This content originally appeared on DEV Community and was authored by Prakhar Yadav
After having read about callbacks, promises & async-await multiple times in not so easy to comprehensions, I finally have wrapped my head around them.
And today I'd share it in simpler terms that I am able to remember & understand.
Callbacks
Callbacks are functions being passed as arguments. That's it. That doesn't satisfy you then read these one page articles (to arrive on same conclusion of course):
Promises:
functions that are not run sequentially. They are run when possible.
const fun = new Promise ((resolve, reject) => {
if(<some condition>)
resolve("some success message");
else
reject("some failure message");
});
fun()
.then(msg => console.log(msg)) // some success message
.catch(msg => console.log(msg)); // some failure message
Resolve is called to indicate & return success status of the Promise, & Reject is called when we need to show failure.
Once returned we need to handle the stuff as well right?
-
then()is used to handle the resolved state -
catch()is used to handle the rejected state
See! Simple.
Async - Await
Just a wrapper around Promise. Async-Await use Promises in the background.
Why was this done?
Because sometimes developers tend to nest things up. Nesting Promises makes it difficult to write, read, follow & understand at one look.
So why not make it easy to read & understand.
const fun = async () => {
await functionToPerformAsynchronously();
}
That easy. You can easily make any function run asynchronously, by appending a await before it. Just remember that await statement must be in a function declared as async.
And you know what! async functions are nothing but promises (well not really; they return promises).
Don't confuse yourself, just read along and find more detail in the links mentioned at the end.
Which is why you can do this:
const fun = async () => {
await functionToPerformAsynchronously();
}
fun()
.then(<whatever you want to do here>) //runs when fun() has run successfully and did not encounter any issue while it ran
.catch(errorMsg => console.log(errorMsg)); // runs when fun() had some problem running. We can display the problem as an error message is returned & can be displayed like in this example.
Want to dig deeper?
Here are one of the best & easy to follow official Node documentation on each one of the three:
- JavaScript Asynchronous Programming and Callbacks
- Understanding JavaScript Promises
- Modern Asynchronous JavaScript with Async and Await
This content originally appeared on DEV Community and was authored by Prakhar Yadav
Prakhar Yadav | Sciencx (2021-08-28T22:05:36+00:00) Easy Callback, Promises, Async-Await. Retrieved from https://www.scien.cx/2021/08/28/easy-callback-promises-async-await/
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