. Later source wins → a becomes 3.
assignWith. 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 — Sum Matching Numbers

Customizer can combine values instead of replacing them.

Lodash

function sumNumbers(objValue, srcValue) {
  if (_.isNumber(objValue) && _.isNumber(srcValue)) {
    return objValue + srcValue;
  }
}

_.assignWith({ a: 1, b: "x" }, { a: 2, b: "y", c: 3 }, sumNumbers);
// → { a: 3, b: "y", c: 3 }

Try It Yourself

How It Works

Both numbers on key a → return sum 3.
Strings on b → customizer returns undefined → default "y".
New key c → objValue undefined → default 3.
Pattern. Override only the pairs you care about.

Example 5 — Own Keys Only (vs `assignInWith`)

Prototype keys are skipped — that is the main difference from assignInWith.

Lodash

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

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

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

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

Try It Yourself

How It Works

Foo instance owns a; inherits b.
assignWith. Skips inherited b entirely.
assignInWith. Includes b, then customizes.
Pick assignWith when you do not want prototype surprises.

Compatibility

Browser & Runtime Support

_.assignWith is a Lodash Object helper (since 4.0.0). It runs anywhere Lodash 4.x runs — modern browsers (CDN or bundle) and Node.js / Bun / Deno with npm install lodash.

✓ Stable · Lodash 4.0.0+

_.assignWith()

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

_.assignWith Lodash 4.0.0+

Bottom line: Safe for production once Lodash is loaded. Prefer import assignWith from "lodash/assignWith". Use assignInWith when inherited keys matter, or mergeWith for deep custom merges.

Frequently Asked Questions

What does _.assignWith() do?

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

Which arguments does the customizer receive?

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

How is it different from _.assignInWith?

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

Does it mutate the destination?

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

Is it a deep merge?

No. Assignment is shallow. For nested merges with a customizer, use _.mergeWith.

How do I import only assignWith?

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

Next: _.at()

Create an array of values corresponding to paths of an object.

_.at() 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)

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Lodash _.assignInWith() Method — Custom Inherited Assign Lodash _.at() Method — Pick Values at Paths

Lodash _.assignWith() Method

Definition

Syntax

Parameters

Return Value

Lodash Examples

Example 1 — Official: Defaults-Style Customizer

How It Works

Example 2 — Direct assignWith Call

How It Works

Example 3 — Without Customizer (assign)

How It Works

Example 4 — Sum Matching Numbers

How It Works

Example 5 — Own Keys Only (vs assignInWith)

How It Works

Browser & Runtime Support

_.assignWith()

Frequently Asked Questions

Related Topics

_.assignInWith()

_.at()

_.assign()

_.mergeWith()

_.defaults()

_.assignIn()

Next: _.at()

About the author

Example 2 — Direct `assignWith` Call

Example 3 — Without Customizer (`assign`)

Example 5 — Own Keys Only (vs `assignInWith`)