Excerpt taken from the official doc on $q: then(successCallback, errorCallback) – regardless of when the promise was or will be resolved or rejected calls one of the success or error callbacks asynchronously as soon as the result is available. The callbacks are called with a single argument the result or rejection reason. This method returns … Read more
Logging the error object won’t print error’s stack trace. You need to ask for it specifically: console.error(‘You had an error: ‘, err.stack);
promiseMeSomething() is going to return a Q promise object, which will have then function in it, which is defined, like this Promise.prototype.then = function (fulfilled, rejected, progressed) { The simplest way to create a Promise object would be to use the Q function constructor, like this new Q(value) will create a new promise object. You … Read more
Since promises can only resolve once (to either fulfilled or rejected), the first resolution wins and any further calls will be ignored. From the docs: In all cases where a promise is resolved (i.e. either fulfilled or rejected), the resolution is permanent and cannot be reset. Attempting to call resolve, reject, or notify if promise … Read more
There’s a nice clean way to to this with [].reduce. var chain = itemsToProcess.reduce(function (previous, item) { return previous.then(function (previousValue) { // do what you want with previous value // return your async operation return Q.delay(100); }) }, Q.resolve(/* set the first “previousValue” here */)); chain.then(function (lastResult) { // … }); reduce iterates through the … Read more
Both are based on the Promises/A standard and implement a then method (though only current jQuery, they once had a incompatible pipe instead of then). However, there are a few differences: Q has exception handling. All thrown errors in the async then callbacks will be caught and reject the promise (and will only get re-thrown … Read more
Given that node is by default only single threaded there isn’t really an easy way to solve this. There is one though. Bases on generators/fibers you can add a sort of concurrent execution to node. There is a waitfor implementation based on that.
Florian provided a good answer For the sake of your original question, there are several ways to start a chain with Bluebird. One of the simplest is calling Promise.resolve() on nothing: var queue = Promise.resolve(); //resolve a promise with nothing or cast a value or Promise.try(function(…){ return …//chain here }); So you can do: var … Read more
This is a case where you will need to branch, which does mean either nesting or creating a subroutine. function doTask(task, callback) { return Q.ncall(task.step1, task) .then(function(result1) { if (result1) return result1; return Q.ncall(task.step2, task) .then(function(result2) { return Q.ncall(task.step3, task); }) }) .nodeify(callback) } Or function doTask(task, callback) { return Q.ncall(task.step1, task) .then(function(result1) { if … Read more
Update: jQuery 3.0 has fixed the problems outlined below. It is truly Promises/A+ compliant. Yes, jQuery promises have serious and inherent problems. That said, since the article was written jQuery made significant efforts to be more Promises/Aplus complaint and they now have a .then method that chains. So even in jQuery returnsPromise().then(a).then(b) for promise returning … Read more