Trying to understand this function: Array.prototype.reverse = function() {

Question

When this method is called, it reverses the order of the items in the original array. Then then it returns that same, original array. No new arrays should need to be created to pass this kata. However, I'm trying to figure out how this.push(arr.pop()); works in this function.

Array.prototype.reverse = function() {
  var arr = this.splice(0);  //I understand first we remove all the items in the array starting from [0]

  while(arr.length) {    //then we run the arr.length of all the items? 
    this.push(arr.pop());  //then add the deleted last item? doesn't make sense...
  }   

  return this;
};

test cases:

Test.assertSimilar([1, 2, 3, 4].reverse(), [4,3,2,1]);
Test.assertSimilar(["a", "b", "c"].reverse(), ["c", "b", "a"]);
Test.assertSimilar([].reverse(), []);

or please write a function you think is a better solution

T.J. Crowder · Accepted Answer · 2015-09-11 18:10:21Z

I've added comments:

Array.prototype.reverse = function() {
  var arr = this.splice(0);  // Removes all entries from `this` array AND returns
                             // them in a new array

  while(arr.length) {        // For as long as that new array still has items
                             // (length is "truthy" until it's 0)
    this.push(arr.pop());    // `pop` removes the LAST entry from `arr`, and
                             // `push` adds it to `this` as the next entry
  }   

  return this;
};

So say we have [1, 2, 3, 4, 5]:

First those are removed from this and put in arr.
Then, because arr.length is 5, we enter the body of the loop.
arr.pop() removes 5 from arr.
this.push() adds 5 to the next available location in this, which is at the beginning
arr.length is now 4, so we go into the body again
arr.pop() removes 4 from arr.
this.push() adds 4 to the next available location in this, which is just after 5
Rinse, repeat
When arr.length is 0, it's not truthy anymore, and we exit the loop

3 revs, 2 users 94% · Accepted Answer · 2015-09-12 18:25:07Z

1

"or please write a function you think is a better solution"

Here's a more efficient and simpler solution:

Array.prototype.reverse = function() {
  for (var i = 0, j = this.length - 1; i < j; i++, j--) {
    var tmp = this[i];
    this[i] = this[j];
    this[j] = tmp;
  }
  return this;
};

In browsers that support ECMAScript 6, you can shorten it to this:

Array.prototype.reverse = function() {
  for (var i = 0, j = this.length - 1; i < j; i++, j--) {
    [this[i], this[j]] = [this[j], this[i]];
  }
  return this;
};

Not sure if there's any extra overhead.

edited Sep 12, 2015 at 18:25

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3 revs, 2 users 94%
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