1.模拟next方法返回值的例子

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", done: false}
it.next(); // {value: "b", done: true}
it.next(); // {value: undefined, done: true}