What will happen when return new Promise((resolve, reject) => {}) forgot to call either resolve or reject? [duplicate]

Question

This question already has an answer here:

• Are JavaScript forever-pending promises bad? 2 answers

The problem is like this

function demo() {
    return new Promise((resolve, reject) => {
        ...
        // The problem here!!
        //I just found in some rare case we failed to call resolve or reject
    })
}

demo()
    .then(res => {
        console.log('resolve')
        console.log(res)
    })
    .catch(rej => {
        console.log('reject')
        console.log(rej)
    })
    .finally(() => {
        console.log('why')
    })

When I failed to call resolve or reject, even the finally block is not called! Why ?

I had thought it was a bug then I found the original author seemed to do that on purpose that if he did not call either resolve or reject, none of then/catch/finally should be called, i.e. in that case no follow-up action should be taken.

But is this a valid way to handle the situation that no follow-up action should be taken ? Will it cause any trouble ?

----- update -----

Even though my question was marked duplicated I am still not satisfied with the answers I got. Originally I had thought it was a bad idea to let promise stay in pending state forever.

But the answer in that SO said "There should be no side effect."
Does never resolved promise cause memory leak? also said "In short - at least in modern browsers - you don't have to worry about unresolved promises as long as you don't have external references to them". So it seems ok to let promise stay in pending if that is the purpose.

Answer 1

Internally, a promise can be in one of three states:

Pending, when the final value is not available yet. This is the only state that may transition to one of the other two states. Fulfilled, when and if the final value becomes available. A fulfillment value becomes permanently associated with the promise. This may be any value, including undefined. Rejected, if an error prevented the final value from being determined. A rejection reason becomes permanently associated with the promise. This may be any value, including undefined, though it is generally an Error object, like in exception handling.

https://developer.mozilla.org/en-US/docs/Mozilla/JavaScript_code_modules/Promise.jsm/Promise

In your case, the promise is in the pending state and calling demo function will always in wait for the promise status to be fulfilled or rejected.

Answer 2

A promise is always expected to either resolve or reject. If you intend to do a no follow up, you may resolve with an empty dataset or reject with an error code that suits your use case.

Answer 3

You may use Promise.race to check if a promise was finished on time. So if you forgot to call resolve or reject into your promise, then Promise.race still be resolved or rejected after delay.

var promise1 = new Promise(function(resolve, reject) {
    setTimeout(reject, 500);
});

var promise2 = new Promise(function(resolve, reject) {

});

Promise.race([promise1, promise2]).then(function(value) {
  console.log(value);
}).catch(err => console.log('promise rejected'));