I have a nodeJS project where I wish to use asynchronous functions. Namely, I'm using the Q library.
I have a function called someFunction(), that I wish to return a promise. With the then -function, I can then check whether the promise was resolved or rejected like so:
someFunction()
.then(
function(results) {
console.log("Success!");
},
function (error) {
console.log("An error occurred, and I would wish to log it for example");
}
);
function someFunction() {
var deferred = Q.defer();
throw new Error("Can't bar.");
deferred.resolve("success");
}
it won't run the error function. Why is that?
Because you synchronously threw an exception, instead of returning a promise. Which you never should.
Isn't the partial point of the promise Q.deferred to catch errors?
No. then
and catch
implicitly catch exceptions in their callbacks, defferreds don't - they're just a (deprecated) API to create promises.
Why should I manually reject every error that happens?
Because asynchronous errors are expected to be passed to callbacks anyway, instead of being thrown.
I know I could set the whole content of someFunction to try/catch clause, and then reject the deferred, but that feels so wrong! There must be a better way!
There is: the Q.Promise
constructor, the standard (ES6) promise creation API. It has the benefit of being able to catch synchronous exceptions from the executor function:
function someFunction() {
return new Q.Promise(function(resolve) {
throw new Error("Can't bar.");
resolve("success");
});
}