TypeScript is an open source programming language created by Microsoft in 2012, which adds a static type system to JavaScript. TypeScript code is converted into standard JavaScript that runs in any browser or in an environment such as Node.js.
Types describe the shape of data: what a function expects, what it returns, which fields an object contains. The compiler flags inconsistencies before the code runs, the editor offers precise autocompletion, and changes across a large codebase become much safer. Typing is gradual: an existing JavaScript project can be migrated file by file.
TypeScript has become the default choice for professional JavaScript projects, on the front end (React, Angular, Vue) as well as on the back end (Node.js, Deno, Bun).
Why it matters when hiring
Almost every JavaScript developer claims to know TypeScript: the mention on a resume says little. The real level shows in how they model data (precise types rather than "any" everywhere, generics, discriminated unions) and in their ability to explain what TypeScript does not guarantee at runtime. Have them read a typed code excerpt and ask what could break.