Solution to sum function challenge. Here are some examples: ( add(2) == 2 ) ( add(1)(2)(3) == 6 )

add-function-es5.js

var add = function add() {
    var args = Array.prototype.slice.call(arguments);
    var fn = Function.prototype.bind.apply(add, [null].concat(args));
	fn.valueOf = function() {
        return args.reduce(function(a, b) {
            return a + parseInt(b);
        }, 0);
    };
    return fn;
}

add-function-es6.js

const add = function add(...args) {
    const fn = Function.prototype.bind.apply(add, [null, ...args]);
    fn.valueOf = () => args.reduce((a, b) => (a + parseInt(b)), 0);
    return fn;
}