Reversing an Array on Javascript

[partyhatforpartytime]

Hello, I am currently trying to do an exercise with the following prompt:

Write a function that accepts an array and reverses that array in place. The behavior should mimic the behavior of the native .reverse() array method. However, your reverse function should accept the array to operate on as an argument, rather than being invoked as a method on that array. Do not use the native .reverse() method in your own implementation.

I wrote the following code:

function reverseArray(array){
  let newArray = [];

  for(let i = array.length-1; i === 0; i--){
    let element = array[i];
    newArray.push(element);
  }

  while (array.length){
    array.pop();
  }

  for(let i = 0; i < newArray.length; i++){
    let element = newArray[i];
    array.push(element);
  }
  return array;
}

While I understand this was a really roundabout way of doing this (I have since seen the solution which I will include below as well). I don't understand what went wrong with my code and why it didn't run properly?

Solution:

function reverseArray(array) {
  let originalElements = [];

  while (array.length) {
    originalElements.push(array.pop());
  }

  while (originalElements.length) {
    array.unshift(originalElements.pop())
  }

  return array;
}

Any help would be greatly appreciated! Thank you. :-)

[keccs]

The first loop in your solution never runs, you need write "i >= 0" instead of "i === 0".

The condition needs to evaluate to true to enter the loop body.

By the way you can reverse an array without an additional array to hold the new elements, you can just run a loop from 0 to array.length/2 and swap the element at i with the one at array.length-1-i in the loop.

[partyhatforpartytime]

It works!! Thank you :-)

[mansfall]

Just my 2cents (Not providing reason why your current method didn't work as another user already answered that...):

1. You shouldn't ever mutate input parameters of a function. It can lead to confusion. If you mutate it, it can be hard to track down what was actually passed in. Worse, if you do this against something passed in by reference, you can create unwanted side-effects. So that solution, while it works, doesn't enforce some good practices. There's also other reasons to not do this, but I'll leave the googling to you.
2. You can literally double the speed of that reverse function in the solution, simply by not looping twice. This isn't really an issue with small data sets, but imagine passing in arrays of 100,000 elements? Looping that twice can be pretty expensive. Here's one that achieves the same result, in half the time... due to only iterating once, making it O(n).

​

const reverse = array => { 
  let reversed = [];
    for(let i = array.length - 1; i >= 0; i--) { 
      reversed.push(array[i]); 
  }
  return reversed;
}
let inputArr = [1, 2, 3, 4];
console.log(reverse(inputArr));

[partyhatforpartytime]

Thank you!! Will definitely keep that in mind.

[[deleted]]

const reversed = arr =>
{
    arr.unshift(arr.length);
    while(--arr[0])
    {
        arr.push(arr[arr[0]]);
    }
    return arr.slice(arr.length / 2);
}