CODEWITHSUNDEEP
node.jsexceptionrequire
Eriedor
Eriedor

Reputation: 411

Nodejs + Require instanceof behaviour

I'm encountering a strange behaviour with instanceof in NodeJs. I'm trying to creating a module which throws exceptions which can be caught by consuming modules and dealt with based on the exception type. However the call to test the type "error instanceof mod.MyException" is always returning false.

mymodule.js: 

function MyException() {}
MyException.prototype = new Error();
MyException.prototype.constructor = MyException;

function Foo() {
    throw new MyException();
}

exports.Foo = Foo;
exports.MyException = MyException;

myconsumingmodule.js

var mod = require('mymodule');

try {
    mod.Foo();
} catch (error) {
    if (error instanceof mod.MyException) { // always returns false
         //do error specific handling (never executes)
         return;
    }
    // generic handling
}

If I do the instanceof test in mymodule.js like so it returns true

mymodule.js

var a = new MyException()
console.log(a instanceof MyException); //true

However if I create an exception in the consuming module and test it it correctly identifies it as MyException, it just doesn't when comparing it against an exception that was created by the other module.

myconsumingmodule.js

var mod = require('mymodule')
var b = new mod.MyException();
console.log(b instanceof mod.MyException); // true

Upvotes: 13

Views: 1762

Answers (2)

soundly_typed
soundly_typed

Reputation: 40386

FOUND IT

To use your example, in file one.js I had:

var mod = require('mymodule');

but in another file two.js I had:

var mod = require('Mymodule');

and everything seemed to work exactly the same, except when you compared an error created from one.js using instanceof in two.js!.

Absurdly, node.js allows you to do a case-insensitive require, except it's not really case-insensitive, it actually requires your file twice, which is why instanceof returns false – the constructors are two different functions from two different module instances!

TL;DR

Make sure all your require strings are case-consistent.

Upvotes: 10

djKartright
djKartright

Reputation: 1067

Cannot reproduce the problem. Working for me using version 0.12 and 4.0.0 (tested both).

What you should consider in future is that you reference your OWN modules using relative paths (absolute ones are not system independent). If you don't, you normally reference core modules of Node.js directly or NPM installed ones. So change 'mymodule' to './mymodule' (change path accordingly).

Upvotes: 0

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