Vector javascript kata

vector.js

// http://www.codewars.com/kata/vector-class/train/javascript

var Vector = function(a) {
    return {
        get: function(key) {
            return a[key];
        },
        getAll: function() {
            return a;
        },
        checkSize: function(b) {
            if (a.length != b.getAll().length) {
                throw 'Size mismatch: '+a+' '+b.getAll();
            }
        },
        negate: function() {
            var c = [];
            a.forEach(function(val) {
                c.push(-val);
            });
            return new Vector(c);
        },
        add: function(b) {
            this.checkSize(b);
            var c = [];
            a.forEach(function(val, key) {
                c.push(val+b.get(key));
            });
            return c;
        },
        subtract: function(b) {
            return this.add(b.negate());
        },
        dot: function(b) {
            this.checkSize(b);
            var c = 0;
            a.forEach(function(val, key) {
                c += val*b.get(key);
            });
            return c;
        },
        norm: function() {
            var c = 0;
            a.forEach(function(val) {
                c += Math.pow(val, 2);
            });
            return Math.sqrt(c);
        },
        toString: function() {
            return '('+a.join(',')+')';
        },
        equals: function(b) {
            return res = a.every(function(val, key) {
                return val == b.get(key);
            });
        }
    }
}