None.

The reference to the original array, now reversed. Note that the array is reversed in place, and no copy is made.

The reverse() method transposes the elements of the calling array object in place, mutating the array, and returning a reference to the array.

The reverse() method preserves empty slots. If the source array is sparse, the empty slots' corresponding new indices are deleted and also become empty slots.

The reverse() method is generic. It only expects the this value to have a length property and integer-keyed properties. Although strings are also array-like, this method is not suitable to be applied on them, as strings are immutable.

The following example creates an array items, containing three elements, then reverses the array. The call to reverse() returns a reference to the reversed array items.

const items = [1, 2, 3];
console.log(items); // [1, 2, 3]

items.reverse();
console.log(items); // [3, 2, 1]

The reverse() method returns reference to the original array, so mutating the returned array will mutate the original array as well.

const numbers = [3, 2, 4, 1, 5];
const reversed = numbers.reverse();
// numbers and reversed are both in reversed order [5, 1, 4, 2, 3]
reversed[0] = 5;
console.log(numbers[0]); // 5

In case you want reverse() to not mutate the original array, but return a shallow-copied array like other array methods (e.g. map()) do, use the toReversed() method. Alternatively, you can do a shallow copy before calling reverse(), using the spread syntax or Array.from().

const numbers = [3, 2, 4, 1, 5];
// [...numbers] creates a shallow copy, so reverse() does not mutate the original
const reverted = [...numbers].reverse();
reverted[0] = 5;
console.log(numbers[0]); // 3

Sparse arrays remain sparse after calling reverse(). Empty slots are copied over to their respective new indices as empty slots.

console.log([1, , 3].reverse()); // [3, empty, 1]
console.log([1, , 3, 4].reverse()); // [4, 3, empty, 1]

The reverse() method reads the length property of this. It then visits each property having an integer key between 0 and length / 2, and swaps the two corresponding indices on both ends, deleting any destination property for which the source property did not exist.

const arrayLike = {
  length: 3,
  unrelated: "foo",
  2: 4,
  3: 33, // ignored by reverse() since length is 3
};
console.log(Array.prototype.reverse.call(arrayLike));
// { 0: 4, 3: 33, length: 3, unrelated: 'foo' }
// The index 2 is deleted because there was no index 0 present originally
// The index 3 is unchanged since the length is 3

• Polyfill of Array.prototype.reverse in core-js
• Indexed collections guide
• Array
• Array.prototype.join()
• Array.prototype.sort()
• Array.prototype.toReversed()
• TypedArray.prototype.reverse()