This article was originally published on webinuse.com
In 2015 ES6 (ECMA Script 6) was introduced. It brought a lot of new and improved features. Two features, among others, that we got with ES6 are rest and spread operators.

If you’ve been learning JavaScript or have tried some general programming, you’ve probably come across these handy little tools. In this article, we’ll explore what rest and spread operators are, the differences between them, and how you can use them to level up your coding skills. So, let’s jump right in!

What are Rest and Spread Operators?

Before we get into the details, let’s clarify what we’re talking about when we mention rest and spread operators. Both operators came with the ES6 (ECMAScript 2015) JavaScript standard.

As you can see from the name, ECMAScript 2015 (ES6 version) was introduced in 2015 and it was a major upgrade from the previous version of JavaScript.

Rest and spread operators are designed to make your life easier when working with arrays and objects. Now, let’s finally see how and why you should learn them.

Rest Operator

The rest operator is denoted by three consecutive dots (…). Its main purpose is to collect the remaining elements of an array or object into a new array or object.

It’s especially useful when you’re dealing with functions that can accept a varying number of arguments.

For example, let’s say you have a function that calculates the sum of all its arguments:

function sum(...numbers) {
  return numbers.reduce((total, num) => total + num, 0);
}

console.log(sum(1, 2, 3, 4, 5)); // Output: 15

In this example, the rest operator collects all the arguments passed to the sum function into an array called numbers.

This makes it incredibly easy to handle functions with a variable number of arguments.

Rest operator helps us to, literally, have as many arguments to our function as we want. Read more about an unlimited number of arguments in a JavaScript function.

Spread Operator

Like the rest operator, the spread operator is also represented by three consecutive dots (…). However, its purpose is different. The spread operator is used to expand the elements of an array or object, making it simpler to manipulate and merge data structures.

For instance, if you want to merge two arrays, you can use the spread operator like this:

const array1 = [1, 2, 3];
const array2 = [4, 5, 6];

const mergedArray = [...array1, ...array2];
console.log(mergedArray); // Output: [1, 2, 3, 4, 5, 6]

As you can see, the spread operator helps you combine arrays with ease, making your code cleaner and more concise.

This syntax is pretty popular with React developers. It is the most commonly used while manipulating React state.

It is also well utilized by the type of developers who propagate that data should be immutable. Meaning that once we set a variable it should hold its data and data’s structure forever, without ever changing it.

This approach to development usually means more typing. But with a spread operator, it’s much easier.

The Difference Between Rest and Spread Operators

At this point, you might be thinking, “Wait, both operators look the same. How do I know when to use which one?” That’s a great question! The key difference between rest and spread operators lies in the way how you use them:

1. The rest operator collects elements into a new array or object.

2. The spread operator expands elements from an existing array or object.

In other words, you can think of the rest operator as a “gatherer” and the spread operator as an “expander”.

Imagine you’re working on a jigsaw puzzle. The rest operator is like the process of gathering all the scattered puzzle pieces from the table and placing them into a neat pile.

On the other hand, the spread operator is like taking the pieces from the pile and spreading them out, making it easier to see each piece and find where it fits in the bigger picture.

Both operators have their unique purposes, and understanding their roles will help even complete beginners grasp their usage in the world of programming.

Now that we’ve covered the basics let’s dive deeper into each operator and see how they can help you level up your coding game.

Rest Operator in Detail

As we mentioned earlier, the rest operator is incredibly useful when dealing with functions that accept a varying number of arguments. Let’s take a closer look at how it works and how you can use it in your code.

Collecting Remaining Elements in an Array

One of the most common use cases for the rest operator is to collect the remaining elements in an array. Let’s say you have an array of numbers, and you want to separate the first element from the rest of the elements:

const numbers = [1, 2, 3, 4, 5];
const [first, ...rest] = numbers;

console.log(first); // Output: 1
console.log(rest); // Output: [2, 3, 4, 5]

In this example, the rest operator collects all the remaining elements in the numbers array (excluding the first element) into a new array called rest.

This technique is particularly useful when working with array destructuring. It helps us extract values in an easy and convenient way. Before the rest operator, you would probably use array loops, or some trick with a .length property.

Simplifying Function Arguments

Another powerful use case for the rest operator is simplifying function arguments. As we saw earlier, the rest operator can easily handle a variable number of function arguments, making it an invaluable tool for writing flexible, reusable code.

Consider the following example, where we create a function that accepts multiple strings and returns a new string with all the input strings concatenated together:

function concatenateStrings(separator, ...strings) {
  return strings.join(separator);
}

console.log(concatenateStrings(" ", "Hello", "world!")); // Output: "Hello world!"

In this example, the rest operator collects all the arguments passed to the concatenateStrings function (excluding the first argument, separator) into an array called strings. This allows the function to accept any number of input strings, making it incredibly flexible and reusable.

The example above is a great example of how we can prevent some unwanted errors.

Instead of using the rest syntax, imagine we used concatenateStrings(separator, stringOne, stringTwo). This would limit us to only two strings. And if you omit any of those, it would throw an error. If you pass more than expected, nothing would happen.

