Variable declaration using const in Javascript

Introduction

We use const to create variables that should not change. It helps keep our code safe and clear.

When you want to store a value that should never change, like a fixed number or a name.

When you want to avoid accidentally changing important data in your program.

When you want to make your code easier to understand by showing which values stay the same.

When working with values that represent constants, like configuration settings or math constants.

When you want to improve code reliability by preventing reassignment of variables.

Syntax

Javascript

const variableName = value;

The variable must be assigned a value when declared.

You cannot change the value of a const variable later.

Examples

This creates a constant named pi with the value 3.14.

Javascript

const pi = 3.14;

This creates a constant string variable name with the value "Alice".

Javascript

const name = "Alice";

This creates a constant boolean variable isActive set to true.

Javascript

const isActive = true;

Sample Program

This program creates a constant variable greeting and prints it. It shows that you cannot change the value later.

Javascript

const greeting = "Hello";
console.log(greeting);

// Trying to change the value will cause an error
// greeting = "Hi"; // Uncommenting this line will cause an error

Important Notes

Even though you cannot reassign a const variable, if it holds an object or array, you can still change the contents inside.

Always use const by default, and only use let if you need to change the variable later.

Summary

const creates variables that cannot be reassigned.

It helps keep values safe and your code clear.

Use const for values that should stay the same.

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