Object creation in Javascript - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of Object.create with prototype properties

What is the output of the following code?

Javascript

const proto = { greeting: 'hello' };
const obj = Object.create(proto);
obj.greeting = 'hi';
console.log(obj.greeting);
console.log(proto.greeting);

A "hi" and then "hello"

B "hello" and then "hi"

C undefined and then "hello"

D "hi" and then "hi"

💡 Hint

Remember that Object.create sets the prototype, and own properties override prototype properties.

The object obj has its own property greeting set to 'hi', which overrides the prototype's greeting 'hello'. So obj.greeting logs 'hi'. The prototype object proto remains unchanged, so proto.greeting logs 'hello'.

❓ Predict Output

intermediate

Output of Object.create with null prototype

What will be the output of this code?

Javascript

const obj = Object.create(null);
console.log(typeof obj.toString);

A TypeError

B "function"

C "object"

D "undefined"

💡 Hint

Object.create(null) creates an object with no prototype, so it has no inherited methods.

Since obj has no prototype, it does not inherit toString method. So obj.toString is undefined and typeof undefined is 'undefined'.

🔧 Debug

advanced

Why does this object creation code throw an error?

The following code throws an error. Which option explains the cause?

Javascript

const obj = Object.create();

A Object.create only accepts arrays as arguments.

B Object.create cannot be called without the new keyword.

C Object.create requires a prototype object argument; calling it without arguments causes a TypeError.

D Object.create requires a second argument for property descriptors.

💡 Hint

Check the required parameters for Object.create.

Object.create must be called with a prototype object as the first argument. Omitting it causes a TypeError because undefined is not an object.

❓ Predict Output

advanced

Output of Object.create with property descriptors

What is the output of this code?

Javascript

const obj = Object.create({}, {
  name: {
    value: 'Alice',
    writable: false,
    enumerable: true
  }
});
obj.name = 'Bob';
console.log(obj.name);

A "Alice"

B "Bob"

C undefined

D TypeError

💡 Hint

Check if the property is writable or not.

The property 'name' is created as non-writable, so attempts to change it are ignored. The value remains 'Alice'.

🧠 Conceptual

expert

Which option correctly creates an object with a prototype and own properties?

You want to create an object that inherits from a prototype object and also has its own property 'age' set to 30. Which code does this correctly?

A

const proto = { greet() { return 'hi'; } };
const obj = Object.create(proto, { age: 30 });

B

const proto = { greet() { return 'hi'; } };
const obj = Object.create(proto, { age: { value: 30, writable: true, enumerable: true } });

C

const proto = { greet() { return 'hi'; } };
const obj = Object.create(proto);
obj.age = 30;

D

const proto = { greet() { return 'hi'; } };
const obj = { ...proto, age: 30 };

💡 Hint

Remember Object.create's second argument requires property descriptors.

Option B correctly uses Object.create with a prototype and defines 'age' as an own property with proper descriptors. Option B adds 'age' after creation but does not use property descriptors. Option B incorrectly passes a number instead of a descriptor object. Option B creates a new object copying properties but does not set the prototype.