Hoisting with let and const in Javascript - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this code snippet?

Consider the following JavaScript code:

console.log(a);
let a = 5;

What will be printed to the console?

Javascript

console.log(a);
let a = 5;

A null

B ReferenceError

C 5

D undefined

💡 Hint

Think about the temporal dead zone for let variables.

Variables declared with let are hoisted but not initialized. Accessing them before declaration causes a ReferenceError due to the temporal dead zone.

❓ Predict Output

intermediate

What is the output of this code snippet?

Look at this code:

console.log(b);
const b = 10;

What will be the output?

Javascript

console.log(b);
const b = 10;

A 10

B TypeError

C undefined

D ReferenceError

💡 Hint

Remember how const variables behave before initialization.

const variables are hoisted but not initialized. Accessing them before declaration causes a ReferenceError due to the temporal dead zone.

❓ Predict Output

advanced

What is the output of this code snippet?

Analyze this code:

var x = 1;
{
  console.log(x);
  let x = 2;
}

What will be printed?

Javascript

var x = 1;
{
  console.log(x);
  let x = 2;
}

A ReferenceError

B 2

C 1

D undefined

💡 Hint

Consider the block scope and temporal dead zone of let.

The block declares a new let x which shadows the outer var x. The console.log(x) tries to access the inner x before initialization, causing a ReferenceError.

❓ Predict Output

advanced

What is the output of this code snippet?

Consider this code:

function test() {
  console.log(y);
  const y = 3;
}
test();

What will be printed when test() is called?

Javascript

function test() {
  console.log(y);
  const y = 3;
}
test();

A ReferenceError

B undefined

C 3

D TypeError

💡 Hint

Think about the temporal dead zone inside functions with const.

The variable y is declared with const inside the function. Accessing it before declaration causes a ReferenceError due to the temporal dead zone.

🧠 Conceptual

expert

Which option correctly explains hoisting behavior of let and const?

Choose the statement that best describes how let and const variables behave during hoisting in JavaScript.

A <p><code>let</code> and <code>const</code> variables are hoisted and initialized with <code>undefined</code> before code execution.</p>

B <p><code>let</code> variables are hoisted and initialized, but <code>const</code> variables are not hoisted at all.</p>

C <p><code>let</code> and <code>const</code> variables are hoisted but not initialized, causing a temporal dead zone until their declaration is reached.</p>

D <p><code>let</code> and <code>const</code> variables behave exactly like <code>var</code> during hoisting.</p>

💡 Hint

Recall the temporal dead zone concept for let and const.

Both let and const declarations are hoisted but remain uninitialized until their declaration line is executed. Accessing them before initialization causes a temporal dead zone error.

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