I have lot of confusion in promise. It's a synchronous or asynchronous ?
return new Promise (function(resolved,reject){
//sync or async?
});
I have lot of confusion in promise. It's a synchronous or asynchronous ?
return new Promise (function(resolved,reject){
//sync or async?
});
When you create a promise and pass a call back to it that callback is gonna executed immediately (sync)
But when you resolve it by .then() method it will act in asynchronous way so for example :
Promises are like normal classes in Javascript. Assume you are creating your own Promise implementation, your promise class would roughly look like this. Notice in your constructor you are expecting a method to be passed that you call immediately passing
resolve and reject
as parameters.So when you do
new Promise()
, you are just creating a new object. YourPromise constructor
will run, and it will call the method immediately. So that is why the code inside your promise gets executed synchronously.If inside your function you were calling another function that was async in nature, then that another function would get executed asynchronously, otherwise, everything else gets executed synchronously.
If you had chains in promise using
then
, then it only gets called after your first promise has calledresolve()
.Promises aren't exactly synchronous or asynchronous in and of themselves. When you create a promise the callback you pass to it is immediately executed and no other code can run until that function yields. Consider the following example:
The code outside the promise has to wait for the code inside the promise (which is synchronous) to complete before it can begin execution.
That said, promises are a common way of dealing with asynchronous code. The most common use case for a promise is to represent some value that's being generated or fetched in an asynchronous fashion. Logic that depends on that value can asynchronously wait until the value is available by registering a callback with
.then()
or related Promise methods.The function you pass into the Promise constructor runs synchronously, but anything that depends on its resolution will be called asynchronously. Even if the promise resolves immediately, any handlers will execute asynchronously (similar to when you
setTimeout(fn, 0)
) - the main thread runs to the end first.This is true no matter your Javascript environment - no matter whether you're in Node or a browser.