What is TypeScript and why do we use it?
Quick answer
TypeScript is a typed superset of JavaScript that compiles to plain JavaScript. It catches type errors in the editor and at build time, and it documents the shape of data for the next person who reads the code.
Every valid JavaScript file is already TypeScript, which is why teams can adopt it gradually. The compiler (tsc) erases types and emits JS that runs anywhere JavaScript runs. Types exist only at compile time: they are not available for runtime checks unless you add a library or generate them.
The usual wins are safer refactors, better editor autocomplete, and APIs that are harder to call wrongly. The cost is a build step and time spent modelling awkward JavaScript patterns. Interviews often follow this with “when would you still use JavaScript?” — small scripts, prototyping, or a codebase that is not yet worth converting.