Context

The keyword this refers to the "current execution context", or just the "context" for short.

We mostly use this in classes — its behavior is typically not helpful and somewhat error-prone elsewhere.

Global this

Outside of any function, the current execution context is an environment-specific global variable (window in the browser, global in node).

Function this

Inside a function declared with function, this refers to an arbitrary value set by the caller.

Inside a function declared with =>, this refers to whatever it was in the parent scope.

We can specify the this value with .call().

Automatic this

Functions that are properties of objects will have their this set to the object when called with . syntax. This applies to instances of classes too.

This is the main use case for this. Depending on your TypeScript compiler settings, you can force the type of this to be defined, which helps avoid errors.