Constructors in classes in Javascript - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this class constructor code?

Consider the following JavaScript class with a constructor. What will be printed when creating a new instance?

Javascript

class Person {
  constructor(name) {
    this.name = name;
    console.log(`Hello, ${this.name}!`);
  }
}

const p = new Person('Alice');

A Hello, Alice!

B undefined

C Hello, undefined!

D TypeError

💡 Hint

Look at what the constructor does with the name parameter and the console.log statement.

The constructor receives 'Alice' as the name argument, assigns it to this.name, and then logs 'Hello, Alice!'.

❓ Predict Output

intermediate

What will be the value of the property after creating an instance?

Look at this class and what happens when we create an instance. What is the value of obj.age?

Javascript

class Animal {
  constructor(age) {
    this.age = age + 1;
  }
}

const obj = new Animal(4);

A NaN

B 4

C undefined

D 5

💡 Hint

The constructor adds 1 to the age parameter before assigning.

The constructor adds 1 to the input age (4), so this.age becomes 5.

❓ Predict Output

advanced

What error does this class code produce?

This class tries to use a constructor but has a mistake. What error will happen when running this code?

Javascript

class Car {
  constructor() {
    this.brand = 'Toyota';
  }
}

const myCar = new Car();

A No error

B TypeError

C SyntaxError

D ReferenceError

💡 Hint

Check the syntax of the constructor declaration.

The constructor is missing parentheses '()' after the word constructor, causing a SyntaxError.

❓ Predict Output

advanced

What is the output of this class with multiple constructor calls?

What will be printed when creating two instances of this class?

Javascript

class Counter {
  constructor() {
    if (!Counter.count) {
      Counter.count = 0;
    }
    Counter.count++;
    console.log(`Count is ${Counter.count}`);
  }
}

new Counter();
new Counter();

A Count is 0\nCount is 1

B Count is 1\nCount is 2

C Count is 1\nCount is 1

D undefined\nundefined

💡 Hint

Look at how the static property Counter.count is used and updated.

Counter.count starts undefined, so first instance sets it to 0 then increments to 1. Second instance increments to 2.

🧠 Conceptual

expert

Which option will cause an error when creating an instance?

Given these class definitions, which one will cause an error when you try to create a new instance?

A class B { constructor(x) { this.x = x.toString(); } } const b = new B();

B class A { constructor() { this.x = 1; } } const a = new A();

C class C { constructor() { return {}; } } const c = new C();

D class D { constructor() { this.x = 1; return 5; } } const d = new D();

💡 Hint

Consider what happens if you call a constructor that expects a parameter but you don't provide one.

Option A causes a TypeError because no argument is passed, so x is undefined, and undefined.toString() throws "Cannot read properties of undefined (reading 'toString')". Option A has no required parameter. Option A returns an object (allowed, overrides 'this'). Option A returns a primitive (ignored).