Wrapper around Iterators/Generators in javascript so that we can find out if an iterator has a next value or not.

iter.js

function Iter(machine) {
    this.gen = machine();
    this.current = this.__iterator__();
}
Iter.prototype.hasNext = function() {
    return this.current.hasNext();
};
Iter.prototype.next = function() {        
    return this.current.next();
};
Iter.prototype.__iterator__ = function() {
    var env = this,
        values = [];
    
    function Wrapper() {
        this.hasNext = function() {
            var possibleNext = true;
            try {
                values.push(env.gen.next());
            } catch(e) {
                if (e instanceof StopIteration) possibleNext = false;
                else throw e;
            }
            return possibleNext;
        };
        this.next = function() {
            if (values.length < 1) return env.gen.next();
            else return values.shift();
        };
    }
    
    return new Wrapper();
}

/*
To make it work:

function* a() {
    for (var i = 0; i < 9; i++) {
        yield i;
    }
}

// Either way works!
var iter = new Iter(a);
while(iter.hasNext()) {
    console.log(iter.next());
}
console.log('-------');
for(var i in new Iter(a)) {
    console.log(i);
}
*/

This is very stateful and not very functional, but considering that I believe exceptions should be in an exceptional condition and the proliferation of try...catch to know when a iterator has stopped is going to lead to painful code in most use cases!!

(own opinion not backed up by anything scientific at all)