Why What this keyword represents in Javascript? - Purpose & Use Cases

The Big Idea

Discover how a tiny word like <code>this</code> can make your code smarter and less confusing!

The Scenario

Imagine you have a group of friends, and you want to tell each one their own favorite color. You try to write a note for each friend, but you keep mixing up who likes what color because you don't have a clear way to say "this friend" in your notes.

The Problem

Without a clear way to refer to the current friend, you might write confusing notes or repeat the same color for everyone. This makes your messages slow to write and easy to mess up, especially if you have many friends.

The Solution

The this keyword in JavaScript acts like a pointer to the current object you are working with. It helps you say "this particular friend" clearly, so you can easily access their favorite color or other details without confusion.

Before vs After

✗ Before

function showColor(friend) {
  console.log(friend.name + ' likes ' + friend.color);
}
showColor({name: 'Alice', color: 'blue'});

✓ After

const friend = {
  name: 'Alice',
  color: 'blue',
  showColor() {
    console.log(this.name + ' likes ' + this.color);
  }
};
friend.showColor();

What It Enables

Using this lets you write cleaner, reusable code that automatically knows which object it belongs to, making your programs easier to understand and maintain.

Real Life Example

Think of a car with many buttons. Each button controls a specific feature of that car. The this keyword is like the label on each button that tells it which car it belongs to, so pressing it affects the right car.

Key Takeaways

this points to the current object in context.

It helps avoid confusion when accessing object properties.

Using this makes code cleaner and easier to reuse.

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