Explanation of Promise in asynchronous programming in es6
//引入模块let fs=require('fs');
//异步读文件方法，但是同步执行function read(url) {
//new Promise 需要传入一个executor 执行器 //executor需要传入两个函数 resolve reject return new Promise((resolve,reject)=> {
fs.readFile(url,'utf8',function (err,data) {
if(err) {
reject(err) }
else {
resolve(data);
}
}) }
)
}
;
//缺点 套了一个promise，而且捕获了2次，这两个请求没有依赖关系，时间叠加了// read('./name.txt').then((data)=> {
// let obj= {
}
;
// obj.name=data;
// read('./age.txt').then((data)=> {
// obj.age=data;
// console.log(obj);
// }
,(err)=> {
// console.log(err);
// }
)//
// }
, (err)=> {
// console.log(err);
// }
);
//回调地狱 链式调用//相对好点的方法，同步两个异步依次执行 异常用catch捕获// let obj= {
}
;
// read('./name.txt').then((data)=> {
// obj.name=data;
// return read('./age.txt')// }
).then((data)=> {
//如果promise返回promise可以继续then// obj.age=data;
// return obj //将结果向下继续传递// }
).then((data)=> {
// console.log(data) //单独处理结果// }
).catch((err)=> {
// console.log(err)// }
);
//all方法是promise是类上自带的方法,并发读取,失败一个都失败了，时间只是一个读取的时间//第一个参数 传递的是数组，数组装的是一个个promise对象//调用后会再次返回一个promise实例//最好的写法Promise.all([read('./name.txt'),read('./age.txt')]).then(([name,age])=> {
//data就是promise执行成功的结果类型是数组 console.log( {
name,age
}
);
}
).catch((err)=> {
console.log(err)
}
)//race如果一个失败了，都失败，如果一个成功了都成功,很少用// Promise.race([read('./name.txt'),read('./age1.txt')]).then((data)=> {
// //data就是promise执行成功的结果类型是数组// console.log(data);
// }
).catch((err)=> {
// console.log(err)// }
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