curry in javascript

curry-example.js

var curry = function(foo, arity) {
    var curried = function() {
        // parameters by far are passed with `this`
        // slice()-ing it here makes the curried function stateless 
        var new_args = this.slice(); 
        for(arg_key in arguments) {
            new_args.push(arguments[arg_key]);
        }
        
        if (new_args.length >= arity) {
            return foo.apply(null, new_args);
        } else {
            return curried.bind(new_args);
        }
    };
    
    return curried.bind([]);
};

var sum4 = function(a,b,c,d) { return a+b+c+d; };

var curried = curry(sum4, sum4.length);

console.log(curried(1,2,3)(4));
// -> 10
console.log(curried(1)(2,3)(4));
// -> 10