Vector Manipulation in JS made easy

Vector.js

function Vector (x,y) {
    this.x = x;
    this.y = y;
}

Vector.prototype.length = function() {
    return Math.sqrt(this.x*this.x + this.y*this.y);
};

Vector.prototype.add = function(vector) {
    this.x += vector.x;
    this.y += vector.y;
    return this;
};

Vector.prototype.factor = function(factor) {
    this.x *= factor;
    this.y *= factor;
    return this;
};

Vector.prototype.unit = function() {
    var length = this.length();
    return new Vector(this.x/length, this.y/length);
};

Vector.prototype.clone = function() {
    return new Vector(this.x, this.y);
};

Vector.prototype.angle = function() {
    var unit = this.unit();
    if (unit.y < 0) {
        return - Math.acos(unit.x);
    } else {
        return Math.acos(unit.x);
    }

};

Vector.prototype.to = function(vector) {
    return new Vector(vector.x-this.x, vector.y-this.y)
};

Vector.prototype.set = function(x,y) {
    this.x = x;
    this.y = y;
    return this;
};

Vector.direction = function(x1, y1, x2, y2) {
    return new Vector(x2-x1, y2-y1);
}

Vector.fromAngle = function(alpha) {
    return new Vector(Math.cos(alpha), Math.sin(alpha));
}

Vector.prototype.addAngle = function(alpha) {
    var length = this.length();
    alpha += this.angle();
    this.x = Math.cos(alpha)*length;
    this.y = Math.sin(alpha)*length;
    return this
};