Are JavaScript Promise asynchronous?

by

I think you are working under a misunderstanding. JavaScript code is always* blocking; that is because it runs on a single thread. The advantages of the asynchronous style of coding in Javascript is that external operations like I/O do not require blocking that thread. The callback that processes the response from the I/O is still blocking though and no other JavaScript can run concurrently.

* Unless you consider running multiple processes (or WebWorkers in a browser context).

Now for your specific questions:

Just a quick question of clarifications: is JavaScript Promise asynchronous?

No, the callback passed into the Promise constructor is executed immediately and synchronously, though it is definitely possible to start an asynchronous task, such as a timeout or writing to a file and wait until that asynchronous task has completed before resolving the promise; in fact that is the primary use-case of promises.

Would this approach with setTimeout work to make code non-blocking inside a Promise?

No, all it does is change the order of execution. The rest of your script will execute until completion and then when there is nothing more for it to do the callback for setTimeout will be executed.

For clarification:

    console.log( 'a' );
    
    new Promise( function ( ) {
        console.log( 'b' );
        setTimeout( function ( ) {
            console.log( 'D' );
        }, 0 );
    } );

    // Other synchronous stuff, that possibly takes a very long time to process
    
    console.log( 'c' );

The above program deterministically prints:

a
b
c
D

That is because the callback for the setTimeout won’t execute until the main thread has nothing left to do (after logging ‘c’).

More Related Contents:

  1. How to return many Promises and wait for them all before doing other stuff
  2. Why does .json() return a promise?
  3. async/await always returns promise
  4. How to turn this callback into a promise using async/await?
  5. Asynchronous exception handling with bluebird promises
  6. What is the difference between JavaScript promises and async await?
  7. How to wait for a JavaScript Promise to resolve before resuming function?
  8. Should an async API ever throw synchronously?
  9. Node JS Promise.all and forEach
  10. $.Deferred: How to detect when every promise has been executed
  11. How to async await in react render function?
  12. What does $.when.apply($, someArray) do?
  13. Why does the Promise constructor need an executor?
  14. Why can I not throw inside a Promise.catch handler?
  15. How to block for a javascript promise and return the resolved result? [duplicate]
  16. using async await and .then together
  17. How to implement a pseudo blocking async queue in JS/TS?
  18. How to return from a Promise’s catch/then block?
  19. Javascript – if with asynchronous case
  20. Using mongoose promises with async/await
  21. What are asynchronous functions in JavaScript? What is “async” and “await” in JavaScript?
  22. map() function with async/await
  23. How to use Promise.all with an object as input
  24. Javascript Asynchronous Exception Handling with node.js
  25. Why are javascript promises asynchronous when calling only synchronous functions?
  26. Produce a promise which depends on recursive promises
  27. When does async function actually return a pending promise?
  28. Properly batch nested promises in Node
  29. JavaScript asynchronous programming: promises vs generators
  30. Promise.all is returning an array of undefined and resolves before being done

Leave a Comment