/promises-generators-async-js

Primary LanguageJavaScript

Promises, Generators and Async/Await

  • Promises

    • The promises are objects reponsible for modeling asynchronous behavior, allowing their treatment in an easier and more direct way;
    • To create a promise, just instantiate it, executing the resolve function in case of success, being handled by then;
    • In case of failurethe reject function must be executed,being handled by means of catch;
    • It's possible to centralize the handling of a promise by chaining its returns;
    • We can execute several promises at the same time, returning after all succeed using Promise.all;
    • We can also execute several promises at the same time, returning after the first succeeds using Promise.race, or if there is an error, it returns this one;
    • We can also execute several promises at the same time, returning after the first one succeeds using Promise.any, just returning the first one successfully;

  • Generators

    • The generators make it possible to pause the execution of a given function, allowing the use of the event loop in a cooperative way;
    • Generators use the next method to iterate over the available values during function execution;
    • Upon encountering a yield, function execution is paused until the next method is invoked again;
    • The return of the next method is an object containing value and done, following the iteration protocol;
    • Through yield it's possible to return values similarly to return;
    • In addition, it's also possible to send a value into the generator through the next method;
    • The return method closes the generator and can return a specific value;
    • The throw method throws an exception inside the generator, interrupting the flow of execution if the exception has not been handled properly;
    • Where can generators be used?
      • As generators implement the iteration protocol, it is possible to use them with Symbol.iterator in a simple way;
      • Furthermore, it is possible to use generators to synchronize asynchronous calls in a similar way to async/await;

  • Async/Await

    • async/await facilitates interaction with asynchronous calls, waiting for a given promise to return;
    • To handle possible exceptions associated with asynchronous calls, you can use a try/catch block;
    • Is it possible to iterate using async/await? Yes;
      • Through forEach, the async in the function will allow it to execute, but it executes asynchronously, that's, there cannot be a dependency below the forEach;
      • Through the for of, creating a dependency chain, that's, the method called below the for, can depend, unlike forEach;
      • It's possible to use the for-await-of block to iterate over a promise iterator;
      • To use for-await-of it's necessary to use the --harmony-async-iteration flag in older versions of Node.js;