2018-05-14 genarator实现

function ge(){
    let arr = [1, 2];
    let i = 0;
    return {
        next(){
            let done = (i >= arr.length);
            let value = !done ? arr[i++] : undefined
            return {
                done: done,
                value: value
            }
        }
    }
 }
console.log(ge().next()); //{false, 1}
console.log(ge().next()); //{false, 2}
console.log(ge().next()); //{true, undefined}