class Foo extends Error {

    type: string;

    constructor(type: string, message: string) {
        super(message);

        // Clean up the changes made by super()
        this.constructor = Foo;
        if ((Object as any).setPrototypeOf) {
            (Object as any).setPrototypeOf(this, Foo.prototype);
        } else {
            (this as any).__proto__ = Foo.prototype;
        }

        // Only reference props on this after this point for safety

        this.type = type;
    }

    doStuff() {
        return "I am a " + this.type + " type of error."
    }
}

let e = new Foo("dramatic", "Goodbye, cruel world");

console.clear();
console.log('e instanceof Error', e instanceof Error);
console.log('e instanceof Foo  ', e instanceof Foo);
console.log('e.type            ', e.type);
console.log('typeof e.doStuff  ', typeof e.doStuff);
console.log('');