What happens if you don't resolve or reject a promise?

Question

I have a scenario where I am returning a promise. The promise is basically triggered by an ajax request.

On rejecting the promise it shows an error dialog that there is a server error.

What I want to do is when the response code is 401, I neither want to resolve the promise nor reject it (because it already shows the error dialog). I want to simply redirect to the login page.

My code looks something like this:

function makeRequest(ur, params) {
  return new Promise(function (resolve, reject) {
    fetch(url, params).then((response) => {
      let status = response.status;    
      if (status >= 200 && status < 300) {
        response.json().then((data) => {
          resolve(data);
        });
      } else {
        if (status === 401) {
          redirectToLoginPage();
        } else {
          response.json().then((error) => {
            if (!error.message) {
              error.message = constants.SERVER_ERROR;
            }
            reject({ status, error });
          });
        }
      }
    });
  });
}

As you can see, if the status is 401, I am redirecting to the login page. The promise is neither resolved nor rejected.

Is this code OK, or is there any better way to accomplish this?

Answer 1

A promise is just an object with properties in Javascript. There's no magic to it. So failing to resolve or reject a promise just fails to ever change the state from "pending" to anything else. This doesn't cause any fundamental problem in Javascript because a promise is just a regular Javascript object. The promise will still get garbage collected (even if still pending) if no code keeps a reference to the promise.

The real consequence here is what does that mean to the consumer of the promise if its state is never changed? Any .then() or .catch() listeners for resolve or reject transitions will never get called. Most code that uses promises expects them to resolve or reject at some point in the future (that's why promises are used in the first place). If they don't, then that code generally never gets to finish its work.

It's possible that you could have some other code that finishes the work for that task and the promise is just abandoned without ever doing its thing. There's no internal problem in Javascript if you do it that way, but it is not how promises were designed to work and is generally not how the consumer of promises expect them to work.

As you can see if the status is 401, I am redirecting to login page. Promise is neither resolved nor rejected.

Is this code OK? Or is there any better way to accomplish this.

In this particular case, it's all OK and a redirect is a somewhat special and unique case. A redirect to a new browser page will completely clear the current page state (including all Javascript state) so it's perfectly fine to take a shortcut with the redirect and just leave other things unresolved. The system will completely reinitialize your Javascript state when the new page starts to load so any promises that were still pending will get cleaned up.