Home » JavaScript » Understanding ordering with JavaScript async methods

About Gerard Davison

Gerard Davison

Understanding ordering with JavaScript async methods

So I challenged someone in a code review to prove that there code that made use of async functions wouldn’t be susceptible to a race condition. To that end I came up with a very trivial code example to demonstrate the issue, worth trying to write down the output and line orderings before you read on.

let list;

async function clearList () {
    list = []; // A

async function processList (processList) {
   await clearList(); // B
   list = list.concat(processList); // C

processList([1,2,3]); // D
processList([4,5,6]) // E
   .then(() => {
      console.dir(list); // F

So the two questions here are what is the output of this code and what order do the lines of code get executed in. Now the point of the code was that it would show that the output is
[1,2,3,4,5,6] because of the race condition; but the actual ordering of the execution of the lines of code I had wrong.

My assumption was that you would not enter the async method directly and instead be queued to be performed on a later clock tick. This gave me an execution flow of D,E,B,B,A,A,C,C,F which I was happy with until my coworker Millan Kuchtiak pointed out I was entirely incorrectly.

It turns out that at least in Chrome and Safari that code doesn’t get transferred queue until you reach the first await call. So actually when you run the code through with a debugger the flow is D,B,A,E,B,A,C,C,F. This make sense from a performance point of view, some async methods might never need the change of context, so just in time async.

To summarise an async method is synchronous up until the first await

Published on Web Code Geeks with permission by Gerard Davison, partner at our WCG program. See the original article here: Understanding ordering with JavaScript async methods

Opinions expressed by Web Code Geeks contributors are their own.

(+2 rating, 2 votes)
You need to be a registered member to rate this.
Start the discussion Views Tweet it!
Do you want to know how to develop your skillset to become a Web Rockstar?
Subscribe to our newsletter to start Rocking right now!
To get you started we give you our best selling eBooks for FREE!
1. Building web apps with Node.js
2. HTML5 Programming Cookbook
3. CSS Programming Cookbook
4. AngularJS Programming Cookbook
5. jQuery Programming Cookbook
6. Bootstrap Programming Cookbook
and many more ....
I agree to the Terms and Privacy Policy
Notify of

This site uses Akismet to reduce spam. Learn how your comment data is processed.

Inline Feedbacks
View all comments