CODEWITHSUNDEEP
javascriptnode.jsexpressbluebird
Nepoxx
Nepoxx

Reputation: 5358

Thrown error in promisified ExpressJs function

I'm not quite sure I understand how errors are handled in promises (and I'm fairly new to promises so I'm likely not using them to their full potential).

I have the following code:

app.listenAsync = Promise.promisify(app.listen);

app.listenAsync(config.port)
.then(function done() {
    console.log("We're listening!");
})
.catch(function (err) {
    console.log("Abort abort!");
});

For a reason I don't quite understand, my catch() is never called, even if app.listenAsync throws an error, EADDRINUSE for instance. Why?

Edit: I just noticed that if I do

var listenAsync = Promise.promisify(app.listen);

listenAsync(config.port)
    .then(function done() {
        console.log("We're listening!");
    })
    .catch(function (err) {
        console.log("Abort abort!");
    });

Gives almost the correct behaviour. listenAsync throws an error (that is caught in the catch statement this time) Possibly unhandled TypeError: listener must be a function. Am I missing something?

Upvotes: 2

Views: 817

Answers (1)

Esailija
Esailija

Reputation: 140236

This has nothing to do with promises, it's just basic javascript: you are calling listen as if it was a function but it's a method of app.

Some options:

Promise.promisifyAll(app);
// Note how it's called as a method on app
app.listenAsync(...).then(....)

Or

// Bind the function as a method of app
var appListenAsync = Promise.promisify(app.listen, app);
appListenAsync(...).then(....)

Upvotes: 4

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