Remove items from Array while iterating over it.

Ever encounter unexpected results when items are removed from an array while the array is being iterated over? This might look familiar:

var numbers = [17, 2, 14, 15, 20, 8, 7, 1, 9, 19, 3, 18, 5, 4, 6, 16],
    i;

for (i=0; i < numbers.length; ++i) {
  if (numbers[i] % 2 === 0) {
    numbers.splice(i, 1); // Remove even numbers
  }
}

console.log(numbers); // [17, 14, 15, 8, 7, 1, 9, 19, 3, 5, 6]
                      // WTF: ^       ^                     ^

What's going on here? You'll notice that the even numbers that didn't get removed were each preceded by another even number.

After splice is called, the item at index i is now what was the next item in the list, then the for loop triggers ++i, which skips to the next next item, leaving
an item unchecked. Watch carefully:

[17, 2, 14, 15, 20, 8, 7, 1, 9, 19, 3, 18, 5, 4, 6, 16] // i = 0
 ^
[17, 2, 14, 15, 20, 8, 7, 1, 9, 19, 3, 18, 5, 4, 6, 16] // i = 1
     X
[17, 14, 15, 20, 8, 7, 1, 9, 19, 3, 18, 5, 4, 6, 16]    // i = 2; skipped `14`
         ^
[17, 14, 15, 20, 8, 7, 1, 9, 19, 3, 18, 5, 4, 6, 16]    // i = 3
             X
[17, 14, 15, 8, 7, 1, 9, 19, 3, 18, 5, 4, 6, 16]        // i = 4; skipped `8`
                ^
[17, 14, 15, 8, 7, 1, 9, 19, 3, 18, 5, 4, 6, 16]        // i = 5
                   ^
[17, 14, 15, 8, 7, 1, 9, 19, 3, 18, 5, 4, 6, 16]        // i = 6
                      ^
[17, 14, 15, 8, 7, 1, 9, 19, 3, 18, 5, 4, 6, 16]        // i = 7
                         ^
[17, 14, 15, 8, 7, 1, 9, 19, 3, 18, 5, 4, 6, 16]        // i = 8
                             ^
[17, 14, 15, 8, 7, 1, 9, 19, 3, 18, 5, 4, 6, 16]        // i = 9
                                X
[17, 14, 15, 8, 7, 1, 9, 19, 3, 5, 4, 6, 16]            // i = 10; skipped `5`
                                   X
[17, 14, 15, 8, 7, 1, 9, 19, 3, 5, 6, 16]               // i = 11; skipped `6`
                                      X
[17, 14, 15, 8, 7, 1, 9, 19, 3, 5, 6]                   // End result.

The most "obvious" solution here is to simply decrement i whenever an item is removed:

for (i=0; i < numbers.length; ++i) {
  if (numbers[i] % 2 === 0) {
    numbers.splice(i, 1); // Remove even numbers
    --i; // Correct the index value
  }
}

But we can do better. Simply by walking through the array in reverse, we avoid the issue entirely.

for (i = numbers.length - 1; i >= 0; --i) {
  if (numbers[i] % 2 === 0) {
    numbers.splice(i, 1); // Remove even numbers
  }
}

Now, whenever an item is removed, i automatically "falls" to the correct item. Watch:

[17, 2, 14, 15, 20, 8, 7, 1, 9, 19, 3, 18, 5, 4, 6, 16] // i = 15
                                                    X
[17, 2, 14, 15, 20, 8, 7, 1, 9, 19, 3, 18, 5, 4, 6]     // i = 14
                                                 X
[17, 2, 14, 15, 20, 8, 7, 1, 9, 19, 3, 18, 5, 4]        // i = 13
                                              X
[17, 2, 14, 15, 20, 8, 7, 1, 9, 19, 3, 18, 5]           // i = 12
                                           ^
[17, 2, 14, 15, 20, 8, 7, 1, 9, 19, 3, 18, 5]           // i = 11
                                       X
[17, 2, 14, 15, 20, 8, 7, 1, 9, 19, 3, 5]               // i = 10
                                    ^
[17, 2, 14, 15, 20, 8, 7, 1, 9, 19, 3, 5]               // i = 9
                                ^
[17, 2, 14, 15, 20, 8, 7, 1, 9, 19, 3, 5]               // i = 8
                             ^
[17, 2, 14, 15, 20, 8, 7, 1, 9, 19, 3, 5]               // i = 7
                          ^
[17, 2, 14, 15, 20, 8, 7, 1, 9, 19, 3, 5]               // i = 6
                       ^
[17, 2, 14, 15, 20, 8, 7, 1, 9, 19, 3, 5]               // i = 5
                    X
[17, 2, 14, 15, 20, 7, 1, 9, 19, 3, 5]                  // i = 4
                X
[17, 2, 14, 15, 7, 1, 9, 19, 3, 5]                      // i = 3
            ^
[17, 2, 14, 15, 7, 1, 9, 19, 3, 5]                      // i = 2
        X
[17, 2, 15, 7, 1, 9, 19, 3, 5]                          // i = 1
     X
[17, 15, 7, 1, 9, 19, 3, 5]                             // i = 0; done!
 ^