this in objects in Javascript - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this code?

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

Javascript

const obj = {
  value: 42,
  getValue: function() {
    return this.value;
  }
};
console.log(obj.getValue());

A 42

B undefined

C TypeError

D null

💡 Hint

Remember, inside a method, 'this' refers to the object before the dot.

The method getValue is called on obj, so 'this' inside getValue refers to obj. obj.value is 42, so 42 is returned and logged.

❓ Predict Output

intermediate

What does this code output?

Look at this code snippet. What will it print?

Javascript

const obj = {
  value: 10,
  getValue: () => this.value
};
console.log(obj.getValue());

A 10

B undefined

C TypeError

D 0

💡 Hint

Arrow functions do not have their own 'this'.

Arrow functions inherit 'this' from their surrounding scope, which is the global scope here. Since 'value' is not defined globally, it returns undefined.

❓ Predict Output

advanced

What is the output of this code?

What will this code print to the console?

Javascript

const obj = {
  value: 5,
  getValue: function() {
    const inner = function() {
      return this.value;
    };
    return inner();
  }
};
console.log(obj.getValue());

A undefined

B 5

C TypeError

D ReferenceError

💡 Hint

Regular functions have their own 'this' depending on how they are called.

The inner function is called without an object, so 'this' inside inner refers to the global object (or undefined in strict mode). Since global.value is undefined, the result is undefined.

❓ Predict Output

advanced

What will this code output?

Analyze the following code and select the correct output.

Javascript

const obj = {
  value: 7,
  getValue: function() {
    const inner = () => this.value;
    return inner();
  }
};
console.log(obj.getValue());

A undefined

B null

C TypeError

D 7

💡 Hint

Arrow functions inherit 'this' from their surrounding context.

The arrow function inner inherits 'this' from getValue, which is called on obj. So 'this.value' is 7.

🧠 Conceptual

expert

Which option causes a TypeError when accessing 'this.value'?

Which of the following code snippets will cause a TypeError when trying to access 'this.value'?

A

const obj = { value: 1 };
const getValue = obj.getValue;
getValue();

B

const obj = { value: 1, getValue() { return this.value; } };
const getValue = obj.getValue.bind(obj);
getValue();

C

const obj = { value: 1, getValue() { return this.value; } };
const getValue = obj.getValue.bind(null);
getValue();

D

const obj = { value: 1, getValue() { return this.value; } };
obj.getValue();

💡 Hint

Binding 'this' to null in strict mode can cause errors.

Binding 'this' to null means inside getValue, 'this' is null. Accessing 'this.value' causes a TypeError because you can't read properties of null.

Practice

1. What does this refer to inside a regular method of an object in JavaScript?

easy

A. The object itself

B. The global window object

C. Undefined

D. The method's name

Solution

1. Step 1: Understand this in object methods

   Inside a regular function defined as a method of an object, this refers to the object that owns the method.

2. Step 2: Compare with other contexts

   Unlike arrow functions or global functions, regular methods bind this to the object itself.

3. Final Answer:

   The object itself -> Option A

4. Quick Check:

   this in object method = object [OK]

Hint: Remember: regular methods use this as the object [OK]

Common Mistakes:

• Thinking this is always global
• Confusing arrow functions with regular functions
• Assuming this is the method name

2. Which of the following is the correct way to define a method that uses this inside an object?

easy

A. const obj = { greet: () => console.log(this.name) };

B. const obj = { greet: function() => console.log(this.name) };

C. const obj = { greet() { console.log(this.name); } };

D. const obj = { greet: function => console.log(this.name) };

Solution

1. Step 1: Identify correct method syntax

   const obj = { greet() { console.log(this.name); } }; uses the concise method syntax with a regular function, which correctly binds this to the object.

2. Step 2: Check other options for errors

   const obj = { greet: () => console.log(this.name) }; uses an arrow function, which does not bind this to the object. Options B and D have invalid syntax for function definitions.

3. Final Answer:

   const obj = { greet() { console.log(this.name); } }; -> Option C

4. Quick Check:

   Regular method syntax = const obj = { greet() { console.log(this.name); } }; [OK]

Hint: Use regular functions, not arrow functions, for methods [OK]

Common Mistakes:

• Using arrow functions for methods
• Incorrect function syntax inside objects
• Confusing method shorthand with arrow functions

3. What will be the output of the following code?

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

medium

A. Alice

B. undefined

C. Error

D. person

Solution

1. Step 1: Identify this inside greet method

   The method greet is a regular function inside the person object, so this refers to person.

2. Step 2: Access the name property

   Since this is person, this.name is 'Alice'. The console logs 'Alice'.

3. Final Answer:

   Alice -> Option A

4. Quick Check:

   this.name = 'Alice' [OK]

Hint: Regular method this points to the object [OK]

Common Mistakes:

• Expecting undefined because of arrow function confusion
• Thinking this is global
• Assuming method returns error

4. Find the error in this code and fix it:

const car = {
  brand: 'Toyota',
  showBrand: () => {
    console.log(this.brand);
  }
};
car.showBrand();

medium

A. No error, code works fine

B. Add .bind(car) to arrow function

C. Replace this.brand with car.brand

D. Change arrow function to regular function: showBrand() { console.log(this.brand); }

Solution

1. Step 1: Identify arrow function issue

   Arrow functions do not have their own this. Here, this.brand is undefined because this is not bound to car.

2. Step 2: Fix by using regular function

   Changing showBrand to a regular method binds this to car, so this.brand correctly accesses 'Toyota'.

3. Final Answer:

   Change arrow function to regular function: showBrand() { console.log(this.brand); } -> Option D

4. Quick Check:

   Regular function binds this correctly [OK]

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

Common Mistakes:

• Using arrow functions for methods
• Trying to bind this on arrow functions
• Ignoring this context rules

5. You want to create an object with a method that returns a greeting using the object's name. Which code correctly uses this to achieve this?

const user = {
  name: 'Bob',
  greet: ???
};
console.log(user.greet());

hard

A. greet: () => `Hello, ${this.name}!`

B. greet() { return `Hello, ${this.name}!`; }

C. greet: function() => `Hello, ${this.name}!`

D. greet: function { return `Hello, ${this.name}!`; }

Solution

1. Step 1: Check method syntax and this binding

   greet() { return `Hello, ${this.name}!`; } uses a regular method syntax, so this correctly refers to the user object.

2. Step 2: Validate other options

   greet: () => `Hello, ${this.name}!` uses an arrow function, which does not bind this to the user object. Options B and C have invalid syntax for function definitions.

3. Final Answer:

   greet() { return `Hello, ${this.name}!`; } -> Option B

4. Quick Check:

   Regular method with this = greet() { return `Hello, ${this.name}!`; } [OK]

Hint: Use regular methods for this access in objects [OK]

Common Mistakes:

• Using arrow functions for methods
• Incorrect function syntax
• Expecting this to work in arrow functions