What is TypeScript?

TypeScript is JavaScript with a type system added on top. It’s a superset of JavaScript — every valid JS file is also valid TypeScript — but it lets you declare data types so the compiler can catch mistakes before the code runs, instead of at runtime. This site (and its .astro components) are written in TypeScript.

The problem it solves

Plain JavaScript doesn’t check types, so mistakes like a mistyped field, passing the wrong argument type, or forgetting to handle null only surface when the code actually runs — or worse, once it’s already in production. TypeScript catches most of these while you’re still writing the code, right in the editor.

Basic syntax

// Type-annotate variables, parameters, return values
function greet(name: string): string {
  return `Hello, ${name}`;
}

// Interface / type describing an object's shape
interface Post {
  title: string;
  pubDate: Date;
  tags: string[];
}

// Union type — a value that can be one of several types
type Locale = "en" | "vi";

Type inference

You don’t always need explicit annotations — TypeScript infers types from the assigned value:

const count = 3; // TypeScript infers this as number

Why it’s used here

Astro Content Collections use Zod schemas to validate Markdown frontmatter and auto-generate types for post.data.title, post.data.pubDate, etc. — a typo in a field name fails at build time, not after the site is already deployed.
Shared helpers like src/lib/content.ts declare explicit types (Locale, CollectionEntry<"blog">…), giving accurate autocomplete when writing new components.
npm run build always runs astro check first — a type error stops the build instead of silently shipping broken code.

When TypeScript pays off