Home Web Front-end JS Tutorial Explanation of Promise in asynchronous programming in es6

Explanation of Promise in asynchronous programming in es6

Sep 30, 2017 am 09:25 AM
promise programming explain

//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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