Hoisting with let and const in Javascript - Mini Project: Build & Apply

Understanding Hoisting with let and const

📖 Scenario: Imagine you are organizing a small event and you want to keep track of the number of guests and the event name. You will learn how JavaScript handles variable declarations with let and const before they are used in the code, a concept called hoisting.

🎯 Goal: You will create variables using let and const, try to access them before declaration to see what happens, and then correctly use them after declaration.

📋 What You'll Learn

Create variables using let and const with exact names and values

Add a helper variable to control access timing

Use a try...catch block to safely test accessing variables before declaration

Print the final values of the variables after declaration

💡 Why This Matters

🌍 Real World

Understanding hoisting helps avoid bugs when using variables in JavaScript, especially in larger programs where order of code matters.

💼 Career

Many programming jobs require knowledge of JavaScript variable behavior to write clean, error-free code and debug issues efficiently.

1

Add a control variable for access timing

Create a variable called accessBeforeDeclaration using let and set it to true. This will help us decide when to try accessing variables before they are declared.

Javascript

// Create a variable called accessBeforeDeclaration using let and set it to true
// Your code here

Hint

Use let accessBeforeDeclaration = true;

2

Try accessing variables before declaration safely

Write a try...catch block that runs only if accessBeforeDeclaration is true. Inside the try, attempt to print guestCount and eventName before they are declared. In the catch, print the error messages.

Javascript

let accessBeforeDeclaration = true;

if (accessBeforeDeclaration) {
    // Use try...catch to print guestCount and eventName before declaration
    // Your code here
}

Hint

Use if (accessBeforeDeclaration) { try { console.log(guestCount); } catch (error) { console.log(error.message); } } and similarly for eventName.

3

Create variables with let and const

Now create a variable called guestCount using let and set it to 50. Also create a constant called eventName using const and set it to 'Birthday Party'.

Javascript

let accessBeforeDeclaration = true;

if (accessBeforeDeclaration) {
    try {
        console.log(guestCount);
    } catch (error) {
        console.log(error.message);
    }
    try {
        console.log(eventName);
    } catch (error) {
        console.log(error.message);
    }
}

// Create a variable called guestCount using let and set it to 50
// Create a constant called eventName using const and set it to 'Birthday Party'
// Your code here

Hint

Use let guestCount = 50; and const eventName = 'Birthday Party';

4

Print variables after declaration

Print the values of guestCount and eventName after their declarations using console.log.

Javascript

let accessBeforeDeclaration = true;

if (accessBeforeDeclaration) {
    try {
        console.log(guestCount);
    } catch (error) {
        console.log(error.message);
    }
    try {
        console.log(eventName);
    } catch (error) {
        console.log(error.message);
    }
}

let guestCount = 50;
const eventName = 'Birthday Party';

// Print guestCount and eventName after declaration
// Your code here

Hint

Use console.log(guestCount); and console.log(eventName); after the if block.

Practice

1. What happens if you try to access a let variable before its declaration in JavaScript?

easy

A. It returns the variable's default value.

B. It throws a ReferenceError due to the temporal dead zone.

C. It returns null.

D. It returns undefined.

Solution

1. Step 1: Understand hoisting behavior of let

   Variables declared with let are hoisted but not initialized immediately.

2. Step 2: Access before declaration causes error

   Accessing such variables before declaration triggers a ReferenceError due to the temporal dead zone.

3. Final Answer:

   It throws a ReferenceError due to the temporal dead zone. -> Option B

4. Quick Check:

   Temporal Dead Zone = ReferenceError [OK]

Hint: Accessing before declaration with let/const causes ReferenceError [OK]

Common Mistakes:

• Thinking it returns undefined like var
• Assuming it returns null
• Believing it has a default value

2. Which of the following is the correct syntax to declare a constant variable in JavaScript?

easy

A. const PI = 3.14;

B. let const PI = 3.14;

C. constant PI = 3.14;

D. var const PI = 3.14;

Solution

1. Step 1: Recall correct const declaration syntax

   The correct way to declare a constant is using const followed by the variable name and assignment.

2. Step 2: Identify invalid syntax options

   Options B, C, and D use invalid keywords or combinations not supported in JavaScript.

3. Final Answer:

   const PI = 3.14; -> Option A

4. Quick Check:

   const declaration syntax = const name = value [OK]

Hint: Use 'const name = value;' to declare constants [OK]

Common Mistakes:

• Using 'constant' keyword instead of 'const'
• Combining let or var with const
• Missing assignment during declaration

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

console.log(x);
let x = 10;

medium

A. 10

B. undefined

C. ReferenceError

D. null

Solution

1. Step 1: Analyze variable declaration and usage

   The variable x is declared with let after the console.log statement.

2. Step 2: Understand temporal dead zone effect

   Accessing x before its declaration causes a ReferenceError due to the temporal dead zone.

3. Final Answer:

   ReferenceError -> Option C

4. Quick Check:

   let before declaration = ReferenceError [OK]

Hint: Accessing let before declaration throws ReferenceError [OK]

Common Mistakes:

• Assuming it prints undefined like var
• Expecting the value 10 immediately
• Thinking it prints null

4. Identify the error in the following code and fix it:

function test() {
  console.log(a);
  const a = 5;
}
test();

medium

A. No error, outputs 5

B. TypeError, fix by initializing a with null

C. SyntaxError, fix by changing const to var

D. ReferenceError, fix by moving const declaration before console.log

Solution

1. Step 1: Identify the cause of error

   Accessing a before its const declaration causes a ReferenceError due to temporal dead zone.

2. Step 2: Fix by reordering declaration

   Moving the const a = 5; line before console.log(a); fixes the error.

3. Final Answer:

   ReferenceError, fix by moving const declaration before console.log -> Option D

4. Quick Check:

   Declare const before use to avoid ReferenceError [OK]

Hint: Declare const before use to avoid ReferenceError [OK]

Common Mistakes:

• Changing const to var without fixing order
• Ignoring temporal dead zone
• Assuming no error occurs

5. Consider the code below. What will be the output and why?

const a = 1;
{
  console.log(a);
  let a = 2;
}

hard

A. ReferenceError

B. 2

C. 1

D. undefined

Solution

1. Step 1: Understand block scope and hoisting

   The block declares a new let a which is hoisted but uninitialized inside the block.

2. Step 2: Accessing a before let a declaration

   Inside the block, console.log(a) tries to access the block-scoped a before its declaration, causing a ReferenceError due to temporal dead zone.

3. Final Answer:

   ReferenceError -> Option A

4. Quick Check:

   Block-scoped let shadows outer const causing ReferenceError [OK]

Hint: Block let shadows outer const; accessing before declaration errors [OK]

Common Mistakes:

• Assuming it prints outer const value 1
• Expecting it prints 2 before declaration
• Thinking it prints undefined