Code recipe: JavaScript merge objects

Here's how you can merge multiple objects as one in JavaScript

Posted on February 11, 2021


There are at least 2 ways to merge two or more JavaScript objects in JavaScript easily:

  • Using spread operator
  • Using Object.assign() method

Let’s see how to use the spread operator first:

Merging objects with the spread operator

You can use the spread operator to extract its properties into a new single property.

Here’s an example of how you can merge human and traits objects to create a person object:

const human = {
  name: "John",
};

const traits = {
  age: 29,
  hobby: "Programming computers",
};

const person = { ...human, ...traits };

The person object will have the following properties:

{
  name: "John",
  age: 29,
  hobby: "Programming computers"
}

Keep in mind that the properties of the previous object will be overwritten with the next object. For example, if the human object already has the age property, it will be overwritten by the age property from the traits object:

const human = {
  name: "John",
  age: 37,
};

const traits = {
  age: 29,
  hobby: "Programming computers",
};

const person = { ...human, ...traits };

console.log(person.age); // 29

The same thing happens when you have a third object that has the age property.

Merging objects using Object.assign()

The Object.assign() method is a built-in JavaScript Object method that allows you to copy defined properties from the source objects into the target object:

Object.assign(target, source);

You can pass as many source objects as you need to the method. Please take note that the Object.assign() method will modify the target object and return it too.

Here’s an example of creating the person object with the assign() method:

const human = {
  name: "John",
  age: 37,
};

const traits = {
  age: 29,
  hobby: "Programming computers",
};

const person = Object.assign(human, traits);

console.log(human); // { name: "John", age: 29, hobby: "Programming computers" }
console.log(person); // { name: "John", age: 29, hobby: "Programming computers" }

As shown in the code above, both human and person objects will have the same properties. If you don’t want to modify the original array, you can pass a new empty object ({}) as the target argument while the rest of the objects are passed as the sources:

const human = {
  name: "John",
  age: 37,
};

const traits = {
  age: 29,
  hobby: "Programming computers",
};

const person = Object.assign({}, human, traits);

console.log(person); // { name: "John", age: 29, hobby: "Programming computers" }
console.log(human); // { name: "John", age: 37 }

Reusable function to merge many objects

As you can see from the examples above, you can use both spread operator or Object.assign() method to merge two or more objects. I will leave you with a reusable function that can merge as many objects passed into its arguments using the rest parameter and reduce() method:

function mergeObj(...arr){
  return arr.reduce((acc, val) => {    
    return { ...acc, ...val  };
  }, {});
}

const human = {
  name: "John",
  age: 37,
};

const traits = {
  age: 29,
  hobby: "Programming computers",
};

const attribute = {
  age: 40,
  nationality: "Belgian"
};

const person = mergeObj(human, traits, attribute);
console.log(person);
// { name: "John", age: 40, hobby: "Programming computers", nationality: "Belgian" }

You can freely use and modify the mergeObj() function in your code.

See also: Understanding JavaScript reduce method

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