Question

So I am new to object definition in Javascript, and am trying to write a program that revolves objects as a practice. My problem is that when I am trying to define the object, some of the object properties are dependent on other parts of the object. I am not sure if this is even permitted, because in all my searching I have not been able to find any examples of it.

My question is basically this: can I use previously defined properties of an object to define that object. The most basic example of this would be something like this:

var alfred = {
    dogs: 1,
    cats:this.dogs+1,
}

Is this permitted? if so is this the right syntax? The reason I need to use a “this.” is because I am pushing newly created objects to a array of objects.The code of mine that is not working is below:

obj.push({
    canvas:document.getElementById(canvasName),

    canvasName:"canvas"+objNum,
    image: img,
    width:objWidth,
    height:objHeight,
    centerX:posX,
    centerY:posY,
    speed:speeds,
    hypSquare:Math.sqrt((this.width*this.width)+(this.height*this.height)),
    angleInSquare:Math.atan(this.height/this.width),
    angle:startAngle,
    angleTotal:this.angle+this.angleInSquare,
    offX:(this.hypSquare* Math.cos(this.anglesTotal))/2,
    offY:(this.hypSquare* Math.sin(this.anglesTotal))/2,
    centeredX:this.centerX-this.offX,
    centeredY:this.centerY-this.offY,
})

when I call a

console.log(obj[objNum].hypSquare);

(where objNum is just the index of the object in the array) I will get NaN even though if I call

 console.log(obj[objNum].width);

I will get the value of objWidth. Is there just a syntactical issue, or is my understanding of objects fundamentally flawed…

Thank you in advance for your time!

Isaac

Answer 1

No, you can’t do that. You have to close the object initializer and then add the other property, e.g.:

var alfred = {
    dogs: 1
};
alfred.cats = alfred.dogs + 1;

So for your obj.push call, you’ll have to use a temporary variable (like alfred above), you can’t just use an inline object initializer.