I created a simple example of a Promise chain. My goal is to fire the 4 functions below to get the output in Order. ie 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?
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);
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.
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.
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