How long a Promise can remain in pending state?

Question

I'm using Promises in Angular (4) project and I have a question about them that I couldn't find a response to it in the docs.

When I create a Promise, I basically wait for an async answer from a service/party. But how long should I expect this Promise to stay in pending state? Is there any mechanism that will terminate it after a while? How reliable is this concept of waiting/pending?

Let's suppose that I need to get some data from a busy service that can answer even after few minutes of waiting, maybe more, no matter if the computing of the response is a resource intensive process or that service is linked with another one that is responding very slow. Is there anything on the client side that will somehow terminate my Promise and determine/force to create another one to ask again for my data?

Someone suggested to upgrade to Observables, and I will do that, but for now I want to keep using Promises, at least for some areas of the code.

Tks a lot

possible duplicate of [Is a promise ever allowed to never resolve or reject?](http://stackoverflow.com/q/34289551/1048572) — Bergi, May 18 '17 at 02:51

score 5 · Accepted Answer · answered May 17 '17 at 15:26

5

A Promise can be in pending state as long as the page is loaded.

You can wrap the call in another Promise where you introduce a timeout like shown in

let wrappingPromise = new Promise((resolve, reject) => {
  var error = false;
  setTimeout(function(){
    reject("some error"); 
  }, 3000);
  this.http.get(...).toPromise().then(res => {
    if(!error) {
      resolve(res.json);
    }
  });
});

This will cause an error when the timeout is reached. It will still wait to receive the full response. An Observable might be able to forward a cancellation and close the connection, so that the result isn't even received anymore when the timeout is reached. This might depend on whether the concrete implementation and the browser used browser API supports that.

answered May 17 '17 at 15:26

Günter Zöchbauer

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When you say 'as long as the page is loaded' you basically mean as long as I don't reload the entire app again (refresh the page)? Being a call to http services, that's something that I do in services (singletons most of the times) and some of them are loaded when the app starts. That means that the Promise can live as long as the singleton is there, or it's life time can span more than of the service that created it? Is there any garbage collection strategy behind this? – orouwk May 18 '17 at 08:57
Browsers have some timeout for HTTP requests http://stackoverflow.com/questions/7297833/how-long-will-the-browser-wait-after-an-ajax-request. When this happens the Future will complete with an error. Until then, it will be in the memory. – Günter Zöchbauer May 18 '17 at 09:10

score 5 · Answer 2 · answered May 18 '17 at 02:27

new Promise(() => {}) will never settle, like a callback never called.

A promise is a return object you attach callbacks to, instead of passing callbacks into the function. That's all. It is not a control surface of the asynchronous operation that was just started.

Instead, look to the asynchronous API you called for such controls, if it has them.

Creating promises

Most people are consumers of promises returned from asynchronous APIs. There's no reason to create a Promise other than to wrap a legacy callback API. In an ideal world there'd be no need.

How long a Promise can remain in pending state?

2 Answers2

Creating promises

Linked