How to create Node.js custom errors and have the stack property return after res.json()

Question

I’m attempting to create custom error objects in Node that use Error as the prototype and then to return these errors to the client via the express res.json() method. I can create an instance of a custom error, set it’s properties but when I call res.json(), the stack property is missing. I believe this is due to JSON.stringify() not following prototypes?

I can’t see a way of creating custom error objects with an arbitrary number of properties and then use JSON.stringify() on these objects to return all the properties and the base class / superclass Error stack property. Please help! An example is below:

(function (customErrorController) {


    function CustomError() {

        this.name = 'CustomError'; // String
        this.message = null; // String
        this.errorCode = null; // String
        // this.stack exists as CustomError inherits from Error
    }

    CustomError.prototype = new Error();


    customErrorController.init = function (app) {

        app.get('/test', function (req, res) {

            var customError = new CustomError();
            customError.message = 'The Message';
            customError.errorCode = 'Err001';

            console.log(customError.stack); // This logs the stack fine

            res.json(customError); // The stack is missing when outputting the object as JSON

        });
    };

})(module.exports);

You've got that right, JSON.stringify does not stringify the prototypes. — adeneo
– adeneo, Commented Aug 18, 2014 at 13:13
Notice that you shouldn't print error stacks to the user, but log them into a serverside file and just respond with a 501 code. — Bergi
– Bergi, Commented Aug 18, 2014 at 13:27
Also, you currently are printing the wrong stack - it is created when new Error, not when new CustomError is called. Have a look at code.google.com/p/v8/wiki/JavaScriptStackTraceApi — Bergi
– Bergi, Commented Aug 18, 2014 at 13:29

Jon · Accepted Answer · 2014-08-19 11:10:44Z

Thanks for the comments. From these and from a bit more digging, I’m now using the following code to create custom errors and have the correct stack on the stack property return after calling res.json() with the error:

// Redefine properties on Error to be enumerable
Object.defineProperty(Error.prototype, 'message', { configurable: true, enumerable: true });
Object.defineProperty(Error.prototype, 'stack', { configurable: true, enumerable: true });


(function (customErrorController) {

    function CustomError(message, errorCode) {

        Error.captureStackTrace(this, CustomError);

        this.name = 'CustomError'; // String
        this.message = message; // String
        this.errorCode = errorCode; // String
    }

    CustomError.prototype = Object.create(Error.prototype);

    customErrorController.init = function (app) {

        app.get('/test', function (req, res) {

            var customError = new CustomError('The Message', 'Err001');
            res.json(customError); // The stack property is now included

        });
    };

})(module.exports);

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