es6（基础十七） Promise

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一、作用：解决异步编程的一种方案，让异步编程写法感觉像是同步

二、用法

new Promise(function(resolve,reject){resolve()}).then(function(){//成功.},function(){//失败}).then(function(){//成功},function(){//失败})

    es6之前的回调函数

{let fn = function(callback){console.info('111')setTimeout(function(){callback()},1000)}fn(function(){console.info("2323")})
}

回调套着回调多的情况下，代码就会嵌套很多层，比较乱

    采用Promise

{let fn = function(){return new Promise((resolve,reject)=>{console.info("aaa")setTimeout(()=>{resolve()},1000)})}fn().then(()=>{console.info("bbb")})
}

{let fn = ()=>{return new Promise((resolve,reject)=>{setTimeout(()=>{resolve("张三");},1000)})}fn().then(function(data){return new Promise((resolve,reject)=>{setTimeout(()=>{reject();},1000)})}).then(function(){console.info("1")},function(){console.info("2")})
}

{let loadImg = function(srcURL){return new Promise((res,rej)=>{let imgs = document.createElement("img");imgs.src = srcURL;imgs.onload = function(){res(imgs)}})}//多个操作一起完成后回调Promise.all([loadImg('https://timgsa.baidu.com/timg?image&quality=80&size=b9999_10000&sec=1522564091129&di=c042d7ee514796ac8e875a92d487dfbd&imgtype=0&src=http%3A%2F%2Fimg.zcool.cn%2Fcommunity%2F010a1b554c01d1000001bf72a68b37.jpg%401280w_1l_2o_100sh.png'),loadImg('https://timgsa.baidu.com/timg?image&quality=80&size=b9999_10000&sec=1522564344979&di=9087da918e46807330109aedc85944c6&imgtype=0&src=http%3A%2F%2Fimg.taopic.com%2Fuploads%2Fallimg%2F120727%2F201995-120HG1030762.jpg'),loadImg('https://timgsa.baidu.com/timg?image&quality=80&size=b9999_10000&sec=1522564091129&di=c042d7ee514796ac8e875a92d487dfbd&imgtype=0&src=http%3A%2F%2Fimg.zcool.cn%2Fcommunity%2F010a1b554c01d1000001bf72a68b37.jpg%401280w_1l_2o_100sh.png'),loadImg('https://timgsa.baidu.com/timg?image&quality=80&size=b9999_10000&sec=1522564091129&di=c042d7ee514796ac8e875a92d487dfbd&imgtype=0&src=http%3A%2F%2Fimg.zcool.cn%2Fcommunity%2F010a1b554c01d1000001bf72a68b37.jpg%401280w_1l_2o_100sh.png')]).then(function(img){img.forEach(item=>{document.body.appendChild(item);})})
}

{let loadImg = function(srcURL){return new Promise((res,rej)=>{let imgs = document.createElement("img");imgs.src = srcURL;imgs.onload = function(){res(imgs)}})}//完成一个就回调Promise.race([loadImg('https://timgsa.baidu.com/timg?image&quality=80&size=b9999_10000&sec=1522564091129&di=c042d7ee514796ac8e875a92d487dfbd&imgtype=0&src=http%3A%2F%2Fimg.zcool.cn%2Fcommunity%2F010a1b554c01d1000001bf72a68b37.jpg%401280w_1l_2o_100sh.png'),loadImg('https://timgsa.baidu.com/timg?image&quality=80&size=b9999_10000&sec=1522564344979&di=9087da918e46807330109aedc85944c6&imgtype=0&src=http%3A%2F%2Fimg.taopic.com%2Fuploads%2Fallimg%2F120727%2F201995-120HG1030762.jpg'),loadImg('https://timgsa.baidu.com/timg?image&quality=80&size=b9999_10000&sec=1522564091129&di=c042d7ee514796ac8e875a92d487dfbd&imgtype=0&src=http%3A%2F%2Fimg.zcool.cn%2Fcommunity%2F010a1b554c01d1000001bf72a68b37.jpg%401280w_1l_2o_100sh.png'),loadImg('https://timgsa.baidu.com/timg?image&quality=80&size=b9999_10000&sec=1522564091129&di=c042d7ee514796ac8e875a92d487dfbd&imgtype=0&src=http%3A%2F%2Fimg.zcool.cn%2Fcommunity%2F010a1b554c01d1000001bf72a68b37.jpg%401280w_1l_2o_100sh.png')]).then(function(img){document.body.appendChild(img);})
}

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