Check whether callback is actually a function

[patrickvuarnoz]

I ran into a problem where I got an error message like TypeError: Cannot read properties of undefined (reading 'apply') in E.emit. This was due to a code change where I changed the name of an event handler but forgot to change it also in the call to emitter.on().

It took me quite some time to find the problem in the code as it is not obvious from the error message which event handler (call to on()) caused the problem.

It would be great if the emitter would check in the on() and once() methods whether callback is actually a function and if not throw an error. This way it would directly indicate which portion of the code was trying to register a non-callable event handler. Something like:

if (typeof callback !== 'function') throw new Error('Callback is not a function');

The issue is easy to reproduce:

emitter.on('some-event', undefined);
emitter.emit('some-event');

emitter.on('some-event', 'string');
emitter.emit('some-event');