// es5中的異步回調(diào)  let ajax = function(callback){   console.log('執(zhí)行')  // 執(zhí)行   setTimeout(() => {    callback&&callback.call();   }, 1000)  };  ajax(function(){   console.log('hello') // 1s后打印hello  });

let ajax = function(){   console.log('執(zhí)行2'); // 執(zhí)行2   return new Promise((resolve,reject)=>{    setTimeout(() => {     resolve() // 執(zhí)行下一步操作， reject  // 中斷當(dāng)前的操作    },5000)   })}ajax().then(()=>{   console.log('promise','timeout2')  // 5s后打印  promise timeout2})

let ajax = function(){   console.log('執(zhí)行3');   return new Promise((resolve,reject)=>{    setTimeout(() => {     resolve();    },5000)   })}ajax().then(function(){   return new Promise(function(resolve,reject){    setTimeout(() => {     resolve()    }, 2000)   })}).then(function(){   console.log('timeouk3') // 7s后打印timeouk3})

// catch 捕獲錯(cuò)誤let ajax = function(num){   console.log('執(zhí)行4');   return new Promise(function(resolve,reject){    if(num>=5){     resolve();    }else{     throw new Error('傳入的數(shù)字比5小')    }   })}ajax(4).then(function(){   console.log('log',4);}).catch(function(err){   console.log('catch',err); // catch Error: 傳入的數(shù)字比5小})

function loadImg(src){   return new Promise((resolve,reject)=>{    let img = document.createElement('img');    img.src = src;    img.onload = function(){     resolve(img);    }    img.onerror = function(err){     reject(err);    }   })}// 對(duì)所有的圖片進(jìn)行遍歷function showImgs(imgs){   console.log(imgs);   imgs.forEach(function(img){    document.body.appendChild(img);   })}// promise.all()的用法Promise.all([   loadImg('http://yanshi.sucaihuo.com/modals/0/23/small.jpg'),   loadImg('http://yanshi.sucaihuo.com/modals/0/20/small.jpg'),   loadImg('http://yanshi.sucaihuo.com/modals/0/2/small.jpg')]).then(showImgs)/**解析:把多個(gè)promise實(shí)例當(dāng)做一個(gè)promise實(shí)例，當(dāng)所有的promise實(shí)例發(fā)生變化后新的promise實(shí)例才會(huì)發(fā)生變化*/

// Promise 先加載 有一個(gè)圖片加載完就先添加到頁(yè)面  function loadImg(src){   return new Promise((resolve,reject)=>{    let img = document.createElement('img');    img.src = src;    img.onload = function(){     resolve(img);    }    img.onerror = function(err){     reject(err);    }   })  }  function showImgs(img){   let p = document.createElement('p');   p.appendChild(img);   document.body.appendChild(p);  }  // promise.race()的用法 只要有一個(gè)網(wǎng)速好的加載完，其他的就不加載了  Promise.race([   loadImg('http://yanshi.sucaihuo.com/modals/0/23/small.jpg'),   loadImg('http://yanshi.sucaihuo.com/modals/0/20/small.jpg'),   loadImg('http://yanshi.sucaihuo.com/modals/0/2/small.jpg')  ]).then(showImgs) }