A 2-dimensional vector in javascript.

Vector2.js

(function (root, factory) {
    if (typeof define === 'function' && define.amd) {
        define([], factory);
    } else if (typeof module === 'object' && module.exports) {
        module.exports = factory();
    } else {
        root.Vector2 = factory();
  }
}(this, function () {
    function Vector2(x,y){
        this.x = x || 0;
        this.y = y || 0;
    }

    function isVector(v){
        return v instanceof Vector2;
    }

    function isVectorLike(v){
        return v && v.x !== undefined && v.y !== undefined;
    }

    function args2Vector(args){
        var arg0 = args[0];
        if ( isVector(arg0) ) {
            return arg0;
        } else if ( arg0 && isVectorLike(arg0) ){
            return new Vector2(arg0.x,arg0.y);
        } else {
            return new Vector2(arg0,args[1]);
        }
    }

    function args2VectorLike(args){
        var arg0 = args[0];
        if ( isVectorLike(arg0) ) {
            return arg0;
        } else {
            // return new Vector2(arg0,args[1]);
            return {
                x: +arg0 || 0,
                y: +arg[1] || 0
            }
        }
    }

    Vector2.prototype = {
        constructor: Vector2,
        init: function(){
            return args2Vector(arguments);
        },
        square: function() {
            return this.x * this.x + this.y * this.y;
        },
        length: function() {
            return Math.sqrt(this.square());
        },
        add: function() {
            var v = args2VectorLike(arguments);
            this.x += v.x;
            this.y += v.y;
            return this;
        },
        minus: function() {
            var v = args2VectorLike(arguments);
            this.x -= v.x;
            this.y -= v.y;
            return this;
        },
        multipy: function(scale) {
            if( isVectorLike(scale) ){
                this.x *= scale.x;
                this.y *= scale.y;
            } else {
                this.x *= scale;
                this.y *= scale;
            }
            return this;
        },
        divide: function(scale) {
            return this.multipy(1/scale);
        },
        unit: function(){
            return this.divide(this.length());
        },
        negative: function(){
            return this.multipy(-1);
        },
        clone: function(){
            return new Vector2(this.x,this.y);
        },
        toArray: function(){
            return [this.x,this.y];
        }
    };

    Vector2.fromPoints = function(p1, p2) {
        return new Vector2(p2.x - p1.x, p2.y - p1.y);
    };

    Vector2.fromCoordinate = function(x,y) {
        return new Vector2(x,y);
    };

    Vector2.fromArray = function(arr){
        return new Vector2(arr[0],arr[1]);
    };

    Vector2.isVector = isVector;
    Vector2.isVectorLike = isVectorLike;

    return Vector2;
}));