You learn why generics exist: a fixed type is too narrow and any loses the link between input and result. You write a generic function and watch the compiler infer its type from the arguments, type arrays two ways, and limit a type parameter with a constraint. You use keyof to accept only an object's keys, build a generic container type, and read the generic types you already use: arrays, promises, records and their nested error messages.

You write classes with fields, constructors and methods, and see what the compiler reports when a field is never set. You compare TypeScript's private with JavaScript's own private fields, and make a field read-only. Then you learn what Node removes when it runs TypeScript, why it refuses parameter properties and enums, how a compiler setting rejects the same syntax, and what to write instead: a constant object and a union type made from its values.

You split code across files with ES modules, import TypeScript files by their full file names, and let the compiler rewrite those imports to JavaScript when it builds. You import types on their own, keep imports exactly as written so the compiler reports what Node would refuse, and catch a typo in a configuration object. Then you build a small typed library from six modules that runs from source, builds a JavaScript and a type file per module, and proves it with its contract check.