Question

If I have a function that’s passed this function:

function(work) {
   work(10);
   work(20);
   work(30);
}

(There can be any number of work calls with any number in them.)

work performance some asynchronous activity—say, for this example, it just is a timeout. I have full control over what work does on the completion of this operation (and, in fact, its definition in general).

What’s the best way of determining when all the calls to work are done?

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My current method increments a counter when work is called and decrements it when it completes, and fires the all work done event when the counter is 0 (this is checked after every decrement). However, I worry that this could be a race condition of some sort. If that is not the case, do show my why and that would be a great answer.

Answer 1

There are a ton of ways you can write this program, but your simple technique of using a counter will work just fine.

The important thing to remember, the reason this will work, is because Javascript executes in a single thread. This is true of all browsers and node.js AFAIK.

Based on the thoughtful comments below, the solution works because the JS event loop will execute the functions in an order like:

1. function(work)
2. work(10)
3. counter++
4. Start async function
5. work(20)
6. counter++
7. Start async function
8. work(30)
9. counter++
10. Start async function
11. — back out to event loop —
12. Async function completes
13. counter–
14. — back out to event loop —
15. Async function completes
16. counter–
17. — back out to event loop —
18. Async function completes
19. counter–
20. Counter is 0, so you fire your work done message
21. — back out to event loop —