Interfaces

Interfaces

What it means

An interface describes the shape a value must have — which properties it needs, and their types — without providing any implementation. It’s a compile-time-only contract: any object matching the shape satisfies the interface, regardless of how it was created (this is called “structural typing,” unlike the nominal typing of languages like C# or Java).

Examples

interface User {
  name: string;
  age: number;
  email?: string;   // optional property
}

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

// No class needed -- a plain object matching the shape satisfies the interface
const alice: User = { name: "Alice", age: 30 };
greet(alice);   // works

// Interfaces can extend other interfaces
interface Employee extends User {
  employeeId: string;
}

// Interfaces can also describe function shapes
interface Comparator {
  (a: number, b: number): number;
}
const byAge: Comparator = (a, b) => a - b;

Common mistake

Assuming an interface enforces that a value was created through a specific constructor or class — TypeScript’s structural typing means any object with a matching shape satisfies the interface, even if it has extra properties or came from somewhere else entirely.

interface Point { x: number; y: number; }

function distance(p: Point) {
  return Math.sqrt(p.x ** 2 + p.y ** 2);
}

// Any object shaped like a Point works, even with extra properties
distance({ x: 3, y: 4, z: 5, label: "origin" });   // fine -- structurally compatible

// This surprises developers coming from nominally-typed languages,
// where you'd expect an explicit "implements Point" relationship

Quick practice

1. Does an object need to explicitly declare implements SomeInterface to satisfy that interface in TypeScript?

   Answer

   No — TypeScript uses structural typing, so any object with a matching shape satisfies the interface automatically, with no explicit declaration required.

2. What does a ? after a property name in an interface mean?

   Answer

   The property is optional — objects satisfying the interface may omit it.

3. Can an interface describe a function’s call signature, not just an object’s properties?

   Answer

   Yes — interface Comparator { (a: number, b: number): number; } describes a callable shape, useful for typing callbacks.