Prototype chain in Javascript - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of accessing a property through the prototype chain?

Consider the following code. What will be logged to the console?

Javascript

function Person(name) {
  this.name = name;
}
Person.prototype.greet = function() {
  return `Hello, ${this.name}`;
};

const alice = new Person('Alice');
console.log(alice.greet());

A "Hello, Alice"

B undefined

C "Hello, undefined"

D TypeError: alice.greet is not a function

💡 Hint

Remember that methods defined on the prototype are accessible to instances.

The greet method is defined on Person.prototype, so alice inherits it. Calling alice.greet() returns 'Hello, Alice'.

❓ Predict Output

intermediate

What happens when a property is shadowed on the instance?

Look at this code. What will be the output of console.log(car.color)?

Javascript

function Vehicle() {}
Vehicle.prototype.color = 'red';

const car = new Vehicle();
car.color = 'blue';
console.log(car.color);

A "red"

B TypeError

C undefined

D "blue"

💡 Hint

Instance properties override prototype properties with the same name.

The instance car has its own color property set to 'blue', which shadows the prototype's 'red'. So car.color is 'blue'.

❓ Predict Output

advanced

What is the output when modifying the prototype after instance creation?

What will this code print to the console?

Javascript

function Animal() {}
Animal.prototype.sound = 'generic';

const dog = new Animal();
console.log(dog.sound);

Animal.prototype.sound = 'bark';
console.log(dog.sound);

A "generic" then "generic"

B "generic" then "bark"

C "bark" then "bark"

D TypeError

💡 Hint

Prototype properties are looked up dynamically at access time.

dog.sound first reads 'generic'. After changing Animal.prototype.sound to 'bark', dog.sound reflects the new value.

❓ Predict Output

advanced

What error occurs when accessing a missing property in the prototype chain?

What will happen when this code runs?

Javascript

const obj = {};
console.log(obj.someProperty.toString());

A TypeError: Cannot read properties of undefined (reading 'toString')

B undefined

C null

D ReferenceError

💡 Hint

Accessing a property on undefined causes an error.

obj.someProperty is undefined. Calling toString() on undefined causes a TypeError.

🧠 Conceptual

expert

Which prototype chain lookup order is correct?

Given an object instance, in what order does JavaScript look up properties in the prototype chain?

A Instance properties → Object.prototype → Constructor.prototype → null

B Constructor.prototype → Instance properties → Object.prototype → null

C Instance properties → Constructor.prototype → Object.prototype → null

D Object.prototype → Constructor.prototype → Instance properties → null

💡 Hint

Think about where properties are first checked: own properties or prototype.

JavaScript first checks own properties on the instance, then the constructor's prototype, then Object.prototype, then null.