How to initialize properties in Javascript constructor - Stack Overflow

I am playing with html5 canvas to create bouncing balls. I have this all working but I have to call an initialise function to set certain properties. How can I do this automatically in the constructor without having the initializer firing when the properties are accessed?

var test1 = new Ball(20);
test1.setAngleAndVelocity(); //I dont want to have to call this.

function Ball(speed){
    this.position = new Vector2(canvas.width / 2, canvas.height / 2);
    this.velocity;
    this.speed = speed;
    this.angle;
    this.setAngleAndVelocity = function(){
        this.angle = Math.floor(Math.random() * 360) * 0.0174532925;
        this.velocity = new Vector2(this.speed/10 * Math.cos(this.angle), this.speed/10 * Math.sin(this.angle));
    }
}

I am playing with html5 canvas to create bouncing balls. I have this all working but I have to call an initialise function to set certain properties. How can I do this automatically in the constructor without having the initializer firing when the properties are accessed?

var test1 = new Ball(20);
test1.setAngleAndVelocity(); //I dont want to have to call this.

function Ball(speed){
    this.position = new Vector2(canvas.width / 2, canvas.height / 2);
    this.velocity;
    this.speed = speed;
    this.angle;
    this.setAngleAndVelocity = function(){
        this.angle = Math.floor(Math.random() * 360) * 0.0174532925;
        this.velocity = new Vector2(this.speed/10 * Math.cos(this.angle), this.speed/10 * Math.sin(this.angle));
    }
}

• javascript
• constructor

Share Improve this question Follow asked Sep 24, 2011 at 8:17 skyfootskyfoot 20.8k1010 gold badges5151 silver badges7171 bronze badges

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2 Answers 2

Sorted by: Reset to default 4

Since setAngleAndVelocity() is a static method, I'd remend putting it in the prototype of your Ball class:

function Ball(speed){
    this.position = new Vector2(canvas.width / 2, canvas.height / 2);
    this.speed = speed;
    this.setAngleAndVelocity(); //Sets the additional values
}
Ball.prototype.setAngleAndVelocity = function(speed){
    speed = typeof speed != "undefined" ? speed : this.speed;
    this.angle = Math.floor(Math.random() * 360) * 0.0174532925;
    this.velocity = new Vector2(speed/10 * Math.cos(this.angle), speed/10 * Math.sin(this.angle));
}

this.velocity; and this.angle; aren't necessary: They're not defining anything, and the only use they serve is to show the developer what properties may be defined.

After these modifications, your script has bee more efficient, and can be used in this way:

var test1 = new Ball(20); //Inititalized
test1.setAngleAndVelocity(22); //Optional, a method to adjust the speed value after the init of the class.

Just inline that putation into the constructor.

function Ball(speed){
    this.position = new Vector2(canvas.width / 2, canvas.height / 2);
    this.speed = speed;
    this.angle = Math.floor(Math.random() * 360) * 0.0174532925;
    this.velocity = new Vector2(this.speed/10 * Math.cos(this.angle), this.speed/10 * Math.sin(this.angle));
}

ADDENDUM

If you want to keep the function around to update the angle and velocity at other times in your application, place that function in the prototype:

Ball.prototype.changeSpeed = function (newSpeed) {
    this.speed = newSpeed;
    this.velocity = new Vector2(this.speed/10 * Math.cos(this.angle), this.speed/10 * Math.sin(this.angle));
}

You can call this method from the constructor if you wish:

function Ball(speed){
    this.position = new Vector2(canvas.width / 2, canvas.height / 2);
    this.angle = Math.floor(Math.random() * 360) * 0.0174532925;
    this.changeSpeed(speed);
}

See http://jsfiddle/FnHLX/ for a working example.

You can also write a similar function for angle changes.