As you see, the race() will return the promise instance which is firstly resolved or rejected: var p1 = new Promise(function(resolve, reject) { setTimeout(resolve, 500, ‘one’); }); var p2 = new Promise(function(resolve, reject) { setTimeout(resolve, 100, ‘two’); }); Promise.race([p1, p2]).then(function(value) { console.log(value); // “two” // Both resolve, but p2 is faster }); For a scenes … Read more
There are of course legit reasons to force synchronous execution of a promise in js. The most obvious is when require‘ing a file that needs to initialize using some asynchronous stuff, and optimization is not an overriding concern. Seems like the package synchronized-promise seems like it ought to do the trick. In your case (warning … Read more
The then callback gets called because the catch callback is before it, not after. The rejection has already been handled by catch. If you change the the order (i.e. (promise.then(…).catch(…))), the then callback won’t be executed. MDN says that the .catch() method “returns a new promise resolving to the return value of the callback”. Your … Read more
function test() { return new Promise((resolve, reject) => { return reject(‘rejected’) }) } test().then(function() { //here when you resolve }) .catch(function(rej) { //here when you reject the promise console.log(rej); });
Preface (you probably know this!): await is syntactic sugar for using promise callbacks. (Really, really, really good sugar.) An async function is a function where the JavaScript engine builds the promise chains and such for you. Answer: The relevant thing isn’t so much whether the promise is settled, but whether the promise callbacks (and the … Read more
This is a classic example where inverting your logic makes it much clearer. Your “race” in this case is that you want your rejection behavior to in fact be success behavior. function oneSuccess(promises){ return Promise.all(promises.map(p => { // If a request fails, count that as a resolution so it will keep // waiting for other … Read more
How do I unwrap/yield the value inside that promise You can do it with async/await.Don’t be fooled into thinking that you just went from async to sync, async await it is just a wrapper around .then. functionA(): Promise<string> { // api call returns Promise<string> } async functionB(): Promise<string> { const value = await this.functionA() // … Read more
Yes, you can do that, and it will run the two asynchronous functions in parallel. You’ve just created a promise and thrown it away. However, this means that when the promise is rejected you won’t notice. You’ll just get an unhandledRejection eventually which will crash your process if not handled. Is this OK? How can … Read more
I would do: async function asyncFunc() { try { await somePromise(); } catch (error) { throw error; } } But I think it comes to personal preference I guess? You could always return Promise.reject(new Error(error));.
Why “1” is after “b”? The promise specification states that all promise .then() handlers must be called asynchronously after the call stack has emptied. Thus, both a and b, which are executed synchronously on the call stack will execute before any .then() handlers so 1 will always be after a and b. Some interesting reading: … Read more