How do I create a custom Error in JavaScript?

Question

For some reason it looks like constructor delegation doesn't work in the following snippet:

function NotImplementedError() { Error.apply(this, arguments); }
NotImplementedError.prototype = new Error();
NotImplementedError.prototype.name = 'NotImplementedError';

(function test() {
    function assert(condition, msg) {
        if (!condition) {
            debugger;
            throw new Error('AssertionError: ' + msg);
        }
    }
    function assertEquals(candidate, target, msg) {
        var newMsg = ['candidate: "', candidate.toString(), '"; ',
            'target: "', target.toString(), '"; ',
            msg].join('')
        assert(candidate === target, newMsg);
    }
    var msg = 'too lazy to implement';
    var nie = new NotImplementedError(msg);
    assertEquals(nie.message, msg, 'Message must be present');
    assert(nie instanceof Error, 'Must be Error subclass');
    assert(nie instanceof NotImplementedError, 'Must be NotImplementedError instance');
    // TODO: assertRaises
})();

A run through js.jar fails the test:

$ java -jar js.jar test.js
js: "test.js", line 9: exception from uncaught JavaScript throw: Error: AssertionError: candidate: ""; target: "too lazy to implement"; Message must be present

Any ideas as to why, or if there is a better way to create a new Error subclass? Is there a problem with applying to the native Error constructor that I don't know about?

Answer 1

The constructor needs to be like a factory method and return what you want. If you need additional methods/properties, you can add them to the object before returning it.

function NotImplementedError(message) { return new Error("Not implemented", message); }

x = new NotImplementedError();

Though I'm not sure why you'd need to do this. Why not just use new Error... ? Custom exceptions don't really add much in JavaScript (or probably any untyped language).

Answer 2

Update your code to assign your prototype to the Error.prototype and the instanceof and your asserts work.

function NotImplementedError(message) {
    this.name = "NotImplementedError";
    this.message = (message || "");
}
NotImplementedError.prototype = Error.prototype;

However, I would just throw your own object and just check the name property.

throw {name : "NotImplementedError", message : "too lazy to implement"};

Answer 3

cdleary: Solution at http://blog.getify.com/2010/02/howto-custom-error-types-in-javascript/