Define the toString method on an object's prototype.

This allows a custom string representation to be provided for data objects, which is excellent for debugging - similar to Rust's Debug and Display.

Replace string with a function that takes object to get dynamic behavior (i.e. complete parity with Display/Debug).

Also an example of how to do mixins in JavaScript without using classes.

Mixins are a very useful pattern from languages with multiple inheritance. They make OOP quite tolerable in e.g. Python.

Making TypeScript correctly recognize these left as exercise to reader.