解决异步的几种实现方式

如下代码

const request = function(){
let num = Math.round(Math.random()*100)/100;
console.log(num);
    setTimeout(function(){
    if(num < 0.5){
    return('1234567');
    }else{
    return('abcdefg');
    }
},500)
}
console.log(request())
//输出：0.31
//      undefined

setTimeout为异步函数，所以第二个返回值就打印为了undefined，因为不会等待计时器函数执行完成再执行外层的console.log(request())。

1、promise then

//解决方法一：promise then
const request = function(){
var _this = this;
let num = Math.round(Math.random()*100)/100;
console.log( num );
return new Promise( (resolve, reject) => {
setTimeout(function(){
num < 0.5 ? resolve('1234567') : reject('abcdefg');
}, 500);
});
}

request().then( val => {
console.log( val );
}).catch( err => {
console.log( err );
});

2、回调函数

const request = function(cb){
let num = Math.round(Math.random()*100)/100;
console.log(num);
    setTimeout(function(){
num < 0.5 ? cb('1234567') : cb('abcdefg');
},500)
}

var result = request(function(res){
console.log(res);
})

3、async await

const request = async function(){
let num = Math.round(Math.random()*100)/100;
console.log(num);
    const result = await aa(num);
}

function aa(num){
setTimeout(function(){
console.log(num < 0.5 ? '1234567' : 'abcdefg');
},500)
}

request();