What is TypeScript?

TypeScript is a superset of JavaScript that adds static type annotations. Every valid JavaScript program is also valid TypeScript, but TypeScript catches errors at compile time that JavaScript would only catch at runtime. Type annotations describe the shape of your data: function add(a: number, b: number): number tells the compiler (and your IDE) that add takes two numbers and returns a number. If you try to pass a string, TypeScript flags the error before your code runs.