What this keyword represents in Javascript - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of this in a regular function

What is the output of the following code when calling obj.show()?

Javascript

const obj = {
  name: 'Alice',
  show: function() {
    console.log(this.name);
  }
};
obj.show();

A "Alice"

B undefined

C ReferenceError

D "obj"

💡 Hint

Inside an object method, this refers to the object itself.

When a function is called as a method of an object, this refers to that object. So this.name is obj.name, which is "Alice".

❓ Predict Output

intermediate

Value of this in arrow function inside object

What will be logged when calling obj.show() in this code?

Javascript

const obj = {
  name: 'Bob',
  show: () => {
    console.log(this.name);
  }
};
obj.show();

A "obj"

B "Bob"

C undefined

D ReferenceError

💡 Hint

Arrow functions do not have their own this.

Arrow functions inherit this from their surrounding scope. Here, this is not obj, so this.name is undefined.

❓ Predict Output

advanced

this inside event listener function

What does this refer to inside the event listener function in this code?

Javascript

const button = document.createElement('button');
button.textContent = 'Click me';
document.body.appendChild(button);

button.addEventListener('click', function() {
  console.log(this);
});

A The button element that was clicked

B The global window object

C undefined

D The document object

💡 Hint

In event listeners, this refers to the element the listener is attached to.

In a regular function used as an event listener, this points to the element that received the event, here the button.

❓ Predict Output

advanced

this in class method with setTimeout

What will be logged when calling obj.delayedShow()?

Javascript

class Person {
  constructor(name) {
    this.name = name;
  }
  delayedShow() {
    setTimeout(function() {
      console.log(this.name);
    }, 100);
  }
}
const obj = new Person('Carol');
obj.delayedShow();

A ReferenceError

B undefined

C "Carol"

D TypeError

💡 Hint

Regular functions inside setTimeout have this as the global object or undefined in strict mode.

The function inside setTimeout is a regular function, so this is not the class instance. It is undefined in strict mode, so this.name is undefined.

🧠 Conceptual

expert

Understanding this binding with call, apply, and bind

Given the code below, which option correctly shows the output of greet.call(person, 'Hello')?

Javascript

function greet(greeting) {
  return `${greeting}, my name is ${this.name}`;
}
const person = { name: 'Dave' };

A TypeError

B "Hello, my name is greet"

C "Hello, my name is undefined"

D "Hello, my name is Dave"

💡 Hint

call sets this explicitly to the first argument.

The call method calls the function with this set to person. So this.name is "Dave".

Practice

1. What does the this keyword represent inside a regular function in JavaScript when called as a method of an object?

easy

A. The object that owns the method

B. The global object (like window in browsers)

C. The function itself

D. Undefined

Solution

1. Step 1: Understand method call context

   When a function is called as a method of an object, this refers to that object.

2. Step 2: Apply to example

   Inside the method, this points to the object owning the method, not the global object or function itself.

3. Final Answer:

   The object that owns the method -> Option A

4. Quick Check:

   this in method = owning object [OK]

Hint: In methods, this is the object before the dot [OK]

Common Mistakes:

• Thinking this always points to global object
• Confusing this with the function itself
• Assuming this is undefined in methods

2. Which of the following is the correct way to access this inside an arrow function to refer to the surrounding object?

easy

A. Use this directly inside the arrow function

B. Use this only inside global scope

C. Use this inside a regular function instead

D. Arrow functions do not have their own this, so it inherits from the surrounding scope

Solution

1. Step 1: Understand arrow function this behavior

   Arrow functions do not have their own this; they inherit it from the surrounding scope.

2. Step 2: Apply to object method

   Using this inside an arrow function refers to the this of the outer function or object context.

3. Final Answer:

   Arrow functions do not have their own this, so it inherits from the surrounding scope -> Option D

4. Quick Check:

   Arrow function this = inherited context [OK]

Hint: Arrow functions inherit this from outside [OK]

Common Mistakes:

• Assuming arrow functions have their own this
• Using arrow functions as object methods expecting this to be the object
• Confusing arrow function this with regular function this

3. Consider the code:

const obj = {
  name: 'Alice',
  greet() {
    console.log(this.name);
  }
};
const greetFunc = obj.greet;
greetFunc();

What will be the output when greetFunc() is called?

medium

A. Alice

B. undefined

C. Error

D. null

Solution

1. Step 1: Understand function reference assignment

   Assigning obj.greet to greetFunc detaches the method from its object.

2. Step 2: Call context of greetFunc()

   When called alone, this inside greetFunc is the global object or undefined in strict mode, so this.name is undefined.

3. Final Answer:

   undefined -> Option B

4. Quick Check:

   Detached method this = undefined [OK]

Hint: Detached methods lose their object this [OK]

Common Mistakes:

• Expecting this to still point to the object
• Thinking it prints the name property
• Ignoring strict mode effects on this

4. Identify the error in the following code:

const person = {
  name: 'Bob',
  sayName: () => {
    console.log(this.name);
  }
};
person.sayName();

medium

A. this inside arrow function does not refer to person

B. Missing parentheses in method call

C. Syntax error due to arrow function in object

D. name property is not defined

Solution

1. Step 1: Analyze arrow function this behavior

   Arrow functions do not have their own this; they inherit from the outer scope, which is not person.

2. Step 2: Effect on console.log(this.name)

   Since this is not person, this.name is undefined or from global scope, not 'Bob'.

3. Final Answer:

   this inside arrow function does not refer to person -> Option A

4. Quick Check:

   Arrow function this ≠ object [OK]

Hint: Avoid arrow functions for object methods needing this [OK]

Common Mistakes:

• Thinking arrow functions bind this to object
• Assuming syntax error due to arrow function
• Confusing missing parentheses with this issues

5. You want to create a constructor function Car that sets the make property using this. Which code correctly uses this to assign the property when creating a new car?

hard

A. const Car = (make) => { this.make = make; }; const myCar = new Car('Toyota');

B. function Car(make) { make = this; } const myCar = Car('Toyota');

C. function Car(make) { this.make = make; } const myCar = new Car('Toyota');

D. function Car(make) { this.make = make; } const myCar = Car('Toyota');

Solution

1. Step 1: Understand constructor function usage

   Constructor functions use this to assign properties to the new object created by new.

2. Step 2: Check each option

   function Car(make) { this.make = make; } const myCar = new Car('Toyota'); correctly assigns this.make = make and uses new to create an object. function Car(make) { make = this; } const myCar = Car('Toyota'); assigns incorrectly. const Car = (make) => { this.make = make; }; const myCar = new Car('Toyota'); uses arrow function which does not bind this. function Car(make) { this.make = make; } const myCar = Car('Toyota'); calls constructor without new, so this is not the new object.

3. Final Answer:

   function Car(make) { this.make = make; } const myCar = new Car('Toyota'); -> Option C

4. Quick Check:

   Constructor this needs new [OK]

Hint: Use new with constructor to bind this correctly [OK]

Common Mistakes:

• Calling constructor without new
• Using arrow functions as constructors
• Assigning make = this instead of this.make = make