The Iterator protocol

This is an experimental technology, part of the Harmony (ECMAScript 6) proposal.
Because this technology's specification has not stabilized, check the compatibility table for usage in various browsers. Also note that the syntax and behavior of an experimental technology is subject to change in future version of browsers as the spec changes.

One addition of ECMAScript 6 is not new syntax or a new built-in, but a protocol. This protocol can be implemented by any object respecting some conventions.

Description

An iterator has a method called next. This method returns an object with two properties: value (any value) and done (will be read as a boolean).

Examples

Simple iterator

function makeIterator(array){
    var nextIndex = 0;
    
    return {
       next: function(){
           return nextIndex < array.length ?
               {value: array[nextIndex++], done: false} :
               {done: true};
       }
    }
}

var it = makeIterator(['yo', 'ya']);

console.log(it.next().value); // 'yo'
console.log(it.next().value); // 'ya'
console.log(it.next().done);  // true

Infinite iterator

function idMaker(){
    var index = 0;
    
    return {
       next: function(){
           return {value: index++, done: false};
       }
    }
}

var it = idMaker();

console.log(it.next().value); // '0'
console.log(it.next().value); // '1'
console.log(it.next().value); // '2'
// ...

With a generator

function* makeSimpleGenerator(array){
    var nextIndex = 0;
    
    while(nextIndex < array.length){
        yield array[nextIndex++];
    }
}

var gen = makeSimpleGenerator(['yo', 'ya']);

console.log(gen.next().value); // 'yo'
console.log(gen.next().value); // 'ya'
console.log(gen.next().done);  // true



function* idMaker(){
    var index = 0;
    while(true)
        yield index++;
}

var gen = idMaker();

console.log(gen.next().value); // '0'
console.log(gen.next().value); // '1'
console.log(gen.next().value); // '2'
// ...

For more informations on ES6 generators, see the dedicated page.

The deprecated Firefox-only iterator protocol

Firefox, prior to version 26 implemented another iterator protocol. The value was returned directly as a return value of calls to next (instead of the value property of a placeholder object). Iteration termination was expressed by throwing a StopIteration object. This version of the protocol has been removed.

Simple example with the old protocol

function makeIterator(array){
    var nextIndex = 0;
    
    return {
       next: function(){
           if(nextIndex < array.length){
               return array[nextIndex++];
           else
               throw new StopIteration();
       }
    }
}

var it = makeIterator(['yo', 'ya']);

console.log(it.next()); // 'yo'
console.log(it.next()); // 'ya'
try{
    console.log(it.next());
}
catch(e){
    if(e instanceof StopIteration){
         // iteration over
    }
}