How do I extend a host object (e.g. Error) in TypeScript

Update for TypeScript 1.6:

It’s now possible to directly extend from the Error class, the code in my original answer still works, but there’s no longer a need for the export declare class Error.

Original answer:

Most of the answers here don’t meet my requirements. The originally accepted answer doesn’t compile anymore since 0.9.5 with a duplicate identifier exception. And non of them really have a stack trace (a JavaScript issue, not TypeScript).

For me a more elegant solution is:

module YourModule {
    export declare class Error {
        public name: string;
        public message: string;
        public stack: string;
        constructor(message?: string);
    }

    export class Exception extends Error {

        constructor(public message: string) {
            super(message);
            this.name="Exception";
            this.message = message;
            this.stack = (<any>new Error()).stack;
        }
        toString() {
            return this.name + ': ' + this.message;
        }
    }
}

What you can do with it:

• new Exception("msg") instanceof Error == true
• class SpecificException extends Exception
• catch (e) { console.log(e.stack); }

The only limitation I found was that you have to declare it in a module, and cannot make them global. For me this isn’t an issue since I think a module helps in structuring, and they are there in any application I make.

One improvement you could make is strip your custom code from the stack trace, personally I think stacktraces are only for the eyes of developers, and they know where to look, so it’s no big deal for me.