如下程式碼

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())
//輸出：0.31
//      undefined

setTimeout為非同步函式，所以第二個返回值就列印為了undefined，因為不會等待計時器函式執行完成再執行外層的console.log(request())。

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1、promise then

//解決方法一：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、回撥函式

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();