Array.prototype.reverse()

Summary: in this tutorial, you will learn how to use the JavaScript array reverse() method that reverses the order of elements within an array.

Introduction to JavaScript Array reverse() method

The reverse() method reverses the order of elements within the array and returns the modified array with elements in the reversed order.

Here’s the syntax of the reverse() method:

reverse()Code language: JavaScript (javascript)

The reverse() method does not take any parameters. It reverses the elements of an array in place and returns the reversed array.

The reverse() method is generic. This means you can call it on a non-array object that has the length property and integer-keyed properties.

Note that you cannot call the reverse() on strings because strings are immutable.

To reverse the order of elements in an array and return a copy of the array without modifying the original array, you can use the toReversed() method.

JavaScript Array reverse() method examples

Let’s explore some examples of using the JavaScript array reverse() method.

1) Using JavaScript array reverse() method on string arrays

The following example uses the reverse() method to reverse an array of strings:

const colors = ['red','green','blue'];
colors.reverse();

console.log(colors);Code language: JavaScript (javascript)

Output:

['blue', 'green', 'red']Code language: JavaScript (javascript)

2) Reversing number arrays

The following example uses the reverse() method to reverse the order of numbers in an array:

const scores = [1, 3, 5, 7];
scores.reverse();

console.log(scores);Code language: JavaScript (javascript)

Output:

[7, 5, 3, 1]Code language: JavaScript (javascript)

3) Reversing arrays of objects

The following example uses the reverse() method to reverse the order of objects in an array:

const books = [
  { title: 'Eloquent JavaScript', author: 'Marijn Haverbeke' },
  { title: 'JavaScript: The Good Parts', author: 'Douglas Crockford' },
  { title: 'JavaScript: The Definitive Guide', author: 'David Flanagan' },
];

books.reverse();

console.log(books);Code language: JavaScript (javascript)

Output:

[
    {
        "title": "JavaScript: The Definitive Guide",
        "author": "David Flanagan"
    },
    {
        "title": "JavaScript: The Good Parts",
        "author": "Douglas Crockford"
    },
    {
        "title": "Eloquent JavaScript",
        "author": "Marijn Haverbeke"
    }
]Code language: JavaScript (javascript)

4) Using reverse() on sparse arrays

When you call the reverse() method on a sparse array, the array remains sparse. The reverse() method copies empty slots over their respective indices as empty slots:

const scores = [1,,7,5];
scores.reverse();

console.log(scores);Code language: JavaScript (javascript)

Output:

[5, 7, empty, 1]Code language: JavaScript (javascript)

5) Using JavaScript array reverse() on non-array objects

The following example illustrates how to call the reverse() method on an object that has the length property and integer-keyed properties:

const arrayLike = {
  length: 3,
  unrelated: "bar",
  2: 2,
  3: 3, // ignored by reverse() since length is 3
};
console.log(Array.prototype.reverse.call(arrayLike));Code language: JavaScript (javascript)

Output:

{0: 2, 3: 3, length: 3, unrelated: 'bar'}Code language: JavaScript (javascript)

In this example, the reverse() method starts by accessing the length property of the array. Subsequently, it iterates through each property with an integer key ranging from 0 to length / 2.

During this process, the reverse() method swaps the values at corresponding indices on both ends of the array. Additionally, the method removes any destination property without a corresponding source property.

Summary

• Use the JavaScript array reverse() method to reverse the order of elements of an array in place.