javascript the onload event for images is not being triggered

I can't seem to get my pictures to load properly in my javascript code. The .onload function doesn't appear to be triggering, so the image remains marked as not loaded. Any suggestions on how to fix this issue?

Below is the code snippet causing trouble:

//image loader
var img = function (source){
    this.loaded = false;
    this.image = new Image();
    this.image.onload = function () {
        this.loaded = true;
    };
    this.image.src = source;            
}

//default component definition
var component = function (x, y){
    this.x = x;
    this.y = y;
    this.draw = function (offsetX, offsetY, img){
        if(img.loaded){         
            ctx.drawImage(img.image, 
            this.x + offsetX, 
            this.y + offsetY,
            img.image.width, img.image.height);         
        }       
    }
}

Any help would be greatly appreciated!

Answer №1

Scenario #1

this.element.onload = function () {
    this.loaded = true;
};

In this code snippet, this is referring to [object HTMLElement], not the specific element.

Scenario #2

If we modify the code like so:

this.element.onload = function () {
    this.loaded = true;
}.bind(this);

When you instantiate a new

Element("https://example.com/image.jpg");
, this will now point to your [object Object].

Scenario #3

If you simply call

element("https://example.com/image.jpg");
, without using new, this will be referencing [object Window].

Solution

The behavior of this varies based on the context it's used in.

  • You can utilize bind to alter its binding.
  • Alternatively, declare an external variable such as var _this = this; and then access _this.loaded = true; within the onload event.

Code example: https://jsbin.com/pasumab/edit?js,console

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