What is TypeScript and what does it add to JavaScript?

TypeScript adds an optional static type layer on top of JavaScript. Types are checked during development and then erased during compilation, so the browser or Node.js runs the same JavaScript it would have run without TypeScript. The value is earlier feedback, safer refactors and better editor support.

type User = {
  id: number;
  name: string;
};

function label(user: User): string {
  return "User " + user.id + ": " + user.name;
}

const user: User = { id: 1, name: "Asha" };
console.log(label(user));

What the compiler gives you

• Type errors for mismatched shapes, missing properties and invalid calls
• Editor completion, go-to-definition and safe rename across files
• Downleveling of modern syntax to the target configured in tsconfig
• A machine-checkable description of module boundaries and public APIs

The type system is structural: two values are compatible when their shapes are compatible, regardless of where they were declared. That makes it natural to type plain objects, functions and third-party data without forcing a class hierarchy.