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

  1. String Replace All Appearances
  2. Remove All Whitespace From a String in JavaScript
  3. Generate a Random ID or String in Node.js or JavaScript
  4. Remove Extra Spaces From a String in JavaScript or Node.js
  5. Remove Numbers From a String in JavaScript or Node.js
  6. Get the Part Before a Character in a String in JavaScript or Node.js
  7. Get the Part After a Character in a String in JavaScript or Node.js
  8. How to Check if a Value is a String in JavaScript or Node.js
  9. Check If a String Includes All Strings in JavaScript/Node.js/TypeScript
  10. Check if a Value is a String in JavaScript and Node.js
  11. Limit and Truncate a String to a Given Length in JavaScript and Node.js
  12. Split a String into a List of Characters in JavaScript and Node.js
  13. How to Generage a UUID in Node.js
  14. Reverse a String in JavaScript or Node.js
  15. Split a String into a List of Lines in JavaScript or Node.js
  16. Split a String into a List of Words in JavaScript or Node.js
  17. Detect if a String is in camelCase Format in Javascript or Node.js
  18. Check If a String Is in Lowercase in JavaScript or Node.js
  19. Check If a String is in Uppercase in JavaScript or Node.js
  20. Get the Part After First Occurrence in a String in JavaScript or Node.js
  21. Get the Part Before First Occurrence in a String in JavaScript or Node.js
  22. Get the Part Before Last Occurrence in a String in JavaScript or Node.js
  23. Get the Part After Last Occurrence in a String in JavaScript or Node.js
  24. How to Count Words in a File
  25. How to Shuffle the Characters of a String in JavaScript or Node.js
  26. Append Characters or Words to a String in JavaScript or Node.js (Coming soon)
  27. Check if a String is Empty in JavaScript or Node.js (Coming soon)
  28. Ensure a String Ends with a Given Character in JavaScript or Node.js (Coming soon)
  29. Left-Trim Characters Off a String in JavaScript or Node.js (Coming soon)
  30. Right-Trim Characters Off a String in JavaScript or Node.js (Coming soon)
  31. Lowercase the First Character of a String in JavaScript or Node.js (Coming soon)
  32. Uppercase the First Character of a String in JavaScript or Node.js (Coming soon)
  33. Prepend Characters or Words to a String in JavaScript or Node.js (Coming soon)
  1. Get Number of Seconds Since Epoch in JavaScript
  2. Get Tomorrow’s Date in JavaScript
  3. Increase a Date in JavaScript by One Week
  4. Add Seconds to a Date in Node.js and JavaScript
  5. Add Month(s) to a Date in JavaScript or Node.js
  6. Add Week(s) to a Date in JavaScript or Node.js
  7. Get the Current Year in JavaScript or Node.js
  8. How to Get a UNIX Timestamp in JavaScript or Node.js
  9. How to Convert a UNIX Timestamp to a Date in JavaScript or Node.js
  10. Add Days to a Date in JavaScript or Node.js
  11. Get Yesterday's Date in JavaScript or Node.js
  12. Add Minutes to a Date in JavaScript or Node.js (Coming soon)
  13. Add Hours to a Date in JavaScript or Node.js (Coming soon)
  14. Check If a Date Is Today in JavaScript or Node.js (Coming soon)
  15. Check If a Date is Tomorrow in JavaScript or Node.js (Coming soon)
  16. Check If a Date is Yesterday in JavaScript or Node.js (Coming soon)
  17. How to Format a Date YYYY-MM-DD in JavaScript or Node.js (Coming soon)

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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