1. Default:
`this` will be window.

2. In a method:
`this` will point to the object method is on.
but once you go into another function,
this will default back to window.

3. Explicitly define `this`.
Use `bind`, `apply`, `call`.

4. When passing in a callback function, value of `this`
inside callback depends on whether the outer function
explicitly sets a `this` value. If not,
it'll default to window.

5. Constructors
`this` points to the newly created object.