I'm really confused with an array im making, its gotten a bit confusing to follow but i believe its the only way for me to approach what im trying to do.

My idea here is to store getImageData in an array so i can link each image to an array of data.

This is how i create my array:

var listObj = {
    id: uid,
    extra: [
            ctx2.getImageData(abposx,abposy,imgwidth,imgheight)
        ]

    };
pixeld.push(listObj);

So now im trying to run through the array "pixeld" to find the uid but i don't know how the syntax should be.... this is what i tried:

for (j = (pixeld.length-1); j > -1; --j){
        if(pixeld[j].extra.data[3] !==0){ //line 220
            matches.push(pixeld[j].id);
        }

    }

The problem is i get this error:

Uncaught TypeError: Cannot read property '3' of undefined on line 220

I have marked which line is 220 in the for loop.

Any one know what my mistake is ?