(I’m not completely happy with the answers so far, so here is my attempt…) I think that Kevin Wright’s comment You can make a Promise and it’s up to you to keep it. When someone else makes you a promise you must wait to see if they honour it in the Future summarizes it pretty … Read more
The return purpose is to terminate the execution of the function after the rejection, and prevent the execution of the code after it. function divide(numerator, denominator) { return new Promise((resolve, reject) => { if (denominator === 0) { reject(“Cannot divide by 0”); return; // The function execution ends here } resolve(numerator / denominator); }); } … Read more
In simple terms, inside a then handler function: A) When x is a value (number, string, etc): return x is equivalent to return Promise.resolve(x) throw x is equivalent to return Promise.reject(x) B) When x is a Promise that is already settled (not pending anymore): return x is equivalent to return Promise.resolve(x), if the Promise was … Read more
Update 2017: I would use an async function if the environment supports it: async function readFiles(files) { for(const file of files) { await readFile(file); } }; If you’d like, you can defer reading the files until you need them using an async generator (if your environment supports it): async function* readFiles(files) { for(const file of … Read more
The problem here is that you are trying to await an array of promises rather than a Promise. This doesn’t do what you expect. When the object passed to await is not a Promise, await simply returns the value as-is immediately instead of trying to resolve it. So since you passed await an array (of … Read more
I understand the situation a bit better now (in no small amount due to the answers here!), so I thought I add a little write-up of my own. There are two distinct, though related, concepts in C++11: Asynchronous computation (a function that is called somewhere else), and concurrent execution (a thread, something that does work … Read more
simple: var promiseResolve, promiseReject; var promise = new Promise(function(resolve, reject){ promiseResolve = resolve; promiseReject = reject; }); promiseResolve();
How a promise library decides If it has a .then function – that’s the only standard promise libraries use. The Promises/A+ specification has a notion called thenable which is basically “an object with a then method”. Promises will and should assimilate anything with a then method. All of the promise implementation you’ve mentioned do this. … Read more
Promises are not callbacks. A promise represents the future result of an asynchronous operation. Of course, writing them the way you do, you get little benefit. But if you write them the way they are meant to be used, you can write asynchronous code in a way that resembles synchronous code and is much more … Read more
The callbacks attached to done() will be fired when the deferred is resolved. The callbacks attached to fail() will be fired when the deferred is rejected. Prior to jQuery 1.8, then() was just syntactic sugar: promise.then( doneCallback, failCallback ) // was equivalent to promise.done( doneCallback ).fail( failCallback ) As of 1.8, then() is an alias … Read more