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Lodash _.assignInWith


The lodash _.assignInWith method is a powerful utility that fills a gap in the functionality provided by the native JavaScript Object.assign() method. Like Object.assign(), _.assignInWith() copies the values of all enumerable own properties from one or more source objects to a target object. However, it differs in two key ways:

  1. It recursively copies nested object properties.
  2. It allows for a customizer function to be passed in, which gives fine-grained control over how the values are merged.


The _.assignInWith() method has the following function signature:

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

The first argument, object, is the target object that will receive the properties. The remaining arguments, sources, are the source objects from which the properties will be copied. Finally, customizer is an optional function that will be called to merge the values of any properties that already exist in the target object. If a customizer function is not provided, then the last sourced value for that property will overwrite earlier values.


Here is a simple example that shows how the lodash _.assignInWith() method can be used:

const _ = require('lodash');
const target = {
    name: 'John',
    age: 40,
    address: {
        city: 'New York',
        street: '123 Main St'
const source = {
    age: 50,
    address: {
        city: 'Boston'
function customizer(objValue, srcValue, key, object, source) {
    // Merge `address` objects, but overwrite all other properties
    if (key === 'address') {
        return _.assignInWith({}, objValue, srcValue, customizer);
const result = _.assignInWith(target, source, customizer);

In this example, we start with a target object that has a name, age, and address property. We also have a source object that has a new age value and an updated city value for the address property. We then define a customizer function that will be called to handle the merging of the address property. This customizer function uses the lodash _.assignInWith() method recursively to merge the nested address objects.

When we run the code, we see that the result object has merged the age and address properties from the source object into the target object, resulting in the following output:

    "name": "John",
    "age": 50,
    "address": {
        "city": "Boston",
        "street": "123 Main St"


The lodash _.assignInWith() method is a powerful utility that provides enhanced functionality for merging object properties. Its ability to recursively merge nested objects and its customizer function make it a valuable tool for managing complex data structures.