microsoft/typescript

TypeScript is a programming language created by Microsoft that builds on top of JavaScript. JavaScript is the language that runs in every web browser and on backend servers via Node.js, but it doesn't natively check whether the values flowing through your code match the kinds of data you actually expect. TypeScript adds optional types to JavaScript, which lets editors and tooling catch many bugs before the code ever runs. The README describes it as a language for application-scale JavaScript. The way it works is by acting as a compiler. You write code in TypeScript files, the compiler checks the types and reports any mismatches, and then it produces clean JavaScript output that can run anywhere ordinary JavaScript runs, in any browser, on any host, on any operating system. Because the produced output is standards-based JavaScript, you don't need a special runtime; existing tools, frameworks, and deployment pipelines all keep working. The official website hosts a Playground where you can try TypeScript directly in the browser. You would use TypeScript when a JavaScript project is growing large enough that silent type mistakes start to slow you down, or when working with a team where having explicit type information makes the codebase easier to navigate and refactor. The README notes that this repository is now restricted to a narrow set of fixes, recent crashes, security issues, language-service crashes, and serious regressions, while most bug fixes are being routed to a separate typescript-go repository, and that feature work is paused until TypeScript 7.0 is completed.