Remove an array of indices from a JavaScript array

Question

I have an array:

var arr = ['A', 'B', 'C', 'D', 'E', 'F', 'G']

and I have an array of indices which I wish to remove:

var remove = [1, 3, 5]

so that the result is :

arr ==== ['A', 'C', 'E', 'G']

I can't do it with splice in a loop:

// WRONG
for (i = 0, l = remove.length; i < l; i++) {
 arr.splice(remove[i]);
}

because after every iteration the index of each element has changed.

So how can I do this?

• David Thomas: Start at the end of the array, and work backwards?
• Marc B: Sort your remove array so the keys are in descending order, so you work from the BACK of the arr array. (That or sort in ascending order, then loop in reverse order).

Answer 1

> arr.filter(function(x,i){return remove.indexOf(i)==-1})
["A", "C", "E", "G"]

To be more efficient, convert remove into an object/hashtable first, like so:

var removeTable = {}
remove.forEach(function(x){removeTable[x]=true})
> arr.filter(function(x,i){return removeTable[i]})
["A", "C", "E", "G"]

Answer 2

To not change your thinking too much- Start at the end.

A B C D E F..

When you remove element 5, it becomes..

A B C D E

Then you remove element 3, it becomes..

A B C E

Which is exactly what you want.

Answer 3

Count backwards:

// RIGHT
for (i = (remove.length-1); i >= 0; i--) {
 arr.splice(remove[i]);
}

• ricochet1k: This assumes the remove array has no duplicates, and is sorted. If the remove array was sorted in reverse it would still have the same problem as before. Also, the init part of the for loop should be i = remove.length - 1
• Marc: Updated for the length-1. And yes, I assumed a sort had taken place based on user's post, but it's a fair point.

Answer 4

start the loop from last and remove the elements from highest index first.

Answer 5

As an alternative suggestion, you could use .push() to send the items you want to keep to a third array. See here for the basics. This would allow you to keep the original array intact, although it seems you don't want/need to do this.