function makeIterator(array) {
  var nextIndex = 0;

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

 var it = makeIterator(['a', 'b']);
 it.next().value; //'a'
 it.next().value; //'b'
 it.next().done; // true

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

 var it = idMaker();
 it.next().value; //'0'
 it.next().value; //'1'
 it.next().value; //'2'

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

 for (var n of it) {
  if (n > 5) {
   break;
   console.log( n );
  }
 }
 //0
 //1
 //2
 //3
 //4
 //5

 const arr = [1, 5, 3, 9];
 for (let v of arr) {
  console.log( v );
 }
 //1
 //5
 //3
 //9

 var arr = ['a', 'b', 'c', 'd'];

 for (a in arr) {
  console.log( a );
 }
 //0
 //1
 //2
 //3

 for (a of arr) {
  console.log( a );
 }
 //0
 //1
 //2
 //3

 var name = ['S', 'D', 'J', 'Z', 'G', 'G', 'G'];
 for ( var e of name) {
  console.log( e );
 }
 //S
 //D
 //J
 //Z
 //G


 var es6 = new Map();
 es6.set('edition', 6);
 es6.set('committee', 'TC39');
 es6.set('standard', 'ECMA-262');
 for(var [name, value] of es6) {
   console.log(name + ": " + value);
 }
 // edition: 6
 // commttee: TC39
 // standard: ECMA-262

 var es6 = {
  name: "G.Dragon",
  year: 22,
  love: "coding"
 };

 for (e in es6) {
  console.log( e );
 }
 //name
 //year
 //love

 for( e of es6) {
  console.log( e );
 }
 // TypeError: es6 is not iterable

 // 字符串例子
 let str = "hello";

 for (let s of str) {
  console.log( s );
 }
 //h
 //e
 //l
 //l
 //o

 // DOM NodeList对象的例子
 let paras = document.getSelectorAll("p");
 for (let p of paras) {
  p.classList.add("test");
 }