vector.js

function Point(x, y) {
	return new Vector2D(x, y)
}

// Number -> Number -> Vector2D
function Vector2D(x, y) {
  this.x = x;
  this.y = y;
}

Vector2D.prototype = {
		// Vector2D -> Vector2D
		add : function(other) { return new Vector2D(this.x + other.x, this.y + other.y) },
		// Vector2D -> Vector2D
		subtract : function(other) { return new Vector2D(this.x - other.x, this.y - other.y) },
		// Unit -> Number
		getLength : function() { return Math.sqrt(this.x * this.x + this.y * this.y) },
		// Number -> Vector2D
		times : function(multiplier) { return new Vector2D(this.x * multiplier, this.y * multiplier) },
		// Unit -> Vector2D
		invert : function() { return new Vector2D(-this.x, -this.y) },
		// Number -> Vector2D
		withLength : function(newLength) { return this.times(newLength / this.getLength()) },
		rotateRad : function(radians) {
			var length = this.getLength()			
			var currentRadians = this.getAngle()
			var resultRadians = radians + currentRadians
			var rotatedUnit = new Vector2D(Math.cos(resultRadians), Math.sin(resultRadians))
			return rotatedUnit.withLength(length)			
		},
		// Number -> Vector2D
		rotateDeg : function(degrees) {
			var radians = degrees * 2 * Math.PI / 360
			return this.rotateRad(radians)
		},
		// Unit -> Number
		getAngle : function() {
			var length = this.getLength()
			unit = this.withLength(1)
			return Math.atan2(unit.y, unit.x)			
		}, 
		getAngleDeg : function() {
		  return this.getAngle() * 360 / (2 * Math.PI)
		}, 
		floor : function() {
			return new Vector2D(Math.floor(this.x), Math.floor(this.y))
		},
		toString : function() {
			return "(" + x + ", " + y + ")"
		}
}