Is it safe to resolve a promise multiple times?

Question

As I understand promises at present, this should be 100% fine. The only thing to understand is that once resolved (or rejected), that is it for a defered object - it is done.

If you call then(...) on its promise again, you immediately get the (first) resolved/rejected result.

Additional calls to resolve() will not have any effect.

Below is an executable snippet that covers those use cases:

var p = new Promise((resolve, reject) => {
  resolve(1);
  reject(2);
  resolve(3);
});

p.then(x => console.log('resolved to ' + x))
 .catch(x => console.log('never called ' + x));

p.then(x => console.log('one more ' + x));
p.then(x => console.log('two more ' + x));
p.then(x => console.log('three more ' + x));

I faced the same thing a while ago, indeed a promise can be only resolved once, another tries will do nothing (no error, no warning, no then invocation).

I decided to work it around like this:

getUsers(users => showThem(users));

getUsers(callback){
    callback(getCachedUsers())
    api.getUsers().then(users => callback(users))
}

just pass your function as a callback and invoke it as many times you wish! Hope that makes sense.

There s no clear way to resolve promises multiple times because since it's resolved it's done. The better approach here is to use observer-observable pattern for example i wrote following code that observes socket client event. You can extend this code to met your need

const evokeObjectMethodWithArgs = (methodName, args) => (src) => src[methodName].apply(null, args);
    const hasMethodName = (name) => (target = {}) => typeof target[name] === 'function';
    const Observable = function (fn) {
        const subscribers = [];
        this.subscribe = subscribers.push.bind(subscribers);
        const observer = {
            next: (...args) => subscribers.filter(hasMethodName('next')).forEach(evokeObjectMethodWithArgs('next', args))
        };
        setTimeout(() => {
            try {
                fn(observer);
            } catch (e) {
                subscribers.filter(hasMethodName('error')).forEach(evokeObjectMethodWithArgs('error', e));
            }
        });

    };

    const fromEvent = (target, eventName) => new Observable((obs) => target.on(eventName, obs.next));

    fromEvent(client, 'document:save').subscribe({
        async next(document, docName) {
            await writeFilePromise(resolve(dataDir, `${docName}`), document);
            client.emit('document:save', document);
        }
    });