A very simple 2d vector implementation.

vector_2d.js

function Vector2D (x, y) {
	this.x = x || 0;
	this.y = y || 0;
}

Vector2D.prototype = {
	constructor: Vector2D,

	set: function set (arg1, y) {
		if( arg1 instanceof Vector2D ) {
			this.x = arg1.x;
			this.y = arg1.y;
		} else {
			if( typeof arg1 !== 'number' || typeof y !== 'number' ) {
				throw 'Must supply eithr a Vector2D or two numbers';
			}

			this.x = arg1 || 0;
			this.y = y || 0;
		}
	},

	add: function add (v2) {
		this.x += v2.x;
		this.y += v2.y;
	},

	sub: function sub (v2) {
		this.x -= v2.x;
		this.y -= v2.y;
	},

	mult: function mult (scale) {
		this.x *= scale;
		this.y *= scale;
	},

	div: function div (factor) {
		if( factor === 0 ) throw 'Division by zero causes problems. Don\'t do it.';
		this.x /= factor;
		this.y /= factor;
	}
}

Vector2D.sum = function sum (v1, v2) {
	return new Vector2D(
		v1.x + v2.x,
		v1.y + v2.y
	);
}