Using promises – Logging stack trace in fail handler

Using promises – Logging stack trace in fail handler

Problem

I am rather new to nodejs so I will explain in a bit more detail what I am trying to do.

I have a webserver. If a request fails I want to log the stack trace of that exception, but deliver a error page and not crash the server.

As an example, the function handling the requests:

var Q = require('q');

var requestHandler = function () {
    // Here I get the data etc. that was requested. As this is not important, just a dummy here
    Q.resolve()
        // Now I answer the request
        .then(function (data) {
            // Dummy Code representing the sending of a response
            console.log('sending response …');
            console.log('oh no! an exception');
            // Now an Exception occurs
            throw new Error('You did something wrong!');
        })
        // If there was any error, show the error page instead
        .fail(function (err) {
            // Dummy Code representing the sending of the error page
            console.log('sending error page');
            // Additionally I want to write the error into a log file
            console.error('You had an error: ', err);
        })
        // If there was an error in the .fail-handler, I deserve the server to crash
        .done();
};

// A request comes in, I want to call my handler
requestHandler();

The console’s output:

sending response …
oh no! an exception
sending error page
You had an error:  [Error: You did something wrong!]

I can’t see a way to access the stack trace. But when I throw an exception in the .fail-handler (or just omit the complete .fail-handler), I get a stack trace on the console (but I’d have to restart the server).

So I guess my question is:

How do I access the stack trace in a promise fail handler?

EDIT: Any tips on how to improve the explanation are welcome, of course. If I didn’t make myself clear, please let me know.

Problem courtesy of: dave

Solution

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);
Solution courtesy of: Mariusz Nowak

Discussion

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