Do Javascript promises block the stack

by

When using Javascript promises, does the event loop get blocked?

No. Promises are only an event notification system. They aren’t an operation themselves. They simply respond to being resolved or rejected by calling the appropriate .then() or .catch() handlers and if chained to other promises, they can delay calling those handlers until the promises they are chained to also resolve/reject. As such a single promise doesn’t block anything and certainly does not block the event loop.

My understanding is that using await & async, makes the stack stop
until the operation has completed. Does it do this by blocking the
stack or does it act similar to a callback and pass of the process to
an API of sorts?

await is simply syntactic sugar that replaces a .then() handler with a bit simpler syntax. But, under the covers the operation is the same. The code that comes after the await is basically put inside an invisible .then() handler and there is no blocking of the event loop, just like there is no blocking with a .then() handler.

Note to address one of the comments below:

Now, if you were to construct code that overwhelms the event loop with continually resolving promises (in some sort of infinite loop as proposed in some comments here), then the event loop will just over and over process those continually resolved promises from the microtask queue and will never get a chance to process macrotasks waiting in the event loop (other types of events). The event loop is still running and is still processing microtasks, but if you are stuffing new microtasks (resolved promises) into it continually, then it may never get to the macrotasks. There seems to be some debate about whether one would call this “blocking the event loop” or not. That’s just a terminology question – what’s more important is what is actually happening. In this example of an infinite loop continually resolving a new promise over and over, the event loop will continue processing those resolved promises and the other events in the event queue will not get processed because they never get to the front of the line to get their turn. This is more often referred to as “starvation” than it is “blocking”, but the point is that macrotasks may not get serviced if you are continually and infinitely putting new microtasks in the queue.

This notion of an infinite loop continually resolving a new promise should be avoided in Javascript. It can starve other events from getting a chance to be serviced.

More Related Contents:

  1. Async function returning promise, instead of value
  2. Use async await with Array.map
  3. Correct Try…Catch Syntax Using Async/Await
  4. Waiting for more than one concurrent await operation
  5. What’s wrong with awaiting a promise chain?
  6. Can I fire and forget a promise in nodejs (ES7)?
  7. async/await always returns promise
  8. Difference between microtask and macrotask within an event loop context
  9. How to turn this callback into a promise using async/await?
  10. Using async/await with a forEach loop
  11. Syntax for an async arrow function
  12. How to wait for a JavaScript Promise to resolve before resuming function?
  13. How to async await in react render function?
  14. Can’t throw error from within an async promise executor function
  15. for await of VS Promise.all
  16. JavaScript array .reduce with async/await
  17. Why do I need to await an async function when it is not supposedly returning a Promise?
  18. Slowdown due to non-parallel awaiting of promises in async generators
  19. using async await and .then together
  20. How to implement a pseudo blocking async queue in JS/TS?
  21. What is the relationship between event loop and Promise [duplicate]
  22. What is the difference between “event loop queue” and “job queue”?
  23. What are asynchronous functions in JavaScript? What is “async” and “await” in JavaScript?
  24. What is the intention behind clause 2.2.4 of Promise/A+ spec?
  25. Wait promise inside for loop
  26. Difference of using async / await vs promises?
  27. Async Await map not awaiting async function to complete inside map function before mapping next item
  28. How can I Interleave / merge async iterables?
  29. Implement async/await in sort function of arrays javascript
  30. Can I catch an error from async without using await?

Leave a Comment