How do i reverse an array in javaScript without .reverse()

Question

I'm trying to reverse the contents of an array. My approach works well when the contents of the said array are of same type. but once they're of different types it just doesn't work.
Constraint is that i can't use the .reverse() method and if possible, without creating a new array.
The answer in this question is close to what i want but i don't understand it.

Here is my code...

reverse.js

#!/usr/bin/node
exports.rev = function (list) {
  for (let i = 0, j = (list.length - 1); i <= (list.length / 2); i++, j--) {
    [list[i], list[j]] = [list[j], list[i]];
  }
  return (list);
};

main.js

#!/usr/bin/node
const rev = require('./reverse').rev;

console.log(rev([1, 2, 3, 4, 5]));
console.log(rev(["School", 89, { id: 12 }, "String"]));

Expected:
[5, 4, 3, 2, 1]
["String", { id: 12 }, 89, "School"]

What I got:
[5, 4, 3, 2, 1]
["String", 89, { id: 12 }, "School"]

Probably you're swapping the central elements twice, and swapping them twice is like not swapping them at all. You should fix it replacing i <= (list.length / 2) with i < (list.length / 2), even if I didn't test it yet — Christian Vincenzo Traina
– Christian Vincenzo Traina, Commented Sep 9, 2022 at 9:48
This also explains why reverting an odd number of elements doesn't cause any problem (in your first example you're swapping 3 twice, which is fine) — Christian Vincenzo Traina
– Christian Vincenzo Traina, Commented Sep 9, 2022 at 9:49
@ChristianVincenzoTraina Oh, yeah! This works! I never imagined that will be the source of the problem. Thanks — BaRzz007
– BaRzz007, Commented Sep 9, 2022 at 9:56

Nick Vu · Accepted Answer · 2022-09-09 09:57:31Z

1

I've found out that your code almost works. You just need to modify the condition a bit to

i < (list.length / 2) //not `<=`

function rev(list) {
  for (let i = 0, j = (list.length - 1); i < (list.length / 2); i++, j--) {
    [list[i], list[j]] = [list[j], list[i]];
  }
  return (list);
};

console.log(rev([1, 2, 3, 4, 5]));
console.log(rev(["School", 89, { id: 12 }, "String"]));

edited Sep 9, 2022 at 9:57

answered Sep 9, 2022 at 9:49

Nick Vu

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3 Comments

Nick Vu Over a year ago

I just modified my question to not clone a new array

BaRzz007 Over a year ago

Not really a requirement though, but what if i wanted this without having to use an extra array.

BaRzz007 Over a year ago

Already posted before i saw your comment

David Ho · Accepted Answer · 2022-09-09 09:46:14Z

0

let array = [1, 2, 3, 4, 5]
let reverse = [];

for (var i = array.length - 1; i >= 0; i--){
    reverse.push(array[i]);
}

answered Sep 9, 2022 at 9:46

David Ho

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1 Comment

Christian Vincenzo Traina Over a year ago

In this way you're creating a new array, not what OP wants

davidhuerta · Accepted Answer · 2022-09-09 09:59:24Z

0

You can try with a for loop like this:

let arr = ["School", 89, { id: 12 }, "String"];

let newArr = [];

for (let i = arr.length - 1; i >= 0; i--) {
  newArr.push(arr[i])
}
console.log(newArr);

Expected output: ["String", [object Object] { id: 12 }, 89, "School"]

edited Sep 9, 2022 at 9:59

answered Sep 9, 2022 at 9:59

davidhuerta

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Comments

Rajesh Dixit · Accepted Answer · 2022-09-09 10:12:34Z

0

You can user a recursion to reverse

You can use Spread operator(...) along with distructure assignment for that.

function rev(arr) {
  const [first, ...rest] = arr
  return arr.length > 1 ? [...rev(rest), first] : arr
}

console.log(rev([1, 2, 3, 4, 5]));
console.log(rev(["School", 89, { id: 12 }, "String"]));

const [first, ...rest] = arr is a shortcut for:

const first = arr[0]
const rest = arr.slice(1)

However, ...rev(rest) will spread items of returned array and spread them. So, [...rev(rest), first] will keep the output of rev first and then push first at the end of array.

If you are comfortable with mutation of original array, try this

function rev(arr) {
  return arr.length > 1 ? [arr.pop(), ...rev(arr)] : arr
}

console.log(rev([1, 2, 3, 4, 5]));
console.log(rev(["School", 89, { id: 12 }, "String"]));

edited Sep 9, 2022 at 10:12

answered Sep 9, 2022 at 10:02

Rajesh Dixit

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2 Comments

BaRzz007 Over a year ago

I'm new to JS so i don't really understand what ... does but i'm interested in this method.

Rajesh Dixit Over a year ago

Updated my answer with explanation. Let me know if you have more queries regarding it

malarres · Accepted Answer · 2022-09-09 10:24:03Z

0

Already good answers (and examples of arrays) here

My try with native methods:

const rev = (arr) => {
  len = arr.length;
  for (i = 1; i<= len; i++) arr.splice(len-i,0,arr.shift());
}

const array = ["School", 89, { id: 12 }, "String"];

console.log("original",array);
rev(array);
console.log("reversed",array);

edited Sep 9, 2022 at 10:24

answered Sep 9, 2022 at 10:17

malarres

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