jquery css without jquery

Element.prototype.css.js

Element.prototype.css = function (a, b) {
    var style = function(t,k) {
        return window.getComputedStyle(t)[k];
    };
    if (typeof b === "undefined") {
        if (typeof a === "string") {
            return style(this,a);
        } else if (Array.isArray(a)) {
            return a.map(function(s) {
                return style(this,s);
            });
        } else {
            for (var k in a) {
                this.style[k] = a[k];
            }
        }
    } else {
        this.style[a] = b;
    }
};