How structural typing differs from nominal typing in Typescript - Practice Exercises

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of structural typing example

What is the output of this TypeScript code using structural typing?

Typescript

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

function printPoint(p: Point) {
  console.log(`x: ${p.x}, y: ${p.y}`);
}

const pointLike = { x: 10, y: 20, z: 30 };
printPoint(pointLike);

A x: 10, y: 20, z: 30

B x: 10, y: 20

C Type error: Argument of type '{ x: number; y: number; z: number; }' is not assignable to parameter of type 'Point'.

D Runtime error: Property 'x' is undefined

💡 Hint

Think about how TypeScript checks object shapes rather than exact types.

TypeScript uses structural typing, so extra properties like z do not cause errors if required properties exist.

❓ Predict Output

intermediate

Nominal typing behavior in TypeScript with classes

What is the output or error of this TypeScript code using nominal typing with classes?

Typescript

class User {
  constructor(public name: string) {}
}

class Admin {
  constructor(public name: string) {}
}

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

const admin = new Admin('Alice');
greet(admin);

A Runtime error: Property 'name' is undefined

B Hello, undefined

C Hello, Alice

D Type error: Argument of type 'Admin' is not assignable to parameter of type 'User'.

💡 Hint

Consider how classes create nominal types in TypeScript.

Classes in TypeScript create nominal types, so an instance of Admin is not assignable to User even if they have the same properties.

🔧 Debug

advanced

Identify the error caused by nominal typing

Why does this TypeScript code cause a type error?

Typescript

class Car {
  constructor(public model: string) {}
}

class Truck {
  constructor(public model: string) {}
}

function showModel(vehicle: Car) {
  console.log(vehicle.model);
}

const myTruck = new Truck('Ford');
showModel(myTruck);

A Because myTruck is undefined.

B Because Truck does not have a model property.

C Because showModel expects a string, not an object.

D Because Truck and Car are different nominal types, myTruck cannot be passed to showModel.

💡 Hint

Think about how TypeScript treats class instances as distinct types.

Even though Truck and Car have the same properties, their class types are nominal and incompatible.

🧠 Conceptual

advanced

Key difference between structural and nominal typing

Which statement best describes the key difference between structural typing and nominal typing?

A Structural typing checks if types have the same shape; nominal typing checks if types have the same name.

B Structural typing requires explicit inheritance; nominal typing does not.

C Nominal typing allows more flexible type assignments than structural typing.

D Nominal typing ignores property names and only checks values.

💡 Hint

Think about how types are compared in each system.

Structural typing compares the shape (properties) of types, while nominal typing compares their declared names or identities.

❓ Predict Output

expert

Output of mixed structural and nominal typing example

What is the output of this TypeScript code that mixes structural and nominal typing?

Typescript

interface Animal {
  name: string;
}

class Dog implements Animal {
  constructor(public name: string) {}
}

class Cat {
  constructor(public name: string) {}
}

function printName(a: Animal) {
  console.log(a.name);
}

const dog = new Dog('Buddy');
const cat = new Cat('Whiskers');
printName(dog);
printName(cat);

A Buddy\nWhiskers

B Buddy\nType error: Argument of type 'Cat' is not assignable to parameter of type 'Animal'.

C Type error: Dog is not assignable to Animal.

D Runtime error: Property 'name' is undefined

💡 Hint

Consider how interfaces and classes interact in TypeScript's structural typing.

TypeScript uses structural typing for interfaces, so Cat matches Animal by shape even without explicit implementation.