承诺-在deferred.reject期间出现包装错误

Question

I'm trying to create a deferred wrapper using q.js such that I can wrap errors (strings) in a custom error class before they're passed back by the promise in then() or fail() . This is what I'm doing at the moment:

var getDeferred = function() {

    var deferred = q.defer();

    var reject = deferred.reject;
    deferred.reject = function(error) {

        if (!(error instanceof MyErrorClass))
            error = new MyErrorClass(error)

        return reject.apply(deferred, arguments);
    }

    return deferred;
}

So the idea is that the user would do something like

var deferred = getDeferred();

deferred.promise.fail(function(err) {
    // err should now be instance of MyErrorClass and NOT a string
})

deferred.reject('A string error')

And expect to get MyErrorClass in the fail() handler, rather than the string passed to deferred.reject .

The above code works, but it's hardly ideal -- I know I shouldn't be monkey-patching deferred.reject. But is there a better way to do this?

Answer 1

It's prettier / more promise oriented like this :

var getDeferred = function() {

    var deferred = q.defer();

    deferred.promise = deferred.promise.then(null, function(error) {
        if (!(error instanceof MyErrorClass))
            error = new MyErrorClass(error)

        throw error
    }

    return deferred;
}

This way you just attach an error handler which will mutate any non MyErrorClass errors. It seems like an odd use case in general though...

Answer 2

Simply map over the fail case with then :

actualPromise.then(null, function(error) {
    if (!(error instanceof MyErrorClass))
        error = new MyErrorClass(error)
    throw error;
})
.fail(function(err) {
    // err is now an instance of MyErrorClass and NOT a string
});

If you need a wrapper functionality, use a function that takes the actualPromise and calls then with the above code on it.