Variable declaration using const in Javascript - Mini Project: Build & Apply

Variable Declaration Using const

📖 Scenario: You are creating a simple program to store information about a favorite fruit. You want to make sure the fruit name does not change accidentally.

🎯 Goal: Learn how to declare a variable using const in JavaScript and understand that its value cannot be changed later.

📋 What You'll Learn

Declare a variable using const

Assign a string value to the const variable

Try to change the value (to see what happens, but this will be commented out)

Print the value of the const variable

💡 Why This Matters

🌍 Real World

Using <code>const</code> helps keep important values safe from accidental changes in programs, like fixed settings or names.

💼 Career

Understanding <code>const</code> is essential for writing reliable JavaScript code in web development and many programming jobs.

1

Declare a const variable

Declare a variable called favoriteFruit using const and assign it the value "Apple".

Javascript

# Declare a const variable called favoriteFruit and assign it "Apple"
// Your code here

Hint

Use const favoriteFruit = "Apple"; to declare the variable.

2

Add a comment about changing the variable

Add a comment below the declaration explaining that favoriteFruit cannot be changed because it is declared with const.

Javascript

const favoriteFruit = "Apple";
// Add a comment explaining favoriteFruit cannot be changed
// Your code here

Hint

Write a comment starting with // explaining the variable is constant.

3

Try to change the const variable (commented out)

Add a line below the comment that tries to change favoriteFruit to "Banana", but comment it out so the program does not break.

Javascript

const favoriteFruit = "Apple";
// favoriteFruit cannot be changed because it is declared with const
// Try to change favoriteFruit to "Banana" but comment it out
// Your code here

Hint

Write // favoriteFruit = "Banana"; to show the change attempt is commented out.

4

Print the value of the const variable

Write a line to print the value of favoriteFruit using console.log.

Javascript

const favoriteFruit = "Apple";
// favoriteFruit cannot be changed because it is declared with const
// favoriteFruit = "Banana"; // This will cause an error if uncommented
// Print favoriteFruit
// Your code here

Hint

Use console.log(favoriteFruit); to print the value.

Practice

1. What does the const keyword do in JavaScript?

easy

A. Declares a variable that cannot be reassigned

B. Declares a variable that can be reassigned anytime

C. Declares a function

D. Declares a variable only inside loops

Solution

1. Step 1: Understand const behavior

   The const keyword creates a variable that cannot be reassigned after its initial value is set.

2. Step 2: Compare options

   Only Declares a variable that cannot be reassigned correctly states that const variables cannot be reassigned.

3. Final Answer:

   Declares a variable that cannot be reassigned -> Option A

4. Quick Check:

   const means no reassignment [OK]

Hint: Remember: const means constant value, no changes allowed [OK]

Common Mistakes:

• Thinking const variables can be reassigned
• Confusing const with let or var
• Assuming const declares functions

2. Which of the following is the correct syntax to declare a constant variable named pi with value 3.14?

easy

A. constant pi = 3.14;

B. const pi := 3.14;

C. const pi = 3.14;

D. let const pi = 3.14;

Solution

1. Step 1: Recall correct const syntax

   In JavaScript, the correct syntax to declare a constant is const variableName = value;.

2. Step 2: Check each option

   Only const pi = 3.14; matches the correct syntax. The other options use invalid keywords or symbols.

3. Final Answer:

   const pi = 3.14; -> Option C

4. Quick Check:

   Correct const syntax uses = and no extra keywords [OK]

Hint: Use const name = value; with equal sign [OK]

Common Mistakes:

• Using := instead of =
• Writing 'constant' instead of 'const'
• Combining let and const keywords

3. What will be the output of this code?

const greeting = 'Hello';
greeting = 'Hi';
console.log(greeting);

medium

A. undefined

B. Hi

C. Hello

D. TypeError

Solution

1. Step 1: Understand reassignment of const variable

   The variable greeting is declared with const, so it cannot be reassigned after initialization.

2. Step 2: Analyze the reassignment line

   Trying to assign 'Hi' to greeting causes a TypeError at runtime.

3. Final Answer:

   TypeError -> Option D

4. Quick Check:

   Reassigning const variable throws TypeError [OK]

Hint: const variables cannot be reassigned; error occurs if tried [OK]

Common Mistakes:

• Assuming const variables can be changed
• Expecting console.log to print new value
• Confusing TypeError with SyntaxError

4. Find the error in this code snippet:

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

medium

A. Missing value when declaring const variable

B. Cannot use console.log with const

C. Variable name 'number' is reserved

D. No error, code runs fine

Solution

1. Step 1: Check const declaration rules

   When declaring a const variable, it must be initialized with a value immediately.

2. Step 2: Identify the problem in code

   The code declares const number; without assigning a value, which causes a SyntaxError.

3. Final Answer:

   Missing value when declaring const variable -> Option A

4. Quick Check:

   const must be initialized on declaration [OK]

Hint: Always assign a value when declaring const [OK]

Common Mistakes:

• Declaring const without initialization
• Thinking console.log causes error
• Assuming variable names like 'number' are invalid

5. You want to store a list of colors that should never change in your program. Which is the best way to declare it using const and why?

const colors = ['red', 'green', 'blue'];
colors.push('yellow');
console.log(colors);

hard

A. Use const and avoid changing array contents; push will cause error

B. Use const for the array; you can change contents but not reassign the variable

C. Use let instead of const to allow adding colors

D. Declare colors as a string instead of array

Solution

1. Step 1: Understand const with objects and arrays

   Using const means the variable cannot be reassigned, but the contents of objects or arrays can be changed.

2. Step 2: Analyze the code behavior

   The array colors is declared with const, but push modifies its contents without reassigning the variable, so it works fine.

3. Final Answer:

   Use const for the array; you can change contents but not reassign the variable -> Option B

4. Quick Check:

   const protects variable binding, not object contents [OK]

Hint: const protects variable name, not array contents [OK]

Common Mistakes:

• Thinking const makes array immutable
• Using let when const is better for fixed references
• Confusing reassignment with content modification