Spread Operator in JavaScript

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JavaScript, the widely scripting language of the web, has been through numerous transformations. Among the game-changing features introduced in ES6 (ECMAScript 2015) is the mighty “spread operator”. This operator is a powerful tool in a developer’s arsenal, and understanding it can significantly enhance your coding efficiency and style.

Understanding the Spread Operator

The spread operator denoted as ... (three dots), is a quick and concise way to ‘spread’ or expand collections like arrays and object literals where zero or more arguments or elements are expected. It’s a versatile operator that shines in scenarios such as function invocation, merging arrays or objects, cloning data structures, and even destructuring.

The Spread Operator in Function Calls

Function calls often require multiple arguments. With the spread operator, we can elegantly pass an array’s contents as individual arguments. Let’s illuminate this with a code snippet:

let numbers = [9, 4, 7, 1];
console.log(Math.min(...numbers));  // Output: 1

In this example, Math.min seeks multiple arguments, not an array. The spread operator unpacks the array into separate arguments, making the function call seamless and clean.

The Spread Operator in Array Literals

When it comes to creating or concatenating arrays, the spread operator comes in handy. It allows you to expand an array into individual elements within another array. Consider this example:

let part = ['two', 'three'];
let numbers = ['one', ...part, 'four', 'five']; // ["one", "two", "three", "four", "five"]

Here, ...part is unfolded into individual elements, resulting in a combined array.

The Spread Operator in Object Literals

Similar to array literals, the spread operator simplifies working with object literals too. It neatly unpacks properties from a provided object onto a new object. Let’s dive into an example:

let obj1 = { foo: 'bar', x: 42 };
let obj2 = { foo: 'baz', y: 13 };

let clonedObj = { ...obj1 }; // Object { foo: "bar", x: 42 }
let mergedObj = { ...obj1, ...obj2 }; // Object { foo: "baz", x: 42, y: 13 }

In these examples, ...obj1 and ...obj2 expand into separate key-value pairs to form the new objects. If both objects share keys, the value from the latter spread object prevails.

Spread Operator with De-structuring

The spread operator synergizes beautifully with another ES6 feature – de-structuring. De-structuring allows you to ‘unpack’ elements or properties from arrays or objects. When combined with the spread operator, you can assign the remaining values to a new array or object. Check out this example:

let arr = ['a', 'b', 'c'];
let [first, ...rest] = arr;
console.log(first); // Output: 'a'
console.log(rest);  // Output: ['b', 'c']

In this snippet, ...rest gathers the remaining values from the array after first is de-structured.

Shallow Copying with the Spread Operator

It’s important to understand that the spread operator performs a “shallow copy“, not a “deep copy“. In essence, when you clone arrays or objects using the spread operator, it duplicates only the top layer. If you’re dealing with nested arrays or objects, the inner layers are copied by reference, not value. Therefore, if you modify a nested object after cloning, it affects the original object as well.

Here’s a detailed example, Let’s consider an object:

let originalObj = {
    name: 'John',
    age: 30,
    address: {
        city: 'New York',
        country: 'USA'

Let’s clone this object using the spread operator:

let clonedObj = { ...originalObj };

At this point, if we make changes to the first-level properties, they will not affect the original object:

clonedObj.name = 'Mike';
console.log(clonedObj.name);      // Output: 'Mike'
console.log(originalObj.name);    // Output: 'John'

However, things change when we try to modify the nested address object:

clonedObj.address.city = 'Los Angeles';
console.log(clonedObj.address.city);     // Output: 'Los Angeles'
console.log(originalObj.address.city);   // Output: 'Los Angeles'

Even though we made the change in clonedObj, the originalObj was also affected. This is because the spread operator performed a shallow copy of the originalObj, and the address property in both objects refers to the same memory space.

Further Reading


The spread operator is a simple yet incredibly potent tool in JavaScript. Mastering its use can make your code more readable, efficient, and fun to write. As with any powerful tool, remember that understanding its intricacies, such as shallow copying, is crucial to prevent unexpected bugs. Happy coding!

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