Duck typing mental model in TypeScript - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this duck typing example?

Consider this TypeScript code using duck typing. What will be printed?

Typescript

interface Quack {
  quack(): void;
}

function makeItQuack(duck: Quack) {
  duck.quack();
}

const notADuck = {
  quack: () => console.log('I am quacking!')
};

makeItQuack(notADuck);

A undefined

B I am quacking!

C Type error: notADuck is not of type Quack

D Runtime error: quack is not a function

💡 Hint

Duck typing means the object just needs to have the right shape, not the exact type.

The object notADuck has a quack method, so it matches the Quack interface. TypeScript allows this, so the function runs and prints the message.

🧠 Conceptual

intermediate

Which statement best describes duck typing in TypeScript?

Choose the best description of duck typing in TypeScript.

A TypeScript checks only the names and types of properties and methods, not the explicit type names.

B TypeScript requires explicit inheritance to match types.

C TypeScript uses runtime type checking to enforce types.

D TypeScript only allows objects to match types if they have the same prototype.

💡 Hint

Think about how TypeScript compares object shapes rather than names.

TypeScript uses structural typing, meaning it compares the shape (properties and methods) of objects rather than their explicit names or inheritance.

🔧 Debug

advanced

Why does this code cause a TypeScript error?

Look at this code snippet. Why does TypeScript report an error?

Typescript

interface Flyer {
  fly(): void;
}

const bird = {
  fly: () => console.log('Flying'),
  swim: () => console.log('Swimming')
};

function makeFly(f: Flyer) {
  f.fly();
}

makeFly({swim: () => console.log('Swimming')});

A The object passed to <code>makeFly</code> lacks the required <code>fly</code> method.

B The object has extra properties which are not allowed.

C The interface <code>Flyer</code> is not implemented explicitly.

D The <code>fly</code> method is private and cannot be accessed.

💡 Hint

Check if the object passed has the required method.

The object passed to makeFly only has a swim method but no fly method. TypeScript requires the object to have all properties of the interface.

📝 Syntax

advanced

Which option correctly defines a duck-typed object matching this interface?

Given this interface, which object correctly matches it by duck typing?

interface Runner {
  run(speed: number): string;
}

A { run(speed: string) { return 'Fast'; } }

B { run: (speed) => speed + ' km/h' }

C { run: (speed: number) => `Running at ${speed} km/h` }

D { run: () => 'Running' }

💡 Hint

Check the method signature matches the interface exactly.

Option C matches the interface exactly: run takes a number and returns a string. Option C misses the parameter type, C uses wrong parameter type, and D has no parameter.

🚀 Application

expert

What is the value of result after this duck typing code runs?

Analyze the code and find the value of result.

Typescript

interface Speaker {
  speak(): string;
}

const obj1 = { speak: () => 'Hello' };
const obj2 = { speak: () => 'World' };

function combineSpeakers(a: Speaker, b: Speaker): string {
  return a.speak() + ' ' + b.speak();
}

const result = combineSpeakers(obj1, obj2);

A "World Hello"

B Type error: obj1 or obj2 does not match Speaker

C "HelloHello"

D "Hello World"

💡 Hint

Both objects have a speak method returning strings.

Both obj1 and obj2 match the Speaker interface by duck typing. The function concatenates their speak results with a space.