Question

Sometimes JavaScript doesn’t make sense to me, consider the following code that generates a photo mosaic based on x/y tiles. I’m trying to set a .Done property to true once each Mosaic image has been downloaded, but it’s always false for some reason, what am I doing wrong?

var tileData = [];

function generate()
{
    var image = new Image();
    image.onload = function()
    {
        // Build up the 'tileData' array with tile objects from this Image

        for (var i = 0; i < tileData.length; i++)
        {
            var tile = tileData[i];

            var tileImage = new Image();
            tileImage.onload = function()
            {
                // Do something with this tile Image
                tile.Done = true;
            };
            tileImage.src = tile.ImageUrl;
        }
    };
    image.src = 'Penguins.jpg';

    tryDisplayMosaic();
}

function tryDisplayMosaic()
{
    setTimeout(function()
    {
        for (var i = 0; i < tileData.length; i++)
        {
            var tile = tileData[i];

            if (!tile.Done)
            {
                tryDisplayMosaic();
                return;
            }
        }

        // If we get here then all the tiles have been downloaded
        alert('All images downloaded');
    }, 2000);
}

Now for some reason the .Done property on the tile object is always false, even though it is explicitly being set to true inside tileImage.onload = function(). And I can ensure you that this function DOES get called because I’ve placed an alert() call inside it. Right now it’s just stuck inside an infinite loop calling tryDisplayMosaic() constantly.

Also if I place a loop just before tryDisplayMosaic() is called, and in that loop I set .Done = true, then .Done property is true and alert('All images downloaded'); will get called.

Answer 1

The variable “tile” in the top loop is shared by every one of the “onload” functions you create; the very same single variable, in other words. Try this, to give each one its own variable:

tileImage.onload = (function(myTile) {
  return function()
        {
            // Do something with this tile Image
            myTile.Done = true;
        };
})(tile);

Why is this so? Because unlike C or Java, declaring “tile” inside the loop like that does not make it scoped to the statement block of the loop body. The only thing that gives you scope in Javascript is a function body.