Game Object Types - Building JavaScript Games for Phones, Tablets, and Desktop - page 115

else if (v.constructor === Number) {
this.x = this.x + v;
this.y = this.y + v;
}
return this;
};
The addTo method adds a vector to an existing vector. You can also define an add method that adds
two vectors and returns a new vector . To do that, you can reuse the copy and addTo methods:
Vector2.prototype.add = function (v) {
var result = this.copy();
return result.addTo(v);
};
You can now do the following:
var position = new Vector2(10, 10); // create a vector (10, 10)
var anotherPosition = new Vector2(20, 20); // create a vector (20, 20)
var sum = position.add(anotherPosition); // creates a new vector (30, 30)
In this example, position and anotherPosition are unchanged in the third instruction. A new vector
object is created that contains the sum of the values in the operand vectors.
Take a look at the Vector2.js file in the Painter6 example, where you can see the full definition of
the Vector2 class. It defines the most common vector operations in this class, including the addition
methods discussed in this section. As a result, using vectors in the Painter game is much easier.
You use the Vector2 type in all the game objects to represent positions and velocities. For example,
this is the new constructor of the Ball class:
function Ball() {
this.position = new Vector2();
this.velocity = new Vector2();
this.origin = new Vector2();
this.currentColor = sprites.ball_red;
this.shooting = false;
}
Thanks to the methods in the Vector2 class, you can intuitively update the ball position according to
its velocity, in a single line of code:
this.position.addTo(this.velocity.multiply(delta));
Default Values for Parameters
Before completing this chapter, let's have one more look at how the Vector2 constructor is defined:
function Vector2(x, y) {
this.x = typeof x !== 'undefined' ? x : 0;
this.y = typeof y !== 'undefined' ? y : 0;
}