3 ms·

The most surprising thing that's bitten me about async/await is that the promise is immediately executed rather than being executed at the point they get awaite

• jezzamon 3y ago
  Definitely something to know, but it's the best way for that to happen when you're waiting on asynchronous requests, as it lets you create the subsequent promises while the first promises is fetching its results
  • plopz 3y ago
    It hurts a lot though when you have a bunch of promises and you want to rate limit the execution
    • vlovich123 3y ago
      Wrap them in a generator.
    • threatofrain 3y ago
      What's the status of cancellable promises in proposal land?
      • capableweb 3y ago
        I mean, why would you want to "cancel" a Promise? You can already cancel HTTP requests, what are some other things you want to cancel really? If you don't want to continue with something inside a Promise, just pass in something that could be changed from `true` to `false` and it cancels itself based on that.
        • coke12 3y ago
          I've had this same cancellation conversation many times with JS programmers. I think it's fair to say there's a need for standardization. Evidence for this need is network request management is really poor in pretty much all JS apps.
          • capableweb 3y ago
            Specifically the case of request management, it's already implemented and available in all browsers practically. It's like four lines of code you have to add/change, to be able to cancel a request. https://developer.mozilla.org/en-US/docs/Web/API/AbortController https://developer.mozilla.org/en-US/docs/Web/API/AbortContro... Regarding other cases, there are already ways of handling those. I don't think just because many ask about something, doesn't mean it's not currently working.
            • coke12 3y ago
              There are a lot of issues with the ergonomics of AbortController. It's very low level.
              • capableweb 3y ago
                Did you actually look at the link I sent you? It's absolutely not low level. If you know how to use window.fetch, you know how to use AbortController already. controller = new AbortController(); const signal = controller.signal; fetch("http://example.com", { signal: signal }) // abort request controller.abort(); It basically couldn't be simpler? What sort of API would you like to be able to cancel requests?
                • coke12 3y ago
                  That's a toy example! In reality in a complex app you'd usually be aborting far from where the fetch is created, and you'd have large promise chains where the semantics of cancellation are non-obvious and even unintuitive.
    • capableweb 3y ago
      Eh, it's so easy to provide your own implementation that does this, but I guess it wouldn't hurt to include in the standard-library since we already have something like Promise.all() Here is the implementation I typically use for doing just that: async function executeQueue(promises, concurrentLimit) { let index = 0; let activePromises = []; async function manageQueue() { if (index >= promises.length && activePromises.length === 0) return; while (activePromises.length < concurrentLimit && index < promises.length) { const promise = promises[index](); activePromises.push(promise); index++; promise.then(() => { activePromises = activePromises.filter(p => p !== promise); manageQueue(); }); } } await manageQueue(); await Promise.all(activePromises); } Used like this: function r(i, resolve) { console.log(`Task ${i}`) resolve(`Task ${resolve}`) } // Example usage const promises = [ () => new Promise(resolve => setTimeout(() => r("Task 1", resolve), 2000)), () => new Promise(resolve => setTimeout(() => r("Task 2", resolve), 1000)), () => new Promise(resolve => setTimeout(() => r("Task 3", resolve), 3000)), () => new Promise(resolve => setTimeout(() => r("Task 4", resolve), 500)), () => new Promise(resolve => setTimeout(() => r("Task 5", resolve), 1500)) ]; executeQueue(promises, 4).then(() => { console.log("All tasks completed!"); }); Would print: "Task Task 4" "Task Task 2" "Task Task 1" "Task Task 5" "Task Task 3" "All tasks completed!"