Conditional Error Handling in Javascript

Question

When deciding if to ignore, handle of re-throw errors in a Try Catch structure, what is the best way to uniquely identify the exact error?

I can't find any standard error number, do I have to parse the name and message properties?

EDIT: In the example below, I want to check if the error is due to the lack of a property on listEvent who's key is the value of evnt.type. If this is the source of the error, I want to ignore it, otherwise I want to re-throw it to let it bubble up.

The only way I can think of is sort of duck-typing the error like this...

try{
    listEvent[evnt.type](procedure)
}catch(error){
    if (error.message.match(evnt.type)) {
        console.log(error.name + ' ' + error.message + ' - ignored')
    } else {
        throw error
    }
}

Answer 1

Create a custom exception and then use instanceof:

function IllegalArgumentException(message) {
    this.message = message;
}

You also want to make it extend the Error prototype:

IllegalArgumentException.prototype = new Error();
IllegalArgumentException.prototype.constructor = Error;

Which you can then use like so:

throw new IllegalArgumentException("Argument cannot be less than zero");

You can then check the type using instanceof:

try {
    // Some code that generates exceptions
} catch (e) {    
    if (e instanceof IllegalArgumentException) {
        // Handle this
    } else if (e instanceof SomeOtherTypeOfException) {
        // Handle this
    }
}

You can also add any other property you want to the constructor of the exception.

As far as your example goes, I'm not sure what you're trying to do. listEvent[evnt.type] will return undefined if event.type is not a property or a key in listEvent. It is better to see if evnt.type even exists by doing something like this:

if (typeof listEvent[evnt.type] !== "undefined") {
   listEvent[evnt.type](procedure);
}