问题:
I created a simple example of a Promise chain. My goal is to fire the 4 functions below to get the output in Order. i.e. 1 2 3 4.
function WriteOne()
{
return new Pro...
可以将文章内容翻译成中文,广告屏蔽插件会导致该功能失效:
问题:
I created a simple example of a Promise chain. My goal is to fire the 4 functions below to get the output in Order. i.e. 1 2 3 4.
function WriteOne()
{
return new Promise(function(resolve, reject)
{
console.log("1");
resolve("true");
})
}
function WriteTwo()
{
return new Promise(function(resolve, reject)
{
setTimeout(function(){console.log("2");}, 3000);
resolve("true");
})
}
function WriteThree()
{
return new Promise(function(resolve, reject)
{
setTimeout(function(){console.log("3");}, 1000);
resolve("true");
})
}
function WriteFour()
{
return new Promise(function(resolve, reject)
{
console.log("4");
resolve("true");
})
}
WriteOne().then(WriteTwo).then(WriteThree).then(WriteFour);
When I execute them, they are firing in this order:
1 4 3 2
What am I doing wrong here?
回答1:
You need to resolve inside setTimeout callback
function WriteOne() {
return new Promise(function (resolve, reject) {
console.log("1");
resolve("true");
});
}
function WriteTwo() {
return new Promise(function (resolve, reject) {
setTimeout(function () {
console.log("2");
resolve("true");
}, 3000);
});
}
function WriteThree() {
return new Promise(function (resolve, reject) {
setTimeout(function () {
console.log("3");
resolve("true");
}, 1000);
});
}
function WriteFour() {
return new Promise(function (resolve, reject) {
console.log("4");
resolve("true");
});
}
WriteOne().then(WriteTwo).then(WriteThree).then(WriteFour);
回答2:
Nothing wrong here, here what's happening:
// t=0s
writeOne()
// 1 is logged
// promise is resolved
.then(writeTwo)
// in 3s, 2 will be logged
// promise is resolved
.then(writeThree)
// in 1s, 3 will be logged
// promise is resolved
.then(writeFour)
// 4 is logged
// t=1s
// 3 is logged
// t=3s
// 2 is logged
Thus the output.
To get the result you're expecting, resolve the promise inside the setTimeout.
回答3:
resolve("true") is called before the console.log() because it's outside the setTimeout().
Since WriteOne() and WriteFour() have no timeout, "1" and "4" appear first. Then "3" 1 second later, Then 2.
You have to put resolve("true"); inside setTimeout() in order to have them appear... in order.
