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Lodash _.assignIn


The lodash _.assignIn method is a utility function in the lodash library that allows you to merge two or more objects together into a single object. This function acts as a wrapper around the native Javascript Object.assign method, but with added functionality and handling some edge cases.

With _.assignIn, you can copy own and inherited properties from the source objects to a target object. This means that any properties that exist in the source objects will be copied to the target object, overwriting any properties with the same name that already exist in the target object. Additionally, if there are properties that exist in an object's prototype chain, those properties will also be copied to the target object.

Here's an example of how to use _.assignIn:

const _ = require('lodash');

const target = {
   name: 'George',
   age: 30,

const source = {
   age: 50,
   occupation: 'Developer',

const result = _.assignIn(target, source);

/* Output:
      name: 'George',
      age: 50,
      occupation: 'Developer',

In this example, we have a target object with two properties (name and age). We also have a source object with two properties (age and occupation). When we use _.assignIn to merge these two objects, the resulting object contains all four properties, with the age property being overwritten by the source object's age value.

There are a few things to keep in mind when using _.assignIn:

  • The first argument passed to the function is treated as the target object, and will be modified in place. If you need to preserve the original target object, you should create a copy of it before using _.assignIn.
  • If you pass in more than two objects to _.assignIn, they will be merged in order, with later objects overwriting properties from earlier objects.
  • If any of the source objects have circular references (where an object refers to itself, either directly or indirectly), _.assignIn may go into an infinite loop and cause the program to crash. Overall, _.assignIn is a useful tool for merging objects in Javascript, and can help simplify code and reduce duplication.