Subclassing Error: adding prototype methods does not take effect

Question

I have made a error class hierarchy, starting with a BaseError class that is purely meant for subclassing. I have set the prototype using Object.create(Error.prototype) and I am rewriting the stack using the name of the current subclass. The name is set in a method called configure that needs to be implemented by the subclasses.

The problem is that although I explicitly add the configure method to the prototype of BaseError it is not actually accessible in the prototype chain.

I think it is somehow related to the __proto__ instance property vs the prototype attribute.

This is the code (transpiled from the Typescript definition)

var BaseError = (function () {
    function BaseError(msg) {
        var err = Error.apply(null, arguments);
        this.message = err.message;

        this.configure();

        if (err.stack) {
            this.stack = rewriteStack(err.stack, this.name);
        }

        if (typeof this.name === 'undefined')
            throw new NotImplementedError('must set "name" property in the configure call');
    }

    return BaseError;
})();

Framework.BaseError = BaseError;

// weird stuff need to happen to make BaseError pass an "instanceof Error"
BaseError.prototype = Object.create(Error.prototype);

BaseError.prototype.configure = function () {
    throw new NotImplementedError(+' This method must be implemented in the overriding class!');
};

Bergi · Accepted Answer · 2015-11-20 14:58:12Z

1

You're doing Object.create(BaseError), not Object.create(BaseError.prototype) as you should.

answered Nov 20, 2015 at 14:58

Bergi

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2 Comments

oligofren Over a year ago

Jeebus. Not the first time I have done something like that, either. Thanks. I see someone giving me a close vote. What do you think? Keep it up or not?

Bergi Over a year ago

It was mine :-) I'm glad I could help, but I don't think the question will be helpful to others.

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