new to javascript, working with arrays

Question

Wondering why i needed to add 4 to the array length in order for it to print out the entire array in reverse?

before i added 4 it was just using the .length property and it was only printing out 6543.

thanks in advance!

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

     for(var i = 0; i <= array.length+4; i++) {
         newArray += array.pop(i);
      }
     return newArray;
 }
var numbers = [1,2,3,4,5,6];
console.log(reverseArray(numbers));

Note that `+=` is **not** how you add an entry to an array; the first time you do it, your array is replaced with a string. — T.J. Crowder, Mar 30 '15 at 17:28
ok so i changed += to push and its an array again thank you! @T.J.Crowder — jordan, Mar 30 '15 at 17:32

GolezTrol · Answer 1 · 2015-03-30T17:42:01.813

array.pop removes (and returns) the last element. This affects the length of the array. The length is checked on every iteration, so since the array is getting shorter every time, the loop is ended early.

You can create a loop and pop items until it is empty, but another thing to take into account, is that it is the original array you are altering. I think a function like reverseArray shouldn't alter the array numbers that was passed to it if it returns another one. So a better solution would be a simple loop that iterates over all items without modifying the array.

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

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

var numbers = [1,2,3,4,5,6];
console.log(reverseArray(numbers));
console.log(numbers); // Should be unaltered.

If you don't mind modifying the array, you can use the reverse() method of the array:

var numbers = [1,2,3,4,5,6];
numbers.reverse();
console.log(numbers);

score 0 · Accepted Answer · answered Mar 30 '15 at 17:31

In Javascript, pop always removes the last element of the array. This shortens length, meaning that i and array.length were converging.

You can do a few things to avoid this behavior:

Store the original length when you start the loop: for (var i = 0 , l = array.length; i < l; i++)
Copy over values without modifying the original array

score 0 · Answer 3 · answered Mar 30 '15 at 17:38

When you pop the items from the array, the item is removed from the array. As you increase the counter and decrease the length, they will meet halfway, so you get only half of the items.

Use push to put the items in the result. If you use += it will produce a string instead of an array.

If you use pop, then you can just loop while there are any items left in the array:

function reverseArray(array) {
    var newArray = [];
    while (array.length > 0) {
        newArray.push(array.pop());
    }
    return newArray;
}

You can leave the original array unchanged by looping through it backwards and add items to the new array:

function reverseArray(array) {
    var newArray = [];
    for (var i = array.length - 1; i >= 0; i--) {
        newArray.push(array[i]);
    }
    return newArray;
}

Amol Kanherkar · Answer 4 · 2017-08-17T06:21:43.593

-1

use following method for same output

function reverseArray(array) {

    var newArray =[];
    var j = array.length-1;

    for(var i = 0; i < array.length; i++)

    {

        newArray[j]= array[i]; j--;

    }

    return newArray;

}

var numbers = [1,2,3,4,5,6];

console.log(reverseArray(numbers));

edited Aug 17 '17 at 06:21

answered Mar 30 '15 at 17:36

Amol Kanherkar

79
6

Your loop does reverse the array, but adds `undefined` to the beginning because you're not using `array.length - 1`. – Pluto Mar 30 '15 at 17:42
array.length - 1 is not needed if we replace <= with – Amol Kanherkar Aug 17 '17 at 06:22

new to javascript, working with arrays

4 Answers4