Constructor functions in Javascript - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a simple constructor function

What is the output of the following code?

Javascript

function Person(name, age) {
  this.name = name;
  this.age = age;
}

const p = new Person('Alice', 30);
console.log(p.name + ' is ' + p.age + ' years old.');

A TypeError

B "Alice is 30 years old."

C "Alice is undefined years old."

D "undefined is 30 years old."

💡 Hint

Remember that the new keyword creates a new object and sets this inside the function.

The constructor function assigns name and age to the new object. So p.name is 'Alice' and p.age is 30.

❓ Predict Output

intermediate

Effect of missing new keyword

What happens when you call a constructor function without the new keyword?

Javascript

function Car(make, model) {
  this.make = make;
  this.model = model;
}

const c = Car('Toyota', 'Corolla');
console.log(c);

A undefined

B An object with make and model properties

C TypeError

D ReferenceError

💡 Hint

Without new, the function does not return an object unless explicitly returned.

Calling Car without new runs the function but does not create or return an object. So c is undefined.

❓ Predict Output

advanced

Prototype method behavior

What is the output of this code?

Javascript

function Animal(type) {
  this.type = type;
}
Animal.prototype.speak = function() {
  return this.type + ' makes a sound';
};
const dog = new Animal('Dog');
console.log(dog.speak());

A TypeError

B "undefined makes a sound"

C "Dog makes a sound"

D "Animal makes a sound"

💡 Hint

Prototype methods are shared by all instances.

The speak method uses this.type which is 'Dog' for the dog instance.

🧠 Conceptual

advanced

Understanding constructor return values

What will be the value of obj after running this code?

Javascript

function Gadget(name) {
  this.name = name;
  return { name: 'Override' };
}
const obj = new Gadget('Original');

A { name: 'Original' }

B TypeError

C undefined

D { name: 'Override' }

💡 Hint

If a constructor returns an object explicitly, that object is used instead of this.

Returning an object from a constructor replaces the created object, so obj is the returned object.

🔧 Debug

expert

Fixing prototype method context error

What error will this code produce when calling greet, and why?

Javascript

function User(name) {
  this.name = name;
}
User.prototype.greet = () => {
  return 'Hello, ' + this.name;
};
const u = new User('Sam');
console.log(u.greet());

A "Hello, undefined"

B ReferenceError

C TypeError

D "Hello, Sam"

💡 Hint

Arrow functions do not have their own this; they use the surrounding context.

The arrow function uses the outer this, which is not the User instance, so this.name is undefined.