当我为我的项目构建一些需要实现的逻辑原型时,我注意到async-await和Promise的一些有趣的行为。
// Notice this one returns a Promise
var callMe = function(i) {
return new Promise((resolve, reject) => {
setTimeout(() => {
console.log(i)
resolve(`${i} is called :)`)
}, (i+1)*1000)
})
}
// But this one doesn't
var callYou = function(i) {
setTimeout(() => {
console.log(i)
}, (i+1)*1000)
}
async function run() {
console.log("Start")
for(let i = 0; i < 3; i++) {
let val = await callYou(i)
# also try with callMe()
#let val = await callMe(i)
console.log(val)
}
console.log("End")
}
run()
使用let val = await callYou(i)
,结果看起来像这样
Start
callYou()
callYou()
callYou()
End
0
1
2
而对于let val = await callMe(i)
,结果看起来像这样
Start
0
0 is called :)
1
1 is called :)
2
2 is called :)
End
我期望两个函数的行为相似,因为异步函数本质上返回一个承诺。有人可以解释一下为什么会发生这种情况吗?
async
函数返回承诺,但callYou
或callMe
都不是async
函数,即使它们在哪里,当函数返回时,callYou
将在函数返回时实现,而不是在回调传递给setTimeout
函数时执行。