Async and Await

Re:Async and Await by artursapek on Thursday June 02, 2016 @12:25PM (#11823816)

You can use async/await to implement a Mutex in ES6! It's crazy.

https://gist.github.com/artursapek/70437f6cdb562b7b62e910fd3...

Usage:

    import Mutex from 'lib/mutex';

    class MyObject {
      constructor() {
        this.m = new Mutex();
      }

      async someNetworkCall() {
        let unlock = await this.m.lock();

        doSomethingAsynchronous().then((result) => {
          // handle result
          unlock();
        }).catch((err) => {
          // handle error
          unlock();
        });
      }
    }

    let mo = new MyObject();
    mo.someNetworkCall();
    // The next call will wait until the first call is finished
    mo.someNetworkCall();

I love it. We use it in our production codebase at Codecademy.

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Re:Async and Await by peterjuras on Thursday June 02, 2016 @12:38PM (#11823923)

You can use the await keyword multiple times, and therefore make it easier to handle the error with try catch:

      async someNetworkCall() {
        let unlock = await this.m.lock();

        try {
          const result = await doSomethingAsynchronous();
          // handle result
        } catch (exception) {
          // handle error
        }
        unlock();
      }

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Re:Async and Await by Matthias247 on Friday June 03, 2016 @05:38AM (#11829233)

It's quite an interesting approach for limiting the amount of concurrency in singlethreaded environments. I first saw that in the documentation of the C++ seastar library which is also mostly a singlethreaded eventloop. See the chapter "Limiting parallelism with semaphores" here: http://docs.seastar-project.org/master/md_doc_tutorial.html

Their semaphore approach allows then to start an operation exactly X amounts of time and the next one will wait until another one finishes. Seems like an interesting approach. e.g. to limit the number of parallel connections.

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Re:Async and Await by spriggan3 on Thursday June 02, 2016 @12:41PM (#11823947)

Why do you need mutexes in a single threaded context ? If you need sequential async operations that's what async await are for. What if you need to return something from someNetworkCall ? yes you're f..cked in that case.

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Re:Async and Await by phpnode on Thursday June 02, 2016 @12:54PM (#11824066)

Presumably instances of `MyObject` can be accessed by functions which are running while this function is suspended, waiting for the network call. It's single threaded, as in only one thing happens at a time from JS's point of view, but other functions can be invoked (by a request coming in, or a UI event etc) while an asynchronous operation is awaiting completion.

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Re:Async and Await by dvlsg on Thursday June 02, 2016 @12:56PM (#11824086)

That's not true. You could return (either the promise being used or a directly awaited value) and that value would be made available through await / promise as well.

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Re:Async and Await by andreas_mauer on Thursday June 02, 2016 @02:58PM (#11825130)

I feel like I know nothing about js/node anymore,

Classes in JS!! what kind of witchcraft is that? How do you do it?

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Re:Async and Await by pault on Friday June 03, 2016 @02:35PM (#11832273)

I'll recommend the You Don't Know JS series: https://github.com/getify/You-Dont-Know-JS

It's free to read and a good primer for how things have changed in JS-land in the last few years.

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Re:Async and Await by yathern on Thursday June 02, 2016 @04:35PM (#11825803)

It was introduced in ES6. It's still using prototyping behind the scenes, but it's quite a bit nicer to look at.

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Re:Async and Await by andreas_mauer on Thursday June 02, 2016 @05:04PM (#11826048)

thanks, I will check it.

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