this in objects in Javascript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does this refer to inside a regular object method?

this refers to the object that owns the method. It points to the object before the dot when the method is called.

beginner

How does this behave inside a function called without an object?

In non-strict mode, this refers to the global object (like window in browsers). In strict mode, this is undefined.

intermediate

What happens to this when using arrow functions inside objects?

Arrow functions do not have their own this. They inherit this from the surrounding code where they are defined.

intermediate

How can you explicitly set this when calling a function?

You can use call(), apply(), or bind() methods to set this explicitly.

intermediate

What is a common mistake when using this inside callbacks in objects?

Callbacks often lose the original this context because they are called as regular functions. Using arrow functions or binding this fixes this.

Inside an object method, what does this refer to?

A The function itself

B The global object

C Undefined

D The object owning the method

this inside an object method points to the object that owns the method.

What does this refer to inside an arrow function defined in an object?

A The object itself

B The global object

C The surrounding context's <code>this</code>

D Undefined

Arrow functions inherit this from their surrounding context, not from the object.

Which method can you use to explicitly set this when calling a function?

A set()

B call()

C run()

D execute()

call() lets you specify what this should be inside the function.

What happens if you call a regular function (not a method) in strict mode without setting this?

A <code>this</code> is undefined

B <code>this</code> is the function itself

C <code>this</code> is the global object

D <code>this</code> is the object

In strict mode, this is undefined in a regular function call without an object.

Why might this lose its expected value inside a callback function?

A Because callbacks are called as regular functions, losing the original <code>this</code>

B Because callbacks always use global <code>this</code>

C Because <code>this</code> is always undefined in callbacks

D Because callbacks cannot access <code>this</code>

Callbacks are often called as regular functions, so this does not point to the original object unless handled.

Explain how this works inside an object method and how it differs in arrow functions inside objects.

Think about how the function is called and where arrow functions get their this.

Describe ways to control or change the value of this when calling functions in JavaScript.

Remember the three methods that let you set this explicitly.

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