Node.js — How to Merge Objects

Merging objects in JavaScript is a common task. For example, when starting an application you may load multiple configuration files and merge them into a single configuration object.

This tutorial shows you how to merge objects in JavaScript.

Node.js Series Overview

Merge Objects

Merging objects in JavaScript is possible in different ways. Two common approaches are Object.assign() or the spread operator.

Merge Objects With Object.assign

The outcome of Object.assign() and the spread operator are the same. Using Object.assign() copies properties of one or many source objects into a target object. The target object is the first one in the parameter list:

const first = {  
  name: 'Marcus',
  sub: { eyes: 'blue' }
}

const second = {  
  name: 'Node.js',
  sub: { hair: 'brown' }
}

const merged = Object.assign({}, first, second)  
// { name: 'Node.js',
//   sub: { hair: 'brown' }
// }

Remember that Object.assign modifies the target object (the first parameter). To ensure a new object without changing any of the sources, you should pass an empty object as the first parameter.

A downside of this approach, it merges only the first-level in a hierarchy. It doesn’t merge nested objects.

Merge Objects Using the Spread Operator

You may merge objects using the spread operator. The spread operator composes of three dots in front of an object. Merge two or more objects into a new objects like this:

const first = {  
  name: 'Marcus',
  sub: { eyes: 'blue' }
}

const second = {  
  name: 'Node.js',
  sub: { hair: 'brown' }
}

const spread = { ...first, ...second }  
// { name: 'Node.js',
//   sub: { hair: 'brown' }
// }

Also, merging objects with this approach comes with a downside: it merges only the first-level in a hierarchy. It doesn’t merge nested objects.

Deep Merge

The described ways to merge objects using Object.assign() and the spread operator have the same shortcoming: they don’t merge objects recursively.

What is a deep merge?

A deep merge also merges deeper hierarchies inside of an object and not only the first level. Basically: merging objects inside of objects.

To recursively merge object, you may use the deepmerge package or lodash’s merge method. Here’s an example using the deepmerge package for a deep object merge:

const merge = require('deepmerge')

const first = {  
  name: 'Marcus',
  sub: { eyes: 'blue' }
}

const second = {  
  name: 'Node.js',
  sub: { hair: 'brown' }
}

const merged = merge(first, second)

// { name: 'Node.js',
//   sub: { eyes: 'blue', hair: 'brown' }
// }

Notice the merged result of sub in the merged object. Both properties, eyes and hair, are now available in the result.

Enjoy merging & make it rock!


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