Variable declaration using let in Javascript - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of let variable in block scope

What is the output of this code?

Javascript

let x = 10;
if (true) {
  let x = 20;
  console.log(x);
}
console.log(x);

A 20\n20

B 10\n20

C 20\n10

D 10\n10

💡 Hint

Remember that let variables are block scoped.

The variable x declared inside the if block is different from the one outside. Inside the block, x is 20, outside it remains 10.

❓ Predict Output

intermediate

Reassignment of let variable

What will be the output of this code?

Javascript

let count = 5;
count = 10;
console.log(count);

A 5

B 10

C undefined

D ReferenceError

💡 Hint

Variables declared with let can be reassigned.

The variable count is declared with let and then reassigned to 10, so the output is 10.

❓ Predict Output

advanced

Temporal Dead Zone with let

What error does this code produce?

Javascript

console.log(a);
let a = 3;

A ReferenceError

B SyntaxError

C TypeError

D undefined

💡 Hint

Variables declared with let are not hoisted like var.

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

❓ Predict Output

advanced

let in for loop scope

What is the output of this code?

Javascript

for (let i = 0; i < 3; i++) {
  setTimeout(() => console.log(i), 0);
}

A 0\n1\n2

B 3\n3\n3

C undefined\nundefined\nundefined

D 0\n0\n0

💡 Hint

Each iteration has its own block scope with let.

Using let in the loop creates a new i for each iteration, so the logged values are 0, 1, and 2.

🧠 Conceptual

expert

Why use let instead of var?

Which of the following is the main advantage of using let over var?

A <code>let</code> variables are hoisted and initialized with undefined.

B <code>let</code> variables can be redeclared in the same scope without errors.

C <code>let</code> variables are globally scoped by default.

D <code>let</code> variables are block scoped, preventing unexpected behavior in loops and conditionals.

💡 Hint

Think about how variable scope affects code reliability.

let is block scoped, which helps avoid bugs caused by var's function or global scope, especially in loops and conditionals.

Practice

1. What does the let keyword do in JavaScript?

easy

A. Declares a constant variable that cannot change

B. Declares a variable that can change value and has block scope

C. Declares a variable with global scope only

D. Declares a function

Solution

1. Step 1: Understand let behavior

   let declares variables that can be reassigned new values.

2. Step 2: Understand scope of let

   Variables declared with let are limited to the block they are declared in (block scope).

3. Final Answer:

   Declares a variable that can change value and has block scope -> Option B

4. Quick Check:

   let = variable with block scope [OK]

Hint: Remember: let = changeable variable inside blocks [OK]

Common Mistakes:

• Confusing let with const (constant)
• Thinking let variables have global scope
• Mixing let with function declarations

2. Which of the following is the correct syntax to declare a variable named score using let?

easy

A. let = score 10;

B. let score: 10;

C. score let = 10;

D. let score = 10;

Solution

1. Step 1: Recall correct variable declaration syntax

   The correct syntax is: let variableName = value;

2. Step 2: Match syntax with options

   let score = 10; matches the correct syntax exactly.

3. Final Answer:

   let score = 10; -> Option D

4. Quick Check:

   Correct syntax = let score = 10; [OK]

Hint: Use: let variableName = value; [OK]

Common Mistakes:

• Placing let after variable name
• Using colon instead of equal sign
• Missing semicolon (though optional)

3. What will be the output of this code?

let x = 5;
x = 10;
console.log(x);

medium

A. 10

B. Error

C. undefined

D. 5

Solution

1. Step 1: Understand variable assignment and reassignment

   Variable x is first set to 5, then changed to 10.

2. Step 2: Check console.log output

   Since x was reassigned to 10, console.log(x) prints 10.

3. Final Answer:

   10 -> Option A

4. Quick Check:

   Reassigned let variable prints new value [OK]

Hint: let variables can change value, so print latest [OK]

Common Mistakes:

• Thinking let variables cannot be reassigned
• Expecting original value to print
• Confusing with const behavior

4. Find the error in this code:

if (true) {
  let message = "Hello";
}
console.log(message);

medium

A. ReferenceError: message is not defined

B. SyntaxError: Unexpected token

C. No error, prints Hello

D. TypeError: message is undefined

Solution

1. Step 1: Understand block scope of let

   Variable message is declared inside the if block and only exists there.

2. Step 2: Check console.log outside block

   Trying to access message outside the block causes a ReferenceError.

3. Final Answer:

   ReferenceError: message is not defined -> Option A

4. Quick Check:

   let variables have block scope, so error outside block [OK]

Hint: let variables only exist inside their block [OK]

Common Mistakes:

• Assuming let variables are global
• Confusing ReferenceError with SyntaxError
• Trying to access block variables outside

5. Consider this code:

let total = 0;
for (let i = 0; i < 3; i++) {
  let total = i * 2;
}
console.log(total);

What will be printed and why?

hard

A. 6, because total sums all i*2 values

B. 4, because total accumulates inside the loop

C. 0, because the outer total is unchanged by inner block

D. ReferenceError, total is redeclared

Solution

1. Step 1: Identify variable scopes

   There are two total variables: one outside the loop and one declared inside the loop block.

2. Step 2: Understand variable shadowing

   The inner total inside the loop is a new variable that does not affect the outer total.

3. Step 3: Check final console.log

   The outer total remains 0, so console.log(total) prints 0.

4. Final Answer:

   0, because the outer total is unchanged by inner block -> Option C

5. Quick Check:

   Inner let shadows outer variable, outer stays 0 [OK]

Hint: Inner let creates new variable, outer stays same [OK]

Common Mistakes:

• Thinking inner total changes outer total
• Assuming let variables are global
• Confusing variable shadowing with reassignment