Javascript Array gets filled with NaN unexpectedly

Question

I am currently making a small scrolling canvas game, and I am trying to implement monsters. I keep two arrays for the monsters, one for their x positions and one for their y positions. For some unexpected reason, both of the arrays get filled up to 1152 elements, two of them are predefined, and the others are all "NaN".

Here's a link to the program: http://codepen.io/kenshin791011/pen/WpxQmg

(The function monsterMove is run on key down)

var monsterx = [13, 5],
    monstery = [41, 42];
function monsterMove() {
    for(var i = 0; i < monsterx.length; i++) {
        var z = Math.floor(Math.random() * 4);
        switch(z) {
            case 0:
                monsterx[i] -= 1;
                break;
            case 1:
                monstery[i] -= 1;
                break;
            case 2:
                monsterx[i] += 1;
                break;
            case 3:
                monstery[i] += 1;
                break;
        }
    }
}

Not what you're asking, but did you ever think of storing the X and Y coordinates together: monsters = [{x:13,y:41}, {x:5,y:42}]? Just seems neater to me. — nnnnnn
– nnnnnn, Commented Mar 5, 2017 at 0:30

nnnnnn · Accepted Answer · 2017-03-05 00:33:46Z

1

The following loop from the JS in your codepen is updating the monsterx array for elements 0 through to cGrid.length:

for(var i = 0; i < cGrid.length; i++) {
    pGridx[i] += g;
    monsterx[i] += 1;
}

Given that the first time monsterx[i] is undefined for almost all of those indices, and undefined + 1 is NaN, that's how you get an array full of NaN - except for the first two elements that started with valid numbers.

You have a similar loop that updates monstersy.

(For future reference, if you're not sure how a variable is being updated, the first step would be to search through the code for that variable. That's all I did to find the problem above.)

answered Mar 5, 2017 at 0:33

nnnnnn

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1 Comment

John. Doe Over a year ago

Thanks for your help! I'll make sure that I'll follow your tip whenever something like this happens again.

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