Handling Variable Number of Function Arguments

Similar to the previous example, we can pass only the rest operator as a function parameter.

Imagine you need to create a function that calculates the average of any number of input numbers. The rest operator makes this a breeze:

function average(...numbers) {
  const sum = numbers.reduce((total, num) => total + num, 0);
  return sum / numbers.length;
}

console.log(average(1, 2, 3, 4, 5)); // Output: 3

In this example, the rest operator collects all the arguments passed to the average function into an array called numbers. This allows the function to calculate the average of any number of input numbers with ease.

This function also is not dependent on any number of arguments. It can accept as many as you need and still return the correct result.

Common Pitfalls and Best Practices

As you begin incorporating the rest operator into your code, keep these potential pitfalls and best practices in mind:

1. Remember that the rest operator can only be used once in a destructuring assignment or function parameter list, and it must appear at the end.

2. When using the rest operator in a function, avoid using the arguments object, as it can lead to less readable and less efficient code.

3. Always test your code thoroughly, particularly when using the rest operator with other ES6 features such as destructuring and default parameters.

Spread Operator in Detail

Now that we’ve covered the rest operator, let’s take a closer look at the spread operator and explore its various use cases.

Expanding Elements in an Array or Object

As we mentioned earlier, the spread operator is used to expand the elements of an array or object, making it easier to manipulate and merge data structures. For example, you can use the spread operator to clone an array or object, preserving the original data while creating a new, separate copy:

const originalArray = [1, 2, 3];
const clonedArray = [...originalArray];

console.log(clonedArray); // Output: [1, 2, 3]

In this example, the spread operator expands the elements of the originalArray and creates a new array called clonedArray with the same elements. This technique is particularly useful when you need to work with a copy of an array or object without modifying the original data.

Simplifying the Manipulation of Arrays and Objects

The spread operator can significantly simplify the manipulation of arrays and objects in your code. For example, you can use the spread operator to add elements to an array:

const originalArray = [1, 2, 3];
const newArray = [0, ...originalArray, 4, 5];

console.log(newArray); // Output: [0, 1, 2, 3, 4, 5]

In this example, the spread operator expands the elements of the originalArray and creates a new array called newArray with additional elements at the beginning and end.

Real-Life Examples and Code Snippets

Let’s look at a few more examples of how the spread operator can be used in real-life programming scenarios.

Combining Arrays Using Spread Operator

Suppose you need to combine multiple arrays into a single array. With the spread operator, you can achieve this quickly and easily:

const fruits = ['apple', 'banana', 'orange'];
const vegetables = ['carrot', 'broccoli', 'pepper'];

const combinedArray = [...fruits, ...vegetables];
console.log(combinedArray); // Output: ['apple', 'banana', 'orange', 'carrot', 'broccoli', 'pepper']

Cloning Objects Using Spread Operator

Imagine you need to create a copy of an object with some additional properties. The spread operator makes this a breeze:

const originalObject = {name: 'John', age: 30};
const updatedObject = {...originalObject, city: 'New York', occupation: 'Software Developer'};

console.log(updatedObject); // Output: {name: 'John', age: 30, city: 'New York', occupation: 'Software Developer'}

In this example, the spread operator expands the properties of the originalObject and creates a new object called updatedObject with additional properties.

Using Spread Operator with Strings

The spread operator can also be used with strings, expanding them into arrays of individual characters:

const string = 'hello';
const characters = [...string];

console.log(characters); // Output: ['h', 'e', 'l', 'l', 'o']

In this example, the spread operator expands the characters of the string and creates a new array called characters.

Common Pitfalls and Best Practices

As you begin incorporating the spread operator into your code, keep these potential pitfalls and best practices in mind:

1. Be aware that the spread operator creates a shallow copy of an array or object. This means that if your original data structure contains nested arrays or objects, the spread operator will only create a new reference to the nested data rather than a deep copy. In such cases, you may need to use other methods, such as JSON.parse(JSON.stringify()), to create a deep copy of your data.

2. When using the spread operator with objects, make sure to consider the order of properties. If you have duplicate properties in the original and new objects, the latter property will overwrite the former.

3. Remember that the spread operator can be used in various places, such as function calls, array literals, and object literals. Make sure to familiarize yourself with these different use cases to maximize their potential in your code.

Conclusion

Congratulations! You’ve made it through our comprehensive guide to rest and spread operators. With your newfound understanding of these powerful tools, you’re well-equipped to write cleaner, more efficient, and more versatile code.

As a beginner in web development, it’s essential to get comfortable with rest and spread operators, as they are widely used in modern JavaScript and can significantly enhance your programming skills.

So, don’t hesitate to practice using them in your projects and explore their various applications.

Remember, the rest operator is your go-to “gatherer” when you need to collect elements into a new array or object, while the spread operator is your trusty “expander” when you need to simplify the manipulation of arrays and objects.

Keep honing your coding skills, and before you know it, you’ll be a master of rest and spread operators in no time. Good luck, and happy coding!