Variable declaration using const in Javascript - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a constant named pi with the value 3.14.

Javascript

const [1] = 3.14;

Drag options to blanks, or click blank then click option'

Api

Bcircumference

Carea

Dradius

Attempts:

3 left

💡 Hint

Common Mistakes

Using a variable name that does not describe the value.

Trying to use 'let' or 'var' instead of 'const'.

The keyword const declares a constant variable. Here, pi is the name of the constant.

2fill in blank

medium

Complete the code to declare a constant MAX_USERS with the value 100.

Javascript

const [1] = 100;

Drag options to blanks, or click blank then click option'

AMaxUsers

BmaxUsers

Cmax_users

DMAX_USERS

Attempts:

3 left

💡 Hint

Common Mistakes

Using camelCase instead of uppercase with underscores.

Using lowercase letters for constant names.

Constants are often written in uppercase with underscores to separate words. Here, MAX_USERS is the correct style.

3fill in blank

hard

Fix the error in the code by completing the constant declaration for API_URL.

Javascript

const [1] = 'https://api.example.com';

Drag options to blanks, or click blank then click option'

AApiUrl

BapiUrl

CAPI_URL

Dapi_url

Attempts:

3 left

💡 Hint

Common Mistakes

Using camelCase or lowercase for constant names.

Using variable names that do not follow constant naming conventions.

Constants that represent fixed values like URLs are usually written in uppercase with underscores. API_URL is the correct constant name.

4fill in blank

hard

Fill both blanks to declare constants for SECONDS_IN_MINUTE and MINUTES_IN_HOUR.

Javascript

const [1] = 60;
const [2] = 60;

Drag options to blanks, or click blank then click option'

ASECONDS_IN_MINUTE

BsecondsInMinute

CMINUTES_IN_HOUR

DminutesInHour

Attempts:

3 left

💡 Hint

Common Mistakes

Using camelCase instead of uppercase with underscores.

Mixing up the names or values.

Constants are named in uppercase with underscores. Here, SECONDS_IN_MINUTE and MINUTES_IN_HOUR are the correct names.

5fill in blank

hard

Fill all three blanks to declare constants for PI, EULER, and GRAVITY with their respective values.

Javascript

const [1] = 3.14159;
const [2] = 2.71828;
const [3] = 9.8;

Drag options to blanks, or click blank then click option'

API

BEULER

CGRAVITY

Dgravity

Attempts:

3 left

💡 Hint

Common Mistakes

Using lowercase or camelCase names for constants.

Mixing up the order of constants.

Constants are named in uppercase letters. Here, PI, EULER, and GRAVITY are the correct constant names.

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