Best Cheat Sheet for JavaScript Merge Objects in 2024

by | Jul 9, 2024 | Web Development


JavaScript is a really flexible and handy programming language that plays a key role in modern web development. One common job developers face is merging objects. Whether you’re combining user data, settings, or configurations, knowing how to effectively perform JavaScript merge objects can make your code run smoother and look cleaner. This article will cover the different ways to merge objects in JavaScript, giving you a handy cheat sheet for 2024.

Introduction to JavaScript Merge Objects

JavaScript merge objects is a basic task where you combine two or more objects into one. This is super handy in lots of situations, like when you’re merging settings, putting together user input, or gathering API responses. Getting the hang of merging objects in JavaScript is key to keeping your code neat and efficient. Objects in JavaScript are made up of key-value pairs, with each key being unique within its object.

When you merge objects, you’re combining these key-value pairs from different objects into one. This can be easy or tricky, depending on how complex the objects are. JavaScript gives you several ways to merge objects, each with its own pros and cons. This cheat sheet will walk you through these methods, with examples and explanations to help you pick the right one for your situation.

Methods to Merge Objects

The Object.assign() Method

The Object.assign() method is one of the oldest and simplest ways to merge objects in JavaScript. It copies all the properties from one or more source objects to a target object. People often use it because it’s easy to understand and works with older JavaScript versions.

Here’s how it works:

const obj1 = { a: 1, b: 2 };
const obj2 = { b: 3, c: 4 };
const mergedObj = Object.assign({}, obj1, obj2);
console.log(mergedObj); // Output: { a: 1, b: 3, c: 4 }

In this example, properties from obj2 overwrite those in obj1 when they conflict. This method is particularly useful when you need to perform JavaScript merge objects quickly and efficiently.

The Spread Operator (...)

Best Cheat Sheet for JavaScript Merge Objects in 2024

The spread operator, introduced in ES6, provides a more concise syntax for JavaScript merge objects. It can expand objects inside a new one, making your code cleaner and more modern.

const obj1 = { a: 1, b: 2 };
const obj2 = { b: 3, c: 4 };
const mergedObj = { ...obj1, ...obj2 };
console.log(mergedObj); // Output: { a: 1, b: 3, c: 4 }

In this example:

  • obj1 and obj2 are combined into mergedObj using the spread operator.
  • If there are key conflicts, the value from the last object (obj2 here) takes precedence.

This method is similar to Object.assign(), but it uses a cleaner and more modern syntax for JavaScript merge objects.

The Object.entries() Method

For more complex merging scenarios, you might need to transform objects before merging them. The Object.entries() method, combined with reduce(), can handle such tasks. This approach offers greater flexibility, letting you apply custom logic during the JavaScript merge objects process.

const obj1 = { a: 1, b: 2 };
const obj2 = { b: 3, c: 4 };
const mergedObj = Object.entries({ ...obj1, ...obj2 }).reduce((acc, [key, value]) => {
  acc[key] = value;
  return acc;
}, {});
console.log(mergedObj); // Output: { a: 1, b: 3, c: 4 }

In this example:

  • obj1 and obj2 are first combined using the spread operator.
  • Object.entries() turns the combined object into an array of key-value pairs.
  • reduce() processes each key-value pair and constructs the mergedObj.

This method allows for more control over how objects are merged, making it useful for situations where you need to apply specific rules or transformations during the merge.

Using lodash.merge

Lodash is a popular JavaScript utility library with a bunch of helpful functions for everyday coding tasks. One of its handy tools is _.merge, which performs a deep merge, efficiently handling nested objects.

const _ = require('lodash');
const obj1 = { a: 1, b: { x: 1, y: 2 } };
const obj2 = { b: { y: 3, z: 4 }, c: 5 };
const mergedObj = _.merge({}, obj1, obj2);
console.log(mergedObj); // Output: { a: 1, b: { x: 1, y: 3, z: 4 }, c: 5 }

In this example:

  • obj1 and obj2 are deeply merged into mergedObj using _.merge.
  • The nested objects are merged correctly, with properties from obj2 overriding those in obj1 where there’s a conflict.

Using lodash.merge makes deep merging tasks easier and removes the need for custom merge functions in JavaScript merge objects complex scenarios.

Handling Deep Merging

Best Cheat Sheet for JavaScript Merge Objects in 2024

While the earlier methods work well for shallow merges, they fall short with nested objects. Deep merging requires a recursive approach to ensure all levels of nested objects are merged correctly during JavaScript merge objects operations.

