can someone explain this Image loader function involving callback

Question

The Code:

function loadImages(sources, callback){
    var images = {};
    var loadedImages = 0;
    var numImages = 0;
    for (var src in sources) {
        numImages++;
    }
    for (var src in sources) {
        images[src] = new Image();
        images[src].onload = function(){
            if (++loadedImages >= numImages) {
                callback(images);
            }
        };
        images[src].src = sources[src];
    }
}
window.onload = function(images){
    var canvas = document.getElementById("myCanvas");
    var context = canvas.getContext("2d");
    var sources = {
        darthVader: "darth-vader.jpg",
        yoda: "yoda.jpg"
    };
    loadImages(sources, function(images){
        context.drawImage(images.darthVader, 100, 30, 200, 137);
        context.drawImage(images.yoda, 350, 55, 93, 104);
    });
};

---

The Confusion

function loadImages(sources, callback){

3: two parameters are passed to this function, one being a function in of itself : callback

    var images = {};

4: finally, images is set to ... nul(?)

    var loadedImages = 0;
    var numImages = 0;
    // get num of sources
    for (var src in sources) {
        numImages++;
    }
    for (var src in sources) {
        images[src] = new Image();
        images[src].onload = function(){
            if (++loadedImages >= numImages) {
                callback(images);

5: my brain is confused at this point....

            }
        };
        images[src].src = sources[src];
    }
}
 
window.onload = function(images){

As I understand,

1: the parameter "images" is empty.

    var canvas = document.getElementById("myCanvas");
    var context = canvas.getContext("2d");
 
    var sources = {
        darthVader: "darth-vader.jpg",
        yoda: "yoda.jpg"
    };
 
    loadImages(sources, function(images){

2: now its being passed as a parameter to this inline functions -- still without pointing to anywhere... it now supposedly calls loadImages method which is defined above...

        context.drawImage(images.darthVader, 100, 30, 200, 137);

where doe it get context to darthvader? i only see "sources" have darthVader above

        context.drawImage(images.yoda, 350, 55, 93, 104);
    });
};

source: http://www.html5canvastutorials.com/tutorials/html5-canvas-image-loader/

EDIT: QUESTIONS::

from step 4: to 5: (specifically in the second for loop), a new array (images[src]) is being created and is passed to the callback() function which is defined as inline just before step 2:. Where does it actually gets images from which were in source?

Answer 1

The Loader

function loadImages(sources, callback){
    //images is set to an object literal
    //this is the same as writing "var images = new Object();
    var images = {};

    //the counter for the number of images loaded
    var loadedImages = 0;

    //the total number of images
    var numImages = 0;

    //count the total number of images to load
    for (var src in sources) {
        numImages++;
    }

    //iterate through every image in sources
    //"src" will be the key used in the object passed to the function (i.e. "yoda")
    for (var src in sources) {
        //set image[*keyname*] to a new Image object
        //i.e. images.yoda = new Image(), images.darthVader = new Image();
        images[src] = new Image();

        //attach an onload event listener to the image
        images[src].onload = function(){
            //add one to the number of images loaded
            //if the number of images loaded is equal to the total number of images, call the callback
            if (++loadedImages >= numImages) {
                //pass the object containing the images to load as a parameter of the callback function
                callback(images);
            }
        };
        //set the source of the created image to the src provided in the sources object
        //i.e. images.yoda.src = sources.yoda
        images[src].src = sources[src];
    }
}

The Usage

window.onload = function(images){
    //get the canvas
    var canvas = document.getElementById("myCanvas");

    //get the drawing context of the canvas
    var context = canvas.getContext("2d");

    //initialize a new object with two sources
    //accessible as sources.darthVader and sources.yoda
    var sources = {
        darthVader: "darth-vader.jpg",
        yoda: "yoda.jpg"
    };

    //load the images in sources using the provided callback function
    loadImages(sources, function(images){
        //draw the images that were loaded on the canvas
        context.drawImage(images.darthVader, 100, 30, 200, 137);
        context.drawImage(images.yoda, 350, 55, 93, 104);
    });
};

Answer 2

To your first question:

var images = {};

The braces mean in javascript it creates a new object, so it is not NUL but an empty object. An object in Javascript is similar to an array, but it stores key-value-pairs instead of indexed values.

---

To understand what is happening with the callback:

callback is a function pointer which gets passed to the function "loadImages" by the caller.

---

source is a parameter to the function which is passed by the caller as well.

In the following line, the image URL is read from this array:

    images[src].src = sources[src];

---

a new array (images[src]) is being created

No, but a new item is created inside the array (an array element).

---

This loader function has the disadvantage that it does only tell you in the callback that a image has completed -- but it doesn't tell you which one!

Answer 3

I've added comments to your JavaScript inline:

function loadImages(sources, callback){
    //  "{}" is object literal syntax.  images is a new empty object, not null.
    var images = {};
    var loadedImages = 0;
    var numImages = 0;

    for (var src in sources) {
        numImages++;
    }
    for (var src in sources) {
        //  In a for..in loop, the variable (src) is the key of the object (sources).
        //  The value is retrieved via object[key] (eg, sources[src]).

        //  In the first iteration of this loop, the images object is given a
        //  property of "darthVader", and "yoda" in the second iteration.
        //  Note: images[src] does not create a new array, but adds a property
        //  named the value of src to images using square bracket notation.
        images[src] = new Image();
        images[src].onload = function() {
            if (++loadedImages >= numImages) {
                callback(images);  // "callback" is passed images as a parameter
            }
        };
        images[src].src = sources[src];
    }
}

//  The "images" parameter referenced here is never used.  It's pretty pointless.
//  It would be an Event object, so calling it "images" is a misnomer.
window.onload = function(images){ 
    var canvas = document.getElementById("myCanvas"); 
    var context = canvas.getContext("2d"); 

    var sources = { 
        darthVader: "darth-vader.jpg", 
        yoda: "yoda.jpg" 
    }; 

    //  Don't confuse "images" here with the "images" parameter passed to 
    //  onload.  This is the callback's own private parameter.  It get's its
    //  value from the caller.
    loadImages(sources, function(images){ 
        context.drawImage(images.darthVader, 100, 30, 200, 137); 
        context.drawImage(images.yoda, 350, 55, 93, 104); 
    }); 
};

Edit: A note on square bracket notation. Given this setup:

var obj = {};
var propertyName = "foo";

The following lines are equivelant:

obj.foo = 1;
obj["foo"] = 1;
obj[propertyName] = 1;

Each of the above lines will add a property named "foo" to obj.