Duck typing mental model in TypeScript - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define an object that matches the interface by duck typing.

Typescript

interface Bird {
  fly(): void;
}

const duck: Bird = {
  [1]() {
    console.log('Flying');
  }
};

Drag options to blanks, or click blank then click option'

Aswim

Bfly

Cwalk

Drun

Attempts:

3 left

💡 Hint

Common Mistakes

Using a method name different from the interface, like 'swim' or 'walk'.

The object must have a method named fly to match the Bird interface by duck typing.

2fill in blank

medium

Complete the code to assign an object with extra properties to a variable typed by an interface.

Typescript

interface Person {
  name: string;
}

const user: Person = {
  name: 'Alice',
  [1]: 30
};

Drag options to blanks, or click blank then click option'

Aheight

Baddress

Cweight

Dage

Attempts:

3 left

💡 Hint

Common Mistakes

Trying to omit the required property name.

Using a property name that conflicts with the interface.

Extra properties like age are allowed in duck typing as long as required properties exist.

3fill in blank

hard

Fix the error by completing the code to match the interface using duck typing.

Typescript

interface Logger {
  log(message: string): void;
}

const consoleLogger: Logger = {
  [1](msg: string) {
    console.log(msg);
  }
};

Drag options to blanks, or click blank then click option'

Aprint

Boutput

Clog

Dwrite

Attempts:

3 left

💡 Hint

Common Mistakes

Using a different method name like 'print' or 'write' causes a type error.

The method name must be log to match the Logger interface for duck typing.

4fill in blank

hard

Fill both blanks to create a function that accepts any object with a 'speak' method and calls it.

Typescript

function makeSpeak(obj: { [1](): void }) {
  obj.[2]();
}

Drag options to blanks, or click blank then click option'

Aspeak

Btalk

Csay

Dshout

Attempts:

3 left

💡 Hint

Common Mistakes

Using different method names in the parameter and call causes errors.

The function expects an object with a method named speak and calls that method.

5fill in blank

hard

Fill all three blanks to create a duck-typed object with a method and an extra property.

Typescript

interface Animal {
  [1](): void;
}

const cat: Animal = {
  [2]() {
    console.log('Meow');
  },
  [3]: 'black'
};

Drag options to blanks, or click blank then click option'

AmakeSound

Ccolor

Dsize

Attempts:

3 left

💡 Hint

Common Mistakes

Using a method name different from the interface.

Using an extra property name not intended for this example.

The interface requires a method makeSound. The object has that method and an extra property color.

Practice

1. What does duck typing mean in TypeScript?

easy

A. Only objects created from the same class are compatible.

B. If an object has the required properties or methods, it is accepted regardless of its explicit type.

C. TypeScript requires explicit class inheritance to match types.

D. TypeScript checks the runtime type of objects strictly.

Solution

1. Step 1: Understand duck typing concept

   Duck typing means an object is accepted if it has the needed properties or methods, regardless of its declared type.

2. Step 2: Compare with other options

   The other options describe strict or class-based typing, which is not how TypeScript works.

3. Final Answer:

   If an object has the required properties or methods, it is accepted regardless of its explicit type. -> Option B

4. Quick Check:

   Duck typing = structural compatibility [OK]

Hint: Focus on properties, not explicit types, for duck typing [OK]

Common Mistakes:

• Thinking TypeScript requires class inheritance
• Confusing runtime type checks with compile-time checks
• Assuming exact type names must match

2. Which of the following is a correct TypeScript syntax to define a type that matches any object with a name string property and a quack() method returning void?

easy

A. type Duck = { name: string; quack: () => void; };

B. interface Duck { name: string; quack: void; }

C. type Duck = { name: string; quack(): string; };

D. interface Duck { name: string; quack: () => string; }

Solution

1. Step 1: Check property and method types

   The name property must be a string, and quack must be a method returning void.

2. Step 2: Validate each option

   type Duck = { name: string; quack: () => void; }; correctly defines quack as a function returning void. The other options have incorrect method signatures or return types.

3. Final Answer:

   type Duck = { name: string; quack: () => void; }; -> Option A

4. Quick Check:

   Correct method signature = type Duck = { name: string; quack: () => void; }; [OK]

Hint: Method type must be a function signature with correct return type [OK]

Common Mistakes:

• Using method return type as property type
• Confusing void and string return types
• Incorrect interface method syntax

3. What will be the output of this TypeScript code?

interface Duck { name: string; quack: () => void; }

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

const obj = { name: 'Daffy', quack: () => console.log('Quack!'), swim: () => console.log('Swim') };
makeItQuack(obj);

medium

A. No output

B. Swim

C. Error: obj does not match Duck interface

D. Quack!

Solution

1. Step 1: Check if obj matches Duck interface

   obj has name (string) and quack (function returning void), so it matches by duck typing.

2. Step 2: Call makeItQuack with obj

   Calling duck.quack() runs console.log('Quack!'), printing "Quack!".

3. Final Answer:

   Quack! -> Option D

4. Quick Check:

   Duck typing allows extra properties, output = Quack! [OK]

Hint: Extra properties don't block duck typing; focus on required ones [OK]

Common Mistakes:

• Thinking extra properties cause errors
• Expecting 'Swim' output instead of 'Quack!'
• Assuming runtime type errors

4. Identify the error in this TypeScript code using duck typing:

interface Duck { name: string; quack: () => void; }

const notADuck = { name: 'Donald' };

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

makeItQuack(notADuck);

medium

A. notADuck is missing the quack method required by Duck interface.

B. The name property should be a number, not a string.

C. makeItQuack should accept any object without type restrictions.

D. There is no error; code runs fine.

Solution

1. Step 1: Check properties of notADuck

   notADuck has name but lacks quack method required by Duck interface.

2. Step 2: Understand TypeScript type checking

   TypeScript will report an error because notADuck does not satisfy the Duck type.

3. Final Answer:

   notADuck is missing the quack method required by Duck interface. -> Option A

4. Quick Check:

   Missing required method = Type error [OK]

Hint: Check all required properties/methods exist for the type [OK]

Common Mistakes:

• Ignoring missing methods in objects
• Assuming extra properties cause errors
• Thinking TypeScript allows missing required members

5. Given this code, which object will TypeScript accept as a valid Duck type?

interface Duck { name: string; quack: () => void; }

const a = { name: 'Daisy', quack: () => console.log('Quack!') };
const b = { name: 'Goose', quack: 'loud' };
const c = { name: 'Swan', quack: () => 'quack' };
const d = { name: 'Mallard' };

function testDuck(duck: Duck) { duck.quack(); }

Which call to testDuck will NOT cause a TypeScript error?

hard

A. testDuck(c);

B. testDuck(b);

C. testDuck(a);

D. testDuck(d);

Solution

1. Step 1: Check each object's quack property type

   a.quack is a function returning void (correct). b.quack is a string (wrong). c.quack is a function returning string (wrong return type). d.quack is missing (wrong).

2. Step 2: Determine which matches Duck interface

   Only object a matches the Duck interface exactly.

3. Final Answer:

   testDuck(a); -> Option C

4. Quick Check:

   Correct method signature only in a [OK]

Hint: Method must be function returning void to match Duck [OK]

Common Mistakes:

• Accepting string or wrong return type as method
• Ignoring missing methods
• Confusing method return types