Custom Deep Merge Function

Creating a custom deep merge function ensures nested objects are merged properly. This function recursively checks if a property is an object and merges it accordingly.

function deepMerge(target, source) {
  for (const key in source) {
    if (source[key] instanceof Object && key in target) {
      Object.assign(source[key], deepMerge(target[key], source[key]));
  Object.assign(target || {}, source);
  return target;
const obj1 = { a: 1, b: { x: 1, y: 2 } };
const obj2 = { b: { y: 3, z: 4 }, c: 5 };
const mergedObj = deepMerge(obj1, obj2);
console.log(mergedObj); // Output: { a: 1, b: { x: 1, y: 3, z: 4 }, c: 5 }

In this example:

  • The deepMerge function takes target and source objects.
  • It checks if each property in source is an object. If it is, the function recursively merges it with the corresponding property in target.
  • Finally, Object.assign merges the source into the target.

This function ensures a deep merge, handling all nested objects correctly.

Practical Use Cases

Merging Configuration Settings

In web development, it’s common to merge default settings with user-provided settings. This lets users customize their experience while keeping sensible defaults in JavaScript merge objects.

const defaultSettings = { theme: 'light', layout: 'grid' };
const userSettings = { theme: 'dark' };
const settings = { ...defaultSettings, ...userSettings };
console.log(settings); // Output: { theme: 'dark', layout: 'grid' }

Combining API Responses

When handling multiple API responses, merging them into a single object can simplify data management. This is particularly useful when aggregating data from different endpoints using JavaScript merge objects.

const apiResponse1 = { user: { id: 1, name: 'John' } };
const apiResponse2 = { user: { age: 30, location: 'USA' } };
const combinedResponse = deepMerge(apiResponse1, apiResponse2);
console.log(combinedResponse); // Output: { user: { id: 1, name: 'John', age: 30, location: 'USA' } }

Performance Considerations

When merging objects, performance can be a concern, especially with large datasets or complex objects. Here are some tips to optimize performance in JavaScript merge objects:

  • Avoid Unnecessary Merges: Only merge objects when necessary. Frequent merging can slow down your code.
  • Use Efficient Methods: For simple merges, prefer the spread operator or Object.assign() as they are optimized for performance.
  • Profile Your Code: Use profiling tools to identify and optimize slow merging operations.

Understanding how different merge methods impact performance will help you choose the best approach for your needs.

Common Pitfalls and How to Avoid Them

Overwriting Properties

When merging objects, properties with the same key will be overwritten. Make sure this is intended to avoid losing data.

const obj1 = { a: 1, b: 2 };
const obj2 = { b: 3, c: 4 };
// Merged object will have b: 3
const mergedObj = { ...obj1, ...obj2 };

Handling Arrays

Merging objects with arrays can be tricky, as arrays are replaced rather than merged in JavaScript merge objects operations.

const obj1 = { a: [1, 2] };
const obj2 = { a: [3, 4] };
// Array in obj2 will replace array in obj1
const mergedObj = { ...obj1, ...obj2 };
console.log(mergedObj); // Output: { a: [3, 4] }

For custom array merging, consider implementing a specific merge strategy.

function mergeArrays(target, source) {
  return [, ...source];
const obj1 = { a: [1, 2] };
const obj2 = { a: [3, 4] };
const mergedObj = { a: mergeArrays(obj1.a, obj2.a) };
console.log(mergedObj); // Output: { a: [1, 2, 3, 4] }

Deep Merge Challenges

Deep merging can lead to performance issues and unexpected behaviors. Make sure your deep merge logic handles all edge cases in JavaScript merge objects.

const obj1 = { a: { b: { c: 1 } } };
const obj2 = { a: { b: { d: 2 } } };
const mergedObj = deepMerge(obj1, obj2);
console.log(mergedObj); // Output: { a: { b: { c: 1, d: 2 } } }


Merging objects in JavaScript is a common task every developer should master. Whether you use Object.assign(), the spread operator, or a custom merge function, understanding each method’s nuances is important for writing efficient and maintainable code. With the techniques in this cheat sheet, you can confidently tackle any JavaScript merge objecs challenges you encounter in 2024 and beyond.

Remember, the key to successful JavaScript merge objects is choosing the right method for the job and being aware of potential pitfalls. With this knowledge, you’ll be ready to handle any object merging scenario in your JavaScript projects.

Becoming proficient in merging objects in JavaScript means you can write cleaner, more efficient code that’s easier to maintain and scale. Happy coding!

Understanding and implementing efficient merging techniques will significantly improve your JavaScript development skills. Keep experimenting with different methods and stay updated with the latest practices to ensure your code stays strong and fast.