The `message` field value is not set in `CustomError` class in NodeJS with the classical inheritance through the function-constructor

Question

This case works just fine:

class CustomError2015 extends Error {
    constructor(message) {
        super(message); // here the message is set correctly
        console.log("Message: " + this.message); 
    }
}

throw new CustomError2015("ECMAScript 2015 class inheritance");

I expected that this one would work the same way, but it didn't:

function CustomError(message){
    Error.call(this, message); // here the message IS NOT set
    console.log("Message: " + this.message); 
}
CustomError.prototype = Object.create(Error.prototype);
CustomError.prototype.constructor = CustomError;

throw new CustomError("CustomError function-like inheritance");

I'm wondering why? I there something wrong with my code or what?

^{* These code samples' playground.}

Builtins like `Error` cannot be extended (easily) in ES5. – Bergi Dec 09 '18 at 14:47 — Bergi, Dec 09 '18 at 14:47

Jonas Wilms · Accepted Answer · 2018-12-09T14:48:11.783

0

Built in types (Error, Array) are exotic, that means that their constructor is not normal, and the objects of their instance are not normal, they are special internal objects. Therefore this:

  Error.call(this, message)

does not work as Error has to return an exotic object, while this is a regular object, and the engine can't convert one into the other. Therefore it returns a new exotic error (which you ignore in your code):

  let exotic = Error.call(this);
  exotic === this // false

The message gets set on that exotic object, and not on this.

That works in classes as super(....) gets called before you can access this, and therefore this can be the exotic object, you cannot replicate that behaviour without classes.

read on

edited Dec 09 '18 at 14:48

answered Dec 09 '18 at 14:45

Jonas Wilms

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Since when are errors exotic? They don't have any weird internal slots, do they? Sure, `Error.call` does not work, but that doesn't have anything to do with exotic vs. non-exotic objects. – Bergi Dec 09 '18 at 14:47
@bergi okay maybe not exotic, but *special* ? :) – Jonas Wilms Dec 09 '18 at 14:49
What's interesting is that I read about it not so long ago :) – Serg Dec 09 '18 at 14:52

The `message` field value is not set in `CustomError` class in NodeJS with the classical inheritance through the function-constructor

1 Answers1