Learn in blocks. Build after every lesson.

TypeScript is a language developed by Microsoft in 2012 — it adds a `type system` on top of JavaScript. It exists to catch mistakes while you type instead of after your code ships: you specify what types your variables, functions, and objects should hold, and the `TypeScript compiler` checks every use of that value against its declared type before compiling ("transpiling") the code down to plain JavaScript that runs anywhere JS already runs.

TypeScript's core primitives are `string` (text), `number` (any numeric value — int, float, hex), and `boolean` (true/false).

Arrays hold multiple values of the same `type` — TypeScript enforces this at compile time.

An interface defines the shape of an object — which properties it has, their `types`, and whether they are `optional` or `readonly`. Interfaces are reusable and can be `extended`.

Functions are reusable blocks of code. In TypeScript you can explicitly type `parameters` and `return types`, preventing errors like passing the wrong argument type or misusing a return value.

Union types let a variable hold values of more than one type using the `|` (pipe) operator.

Classes are blueprints for creating objects. TypeScript adds access modifiers and strict typing on top of JavaScript classes.

Generics let you write reusable code that works with different types while staying type-safe. They use the `<T>` syntax.

Enums let you define a set of named constants, making code more readable and self-documenting.

Type narrowing is how TypeScript figures out a more specific type from a broad one using guards or assertions.

Modules split code into separate files using `export` and `import`.

TypeScript ships with built-in generic helper types: `Partial`, `Required`, `Pick`, `Omit`, `Record`, `ReturnType`, `NonNullable`.

Decorators are functions that wrap other code. They start with `@` and sit above a class, method, property, or parameter.

TypeScript uses `tsconfig.json` to understand how to compile your project. Generate it with `tsc --init`.

TypeScript enhances React with typed `props`, typed `state`, typed `refs`, and typed `events`.

TypeScript works seamlessly with Node.js and Express for type-safe server code.

The deep end of TypeScript: conditional types, mapped types, template literal types, and `infer`.

Use strict mode, unknown, never, discriminated results, exhaustive checks, and safe error boundaries.

Separate compile-time types from untrusted runtime data and validate API boundaries before values enter the application.

Test typed functions, design dependency boundaries, use fakes, and keep domain logic independent from frameworks.

Combine strict types, generics, validation, modules, tests, and framework integration in one production-style application.