//promise
let state=1;
function step1(resolve,reject){
    console.log(‘1.開始洗菜做飯‘);
    if(state=1){
        resolve(‘洗菜做飯完成‘)
    }else{
        reject(‘洗菜做飯錯誤‘);
    }
}
function step2(resolve,reject){
    console.log(‘2.開始坐下來吃飯‘);
    if(state=1){
        resolve(‘坐下來吃飯完成‘)
    }else{
        reject(
 
‘坐下來吃飯錯誤‘);
    }
}
function step3(resolve,reject){
    console.log(‘3.開始收拾桌子洗碗‘);
    if(state=1){
        resolve(‘收拾桌子洗碗完成‘)
    }else{
        reject(‘收拾桌子洗碗錯誤‘);
    }
}
new Promise(step1).
then(function(val){
    console.log(val);
    return new Promise(step2);
}),
then(function(val){
    console.log(val);
    
 
return new Promise(step3);
}),
then(function(){
    console.log(val);    
});

ES6 promise對象解決嵌套回調的問題