How to Reverse an Array In-Place and Out-of-Place in JavaScript

Published May 25, 2020

I remember that I used to Google this question quite a bit because it would come up repeatedly during work. Let’s take this time to understand and remember how JavaScript handles its reversals.

In-Place Reversal: reverse()

const arr1 = [1, 2, 3];
const arr2 = arr1.reverse();

console.log(arr1); // [ 3, 2, 1 ]
console.log(arr2); // [ 3, 2, 1 ]

Using reverse() by itself will mutate, or modify, the original array.

Out-of-Place Reversal: slice() and reverse()

const arr1 = [1, 2, 3];
const arr2 = arr1.slice().reverse();

console.log(arr1); // [ 1, 2, 3 ]
console.log(arr2); // [ 3, 2, 1 ]

Using slice() with no arguments will default to slice(0), which returns the array from element 0 to the end of the array.

In other words, slice() returns a copy of the array while reverse() mutates the copied array.

Out-of-Place Reversal: slice() and reverse()

const arr1 = [1, 2, 3];
const arr2 = arr1.slice().reverse();

Using slice() with no arguments will default to slice(0), which returns the array from element 0 to the end of the array.

In other words, slice() returns a copy of the array while reverse() mutates the copied array.

Out-of-Place Reversal: spread and reverse()

const arr1 = [1, 2, 3];
const arr2 = [...arr1].reverse();

In ES6, we can use the spread operator to create a new array that we eventually reverse. The general idea is the same as the method above.

Out-of-Place Reversal: reduceRight()

const arr1 = [1, 2, 3];
const arr2 = arr1.reduceRight((acc, value) => {
  return [...acc, value];
}, []);

We use reduceRight() to apply a function to some accumulator acc and each value of an array, from right to left.

Out-of-Place Reversal: Recursion

const reverse = ([head, ...tail]) => {
  if (tail.length === 0) return [head];
  else return [...reverse(tail), head];
};
const arr1 = [1, 2, 3];
const arr2 = reverse(arr1);

Out-of-Place Reversal: Good ol’ for loop

const arr1 = [1, 2, 3];
let arr2 = [];
for (let i = arr1.length - 1; i >= 0; i--) {
  arr2.push(arr1[i]);
}

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