CODEWITHSUNDEEP
node.js
Chris
Chris

Reputation: 28064

NodeJS is swallowing my exception thrown from a callback

Assume the following code:

myModule.doStuff(options, function(err, results) {
    if (err) console.log('e', err);
    if (err) throw err;
    console.log(results);
});

The offending code is:

return new Promise(function(resolve, reject) {
    try {
        throw new Error('test error');
        resolve('success');
    } catch (ex) {
        console.log('r', ex);
        reject(ex);
    }
});

And then farther up the chain:

self.adapter
    .collect(self.options)
    .then(function(data) {
        self.callback(null, data);
        if (self.runnable) {
            setTimeout(self.collect.bind(self), 2000);
        }
    }, function(error) {
        self.callback(error);
    });

The 'e [error info here]' statement prints, but the throw statement is apparently ignored. The app happily continues doing whatever else it was instructed to do and does not crash as expected. Why?

Upvotes: 2

Views: 571

Answers (1)

Chris
Chris

Reputation: 28064

I never found the answer with the specific library I was using, but after switching to the Q library, it exposed a crucial step that I may have been missing before also. If there is no reject handler at the top level of the promise chain, the exception may be swallowed. According to the Q documentation, the way around this is to always call .done() at the top of the chain which flushes any pending exceptions back to the caller.

Upvotes: 2

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