javascript Array.splice() does nothing

Question

I have the following code in my update function, which is called 30 times per second:

        // Checking collision between rune and the players:
        for(var j = 0; j < this.players.length; j++) {
          if(this.checkCollision(this.players[j], this.runes[i])) {
            this.runes[i].activate(this.players[j]);
            this.runes[i].isHidden = true;
            this.runes[i].onDeactivate = function() {
              console.log(i);
              self.runes.splice(i, 1);
            }
          }
        }

before that I have:

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

the self.runes.splice(i, 1) does nothing to an array... i is being set to some value. I just want to remove the not active rune from array of runes. Any Ideas?

by the way, onDeactivate is being called, i've tested that... — Juraj Carnogursky
– Juraj Carnogursky, Commented Jul 9, 2016 at 10:03
Out of sheer curiosity (I know exactly what's going on) but what does console.log(i) show you? — Niet the Dark Absol
– Niet the Dark Absol, Commented Jul 9, 2016 at 10:05
it depends on which rune I take with player... for example now it's showing 507... I wonder if self isn't a problem — Juraj Carnogursky
– Juraj Carnogursky, Commented Jul 9, 2016 at 10:06
You're calling self.runes.splice(i, 1) in an asynchronous function. By the time it runs, i will be outside the length of the array. — Barmar
– Barmar, Commented Jul 9, 2016 at 10:11

Niet the Dark Absol · Accepted Answer · 2016-07-09 10:10:59Z

You have two problems.

Firstly, at the time of calling onDeactivate, i will have reached this.runes.length and this is what you will be seeing in the console.log(i) call. The classic fix to resolve that issue is something like:

(function(i) {
    // code that relies on i
})(i);

This will essentially "lock" the value of i for the contents of that closure.

The second problem is that splice modifies the array, and you're not accommodating for that. Let's say you have three runes:

[rune_0, rune_1, rune_2]

Now say rune_1 gets deactivated, so the code splice(1,1) is called to remove it. Now your array looks like:

[rune_0, rune_2]

Now rune_2 is deactivated, so splice(2,1) is called. That removes the [2] element from the array... but there isn't one any more.

[rune_0, rune_2]

The rune is still there.

To remove a rune from the array, you can do something like:

this.runes = this.runes.filter(function(rune) {return rune !== toremove;});

Where toremove is the rune you want gone.

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