Question

Is there a clean way to somehow use underscore.js _.extend function (or any other) to create custom Error classes that inherit from base Error class? I’m looking for a backbone-like way to do this.

Tried this :

InternalError = function(message, args) {
    message || (message = {});
    this.initialize(message, args);
};
_.extend(InternalError.prototype, Error.prototype, {
    initialize: function(message, args) {
        this.message = message;
        this.name = 'InternalError';
    }
});

var error1 = new Error('foo');
var error2 = new InternalError('bar');
console.warn(error1, error2);
throw error2;

But it is not working :(.

Answer 1

(Excuse my small parenthesis about prototypal inheritance. You can skip this and see the answer below.

In order for an object to extend another one, the child‘s prototype must be an instance of it’s parent. You can find many good resources on the web about this, but, unfortunately, many bad ones as well, so I recommend you take a peak at this article : http://javascript.crockford.com/prototypal.html .
A new object instantiated via the new keyword : new f() returns a copy of it’s prototype object : f.prototype. Acknowledging this, you realize that in order to extend an object x, your current object’s prototype must be a new x instance :

function Person(){};
Person.prototype.speak = function(){
    alert("I'm a person");
}
function StackoverflowUser(){};
StackoverflowUser.prototype = new Person();
// now StackOverflowUser is a Person too

)

you don’t actually need underscore.js for that :

var InternalError = function(msg,args){
    return this.initialize(msg||{},args);
}

// inherit from the Error object
InternalError.prototype = new Error();

// overwrite the constructor prop too
InternalError.constructor = InternalError;
InternalError.prototype.initialize = function(msg,args){
    this.message = msg;
    this.name = 'InternalError';
}

var err = new InternalError("I'm an internal error!");
alert(err instanceof Error); // true
throw err;

if you really want to use underscore.js :

var InternalError = function(msg,args){
    return this.initialize(msg||{},args);
}
_.extend(InternalError.prototype,new Error(),{
    initialize : function(msg,args){
        this.message = msg;
        this.name = 'InternalError';
    },
    constructor : InternalError
});