Interfaces and Types

As programs grow, describing the shape of your data becomes just as important as describing its logic. TypeScript gives you two main tools for that: interfaces and type aliases.

interface Course {
  id: string;
  title: string;
  lessons: number;
  published?: boolean; // optional
}

type Role = 'STUDENT' | 'INSTRUCTOR' | 'ADMIN'; // union type

function describe(course: Course, role: Role): string {
  return `${role} viewing "${course.title}" (${course.lessons} lessons)`;
}

console.log(describe({ id: 'c1', title: 'TypeScript from JavaScript', lessons: 12 }, 'STUDENT'));

Interfaces describe object shapes

interface Course { ... } says: "a Course is an object with exactly these properties." Passing an object missing title, or with a lessons that's a string instead of a number, is a compile error. The ? after published marks it optional, an object can be a valid Course with or without it.

Union types restrict the possible values

type Role = 'STUDENT' | 'INSTRUCTOR' | 'ADMIN'; means a Role can only ever be one of those three exact strings, nothing else. Try passing 'ADMINISTRATOR' where a Role is expected, and the compiler catches the typo immediately, something plain JavaScript could never do.