es6 iterator cycle

es6-iterators.js

var arr = [1, 2, 3];
var greeting = "hello world";
var m = new Map();
m.set("foo", 42);
m.set({cool: true}, "hello world");

var it = arr[Symbol.iterator]();
it.next();
it.next();
it.next();
it.next(); // {value: undefined; done: true;}

// Make object it iterable
var it = {
  [Symbol.iterator]() { return this; },
  next() { }
};

it[Symbol.iterator]() === it; // true

for (var v of it) {
 console.log(v); 
}

// es5 like iterator interface
for (var v, res; (res = it.next()) && !res.done; ) { // {done: true, value: 42}
  v = res.value;
  console.log(v);
}

// user iterator
var Fib = {
  [Symbol.iterator]() {
    var n1 = 1, n2 = 1;
    
    return {
      // iterator iterable
      [Symbol.iterator]() {
        return this; 
      }
      
      next() {
        var current = n2;
        n2 = n1;
        n1 = n1 + current;
        return {value: current; done: false};
      }
    
      return(v) {
        console.log("Fibbonaci ended");
        return {value: v, done: true};
      }
    }
  }
};

for (var v of Fib) {
  console.log(v);
  if (v > 50) break;
}
// 1 1 2 3 5 8 13 21 34 55