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TypeScript is JavaScript with a type system bolted on — and it's taken over the web development world. Every variable, function parameter, and return value can have a declared type, and the TypeScript compiler catches mismatches before your code ever runs. If you've spent an afternoon debugging `undefined is not a function`, TypeScript exists to prevent that.
Microsoft released TypeScript in 2012, and by now it's the default for serious JavaScript projects. Angular requires it. React and Vue strongly recommend it. Node.js backends increasingly use it. The language adds interfaces, enums, generics, and type inference to JavaScript while compiling away to clean JS that runs anywhere JavaScript runs.
Open .ts files in any text editor — they're plain text. VS Code (also from Microsoft, not coincidentally) has first-class TypeScript support with real-time type checking, auto-completion, and inline error reporting. To compile: `npx tsc file.ts` produces a .js file. In practice, most projects use a bundler (Vite, webpack) that handles compilation automatically.