This was orginally posted at lindaojo.com
To fully appreciate the use of Async-Await you must first understand that by default, JavaScript is synchronous.
Synchronous Functions
In synchronous functions, operations run simultaneously and you can't specify pausing or waiting points.
Example
function solveC() {
const A = 1;
const B = 2;
const C = A + B;
console.log(C);
}
solveC(); // 3
But if for some reason there is a delay in getting the value 'B', JavaScript will execute other lines of code that aren't delayed. This could bring up unexpected results.
In the example below, 'B' is delayed using a promise. Let's check out what the results will be.
function solveC() {
const getB = () => {
const executor = (resolve, _reject) => {
setTimeout(() => resolve(2), 100);
};
return new Promise(executor);
}
const A = 1;
const B = getB();
const C = A + B;
console.log(C);
}
solveC(); // "1[object Promise]"
JavaScript logs the result before the promise above is fulfilled;
What do we do to get the right result even if B is delayed? How do we ask Javascript to pause and wait for 'B'.
The answer is we make the function asynchronous. This is where "async-await" comes in.
Note: there are other ways to write asynchronous code. You could use Callback functions and promises.
Asynchronous Functions using Async-Await
To make a function Asynchronous we declare the function using the "Async" keyword.
The word “async” before a function means the function will always returns a promise.
The async function below...
async function One() {
return 1;
}
is the same as the normal function below that returns a promise.
function One() {
return Promise.resolve(1);
}
We can ask JavaScript to wait for a promise to be fulfilled by using the "await" keyword. It has to be noted that it only makes the async function block wait and not the whole program execution.
The code block below shows how we solve our earlier problem with the use of async-await.
async function solveC() {
const getB = () => {
const executor = (resolve, _reject) => {
setTimeout(() => resolve(2), 100);
};
return new Promise(executor);
}
const A = 1;
const B = await getB(); //waits for promise to be resolved
const C = A + B;
console.log(C);
}
solveC(); // 3
Note: the "await" keyword can only be used within "async" functions.
That's it! Hope this was helpful cause I kept it light for beginners. If you want to read a more advanced explanation of async-await, I recommend this article by Ashay Mandwarya
Updated: Made a few corrections, thank you @peerreynders .
Top comments (11)
The reason
solveC
returnsNaN
is becausegetB
will always returnundefined
.And
1 + undefined
isNaN
.This has nothing to do with the
setTimeout
.And as it happens
Promises
andsetTimeout
are scheduled on separate queues.setTimeout
is scheduled on the task queue while promises are scheduled on the micro-task queue.However promises are often used to get a value from
setTimeout
to a consumerWith async/await one can write
or perhaps
Resources:
I understand, thank you for the detailed correction 💙
Your correction
Simply replaces
c = 1 + undefined
withc = 1 + Promise<pending>
- which similarly doesn't work.The displayed result shows that JavaScript simply concatenated them - with really isn't useful.
Perhaps you meant:
This may be of interest:
No, that was my intention; to show what could go wrong if 'B' was delayed.
Thank you for the resources.
The point is
b
isn't delayed.b
is a promise, not a number.What is delayed is the fulfillment of the promise to a number. In TypeScript this is immediately flagged as an error.
link
This is simply a programming error caused by type confusion
async/await
doesn't "fix" this error.async/await
is simply syntax sugar so you don't have to deal with the promise directly. Butawait
only works on promises - so you have to be aware that you are dealing with promises as it is the case withgetB()
.I'm not advocating the use of TypeScript here but I do think it is essential to "think in types" even when you are using a dynamically typed language.
PS:
await
has its own challenges:Nice and simple introduction. I never had a real-life use case for this, but with this explanation I can think of new ways to implement this. Thanks.
Also, I think (and I may be wrong) that you've made a mistake though.
In the 3rd and 4th code blocks, the async function and the normal function both have the async keyword before them. Is that right?
In the final code block, function getB( ) doesn't have the async keyword, whereas you mention that the await keyword can only be used with async functions.
Thank you, I have fixed the first issue.
Await can only be used "within" an async function. getB() does not have the async keyword because it doesn't use await within it. I hope that's clear.
Thank you so much. This was one of the best explanations.
Thank you 🥰
Great post, it can be quite difficult to wrap your head around promises in JS, and this is a lovely primer.
Didn’t know that it’s possible with just a setTimeout.