Reversing an array in JS [duplicate]

Question

I've looked at SO to see what's been said about this and I've done my homework regarding this subject but, in the solution to this problem from Eloquent JavaScript, I've done the following:

function reversedArray(array) {
    let reversedArray = [];
    for (let i = 0; i <= array.length; i++) {
        reversedArray.push(array[array.length - i]);
    }
    return reversedArray;
}
let arr = [
    1,
    2,
    3,
    4
];
console.log(arr);
console.log(reversedArray(arr));

There, I obtained this:

> 1,2,3,4
> ,4,3,2,1

Which means that the array.length of the reversed array is now longer than the original array by an extra element. Of course, I fixed my code to eliminate the first element of the reversed array like this:

function reversedArray(array) {
    let reversedArray = [];
    for (let i = 0; i <= array.length; i++) {
        reversedArray.push(array[array.length - i]);
    }
    reversedArray.shift();
    return reversedArray;
}
let arr = [
    1,
    2,
    3,
    4
];
console.log(arr);
console.log(reversedArray(arr));

And it worked as expected producing:

> 1,2,3,4
> 4,3,2,1

I am starting to learn JS and I can't see why the reversed array has an extra element. Can somebody explain it to me? I don't feel it is right to just eliminate the element that's extra and move on with life... I have looked at some other solutions and they are fine but I just want to learn please, why is it that my function increases the array length? Many thanks!

let i = 0, array[array.length - i]… What index is array.length - 0…? — deceze
– deceze ♦, Commented Jan 14, 2022 at 14:42
Why reinvent the wheel? What's wrong with array.reverse()? — BenM
– BenM, Commented Jan 14, 2022 at 14:42
Personally, I'd reverse the looping: for (let i = array.length - 1; i >= 0 ; i--) { reversedArray.push(array[i]); } — Andrew Corrigan
– Andrew Corrigan, Commented Jan 14, 2022 at 14:49

Mr. Polywhirl · Accepted Answer · 2022-01-14 14:45:29Z

3

If you loop backwards, you do not need to perform any index subtraction.

const reversedArray = (array) => {
  const result = [];
  for (let i = array.length - 1; i >= 0; i--) {
    result.push(array[i]);
  }
  return result;
};

console.log(reversedArray([1, 2, 3, 4, 5]));

answered Jan 14, 2022 at 14:45

Mr. Polywhirl

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3 Comments

BenM Over a year ago

Or, if we're going down the ES6 route, reverse = (arr) => arr.map(arr.pop, [... arr]); ;)

Mr. Polywhirl Over a year ago

@BenM I tried to honor the for-loop as much as possible. I just rewrote the function definition as an assigned const. :p

Diego Over a year ago

Many thanks! Loopping backwards seems cool and that's a nice approach. I've learned something new today and for that I am thankful...

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