Handling Server-Sent Events with javascript

Now that we’re sending out events, I could point you to the Mozilla pages that explain how the EventSource instance is to be used and call it a day. However, in the year 2023 I don’t find the use of event handlers really idiomatic javascript. Let’s see how we can translate a class that emits events to an async iterable (kind of the javascript counterpart to dotnet’s IAsyncEnumerable).

Normally you can write a generator function over which you can iterate with the asterisk syntax.

function* yieldStuff() {
  for (let i = 0; i < 10; i++) {
    yield i;
  }
}

for (let item of yieldStuff()) {
  console.log(item);
}

This also works in an asynchronous fashion:

async function* yieldStuff() {
  for (let i = 0; i < 10; i++) {
    yield Promise.resolve(i);
  }
}

async function iterating() {
  for await (let item of yieldStuff()) {
    console.log(item);
  }
}

iterating()

The asterisk notation is syntactic sugar for this construction here:

function yieldStuff() {
  let counter = 9;
  return {
    [Symbol.asyncIterator]: () => ({
      next: () => {
        if (counter >= 0) 
          return Promise.resolve({ value: counter--, done: false});
        return Promise.resolve({ done: true })
      }
    })
  }
}

async function iterating() {
  for await (let item of yieldStuff()) {
    console.log(item);
  }
}

iterating()

if an object defines a function that returns an object with a next- function that returns (in Typescript notation)

type Return<T> = { value: T, done: false } | { done: true }

you can iterate over it!

Armed with this knowledge we can build an iterator around an EventSource that we can iterate over events coming out of an EventSource in an asynchronous fashion.

The code is a bit involved as it has to satisfy the needs of an iterator while the event handler of the EventSource can also be called at any moment.

The eventQueue array acts like a queue (we add elements via push and take the first element via shift).

On the iterator side, everytime next is called, we either resolve the first element of the queue if there is one, or signal that the next call is waiting for an element (isWaiting) and await the promise stored under eventAvailableSignal. Once the signal is resolved, the iterator is not waiting anymore and can yield the element that is now available in the queue.

On the event handling side, when an event is received, it is added to the queue. Then it is checked whether the iterator is waiting - if it is, the signal is resolved, and a new signal (aka Promise) is set up for the next round.

As mentioned in the previous post, we allow the server to send a special “Close” event, which is specifically handled such that the event source is closed and an element is enqueued that signals to any iterating code that the iterator will not yield any more elements.

Now we can use the iterateEventSource method to create modern-day idiomatic javascript:

This concludes our small excursion into Server-Sent Events and how you can translate the basic mechanisms into higher level idioms of the respective language.