Is it safe to resolve a promise multiple times?
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.
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));
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);
}
});