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TypeScript6.0

TypeScript Introduction

TypeScript is a superset of JavaScript that adds static types. You write types, TypeScript checks them at compile time, and it compiles down to plain JavaScript that runs anywhere JS does — browsers, Node, everywhere.

Why add types?

  • Catch bugs as you type — typos, wrong arguments, and null mistakes surface *before* running the code.
  • Great editor support — autocomplete, inline docs, and safe rename across a whole codebase.
  • Self-documenting — types describe what a function expects and returns.
  • Scales — large codebases and teams stay maintainable and refactorable.

It's a superset

Every valid JavaScript file is already valid TypeScript — so you can adopt it gradually, adding types where they help most. TypeScript adds a *type layer* on top of JS; it doesn't change how the language runs.

Types disappear at runtime

Warning: Types are a compile-time feature only — they're erased when TypeScript compiles to JavaScript. There's no type checking at runtime, so data crossing your app's boundary (API responses, form input) still needs runtime validation (covered in the Zod chapter).

How it runs

flow
hello.ts  ── tsc (compile) ──▶  hello.js  ──▶  runs in Node/browser
# types are checked during compile, then stripped out
Note: New in 2026: TypeScript 7.0 ships a Go-native compiler that type-checks roughly 8–12× faster and is strict by default. The type system and syntax are unchanged — everything in this course applies.
Tip: Think of TypeScript as JavaScript plus a safety net you opt into. Start by renaming a .js file to .ts and adding types where the payoff is highest — function signatures and data shapes — then tighten from there.
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TypeScript Introduction — TypeScript | Full Stack Learning Simplified