Question

Consider the following code:

function func() {
    var totalWidths = 0;
    for( var i = 0, count = arr.length; i < count; i++ ) {
        var image = arr[i];

        insertElemInDOM(image);

        preloadImage(image,function(){
            var w = image.width();
            totalWidths += w;
        });

    }
    // do something with the variable "totalWidths"
    doSomething(totalWidths)
}

I have 2 problems here. The image will be always the same (first problem), which one can solve with an anonymous function:

    for(...) {
        (function(image) {
            preload(image,function() {
                // now image is the correct one
            });
        })(image);
    }

But how do I manage the totalWidths variable in order to use it later on doSomething(totalWidths)? The previous code would have a value of 0 for totalWidths.
Thanks!

Answer 1

You could timeout the whole loop and the doSomething, that’s much more performant than setting up so many timeouts:

setTimeout(function() {
    var inc = 0;
    for (var i = 0; i < count; i++) {
        var w = arr[i].width();
        inc++;
    }
    doSomething(inc);
}, 1000);

---

However, what you actually seem to want are nested timeouts, i.e. waiting 1s for each iteration step and doing something after all have finished:

var inc = 0, count;
function asyncLoop(i, callback) {
    if (i < count) {
        var w = arr[i].width();
        inc++;
        setTimeout(function() {
            asyncLoop(i+1, callback);
        }, 1000);
    } else {
        callback();
    }
}
asyncLoop(0, function() {
    doSomething(inc);
});

---

OK, now that we know what you need the solution is to check after each load event whether all images are loaded:

var totalWidths = 0,
    count = arr.length,
    loaded = 0;
for (var i = 0; i < count; i++) 
    (function (image) {
        insertElemInDOM(image);
        preload(image, function() {
            totalWidths += image.width();
            // counter:
            loaded++;
            if (loaded == count-1) // the expected value
                doSomething(totalWidths); // call back
        });
    })(arr[i]);