Recursive Array Reversal Javascript

Question

So we are practicing functional javascript in my programming class with this assignment and I just can't get it to work right. Any advice in making this code work would be appreciated. Everything but the body was given for me to use. Here is what I have come up with: (It is always sending me just the first array index content rather than all of them reversed. I tried changing it to

 if(arr.length <= 1) return arr;

but that never hits the base case.)

function ReverseArray(arr) {

//base case

if(arr.length == 1)
{
   return arr[0];
}
if(arr.length == 0)
{
   return 0;
}

var head = arr.pop;
var newArr = [head, ReverseArray(arr)];
return newArr;
}

if(arr.length = 1) WRONG and if(arr.length = 0) wrong, You are assigning — epascarello
– epascarello, Commented Nov 12, 2014 at 17:22
Deleted my answer because I realised you were meant to use recursion. — Andy
– Andy, Commented Nov 12, 2014 at 17:31
Even if your function worked, your function returns an array so after one iteration, you'll have this [end0, [....]]. After two iterations, you'll have this [end0, [end1, [...]]] and it will keep nesting arrays further and further. Your approach will not work. You need a new design. Perhaps you want to use .concat() to add two arrays together rather than embedding one in another. If you want help with a new design, you will have to better describe the rules of the assignment since arr.reverse() already exists to do this. — jfriend00
– jfriend00, Commented Nov 12, 2014 at 17:59
possible duplicate of Reverse array function with one parameter. (JavaScript) — Dave
– Dave, Commented Nov 13, 2014 at 4:51

epascarello · Accepted Answer · 2014-11-12 17:24:10Z

2

x = y <--assignment
z == y <-- comparison

Looking at your code:

if(arr.length = 1)

needs to be

if(arr.length == 1)

same with the zero check

AND you are not calling pop

var head = arr.pop;

you need to parenthesis

var head = arr.pop();

answered Nov 12, 2014 at 17:24

epascarello

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Comments

Arslan shakoor · Accepted Answer · 2017-10-24 18:37:08Z

2

This is the most precise and clean way to do it in a single line with the ternary operator.

function reverse(arr) {
  return arr.length < 2 ? arr : [arr.pop()].concat(reverse(arr));
}
console.log(reverse([4, 3, 3, 1]));

answered Oct 24, 2017 at 18:37

Arslan shakoor

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Dave · Accepted Answer · 2019-05-21 15:23:27Z

2

Here's a runnable example with a working function. FYI there is also a built-in function that will do this for you.

I think other than the confusion with assignment/comparison operators, when you were building the result array, you should be using Array.concat() to concatenate your array, instead you were building a new array where the second item was an array itself.

var a = [1,2,3,4,5];
console.log(ReverseArray(a));

function ReverseArray(arr) {
    if(arr.length < 2) {
        return arr;
    } else {
        return [arr.pop()].concat(ReverseArray(arr));
    }
}

edited May 21, 2019 at 15:23

answered Nov 12, 2014 at 17:38

Dave

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