Class syntax in Javascript - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of method call in a class

What is the output of this code when calling obj.greet()?

Javascript

class Person {
  constructor(name) {
    this.name = name;
  }
  greet() {
    return `Hello, ${this.name}!`;
  }
}
const obj = new Person('Alice');
console.log(obj.greet());

A SyntaxError

B "Hello, undefined!"

C TypeError: obj.greet is not a function

D "Hello, Alice!"

💡 Hint

Remember how the constructor sets the name property and how methods access it with this.

The constructor sets this.name to 'Alice'. The greet method returns a string using this.name, so the output is 'Hello, Alice!'.

❓ Predict Output

intermediate

Output of static method call

What will this code output when calling MathHelper.square(4)?

Javascript

class MathHelper {
  static square(x) {
    return x * x;
  }
}
console.log(MathHelper.square(4));

A 16

B NaN

C TypeError: MathHelper.square is not a function

D undefined

💡 Hint

Static methods are called on the class itself, not on instances.

The static method square multiplies the input by itself. Calling MathHelper.square(4) returns 16.

❓ Predict Output

advanced

Output of inheritance and method override

What is the output of this code?

Javascript

class Animal {
  speak() {
    return 'Animal sound';
  }
}
class Dog extends Animal {
  speak() {
    return 'Woof!';
  }
}
const pet = new Dog();
console.log(pet.speak());

A "Woof!"

B "Animal sound"

C TypeError: pet.speak is not a function

D undefined

💡 Hint

Child classes can replace methods from parent classes.

Dog overrides the speak method from Animal. Calling pet.speak() returns 'Woof!'.

❓ Predict Output

advanced

Output of private field access

What happens when this code runs?

Javascript

class Counter {
  #count = 0;
  increment() {
    this.#count++;
  }
  getCount() {
    return this.#count;
  }
}
const c = new Counter();
c.increment();
c.increment();
console.log(c.getCount());

A 0

B SyntaxError

C 2

D TypeError: Cannot access private field

💡 Hint

Private fields start with # and are only accessible inside the class.

The private field #count starts at 0. Calling increment twice adds 2. getCount returns 2.

❓ Predict Output

expert

Output of method using super in subclass

What is the output of this code?

Javascript

class Vehicle {
  start() {
    return 'Starting vehicle';
  }
}
class Car extends Vehicle {
  start() {
    return super.start() + ' and car engine';
  }
}
const myCar = new Car();
console.log(myCar.start());

A TypeError: super.start is not a function

B "Starting vehicle and car engine"

C undefined

D "Starting vehicle"

💡 Hint

super calls the method from the parent class.

Car's start method calls Vehicle's start using super, then adds text. The output is 'Starting vehicle and car engine'.