Interfaces

Define an interface

An interface lists required property names and types. Objects that match the shape are accepted.

interface User {
  name: string;
  age: number;
}

const learner: User = { name: "Ada", age: 28 };
console.log(learner.name);

Mark optional properties

A trailing ? means the property may be missing.

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

const draft: Course = { title: "TypeScript", lessons: 18 };

Reuse shapes in functions

Pass the interface as a parameter type so callers must provide the right fields.

function greet(user: User): string {
  return `Hello, ${user.name}`;
}

console.log(greet({ name: "Sam", age: 20 }));

Interface habits

• Name interfaces after the data they describe
• Prefer interfaces for object shapes you reuse
• Type aliases (type) also work; interfaces excel at extendable objects
• Excess property checks catch typos on object literals