Variable declaration using const in Javascript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does const mean in JavaScript?

const declares a variable that cannot be reassigned after its initial value is set.

beginner

Can you change the value of a const variable after it is declared?

No, you cannot reassign a new value to a const variable once it is declared.

beginner

Is it possible to declare a const variable without initializing it?

No, const variables must be initialized when declared.

intermediate

Does const make objects or arrays immutable?

No, const prevents reassignment of the variable, but the contents of objects or arrays can still be changed.

beginner

Example: What will happen if you run this code?<br><pre>const x = 5;<br>x = 10;</pre>

This will cause an error because x is declared with const and cannot be reassigned.

What happens if you try to reassign a const variable?

A It ignores the reassignment

B It changes the value successfully

C It converts the variable to let

D It causes an error

Reassigning a const variable causes a TypeError in JavaScript.

Can you declare a const variable without giving it a value?

A No, it must be initialized immediately

B Yes, and assign later

C Yes, but only inside functions

D Only if it is a number

const variables must be initialized when declared.

Which of these is true about const and objects?

A You can change properties but cannot reassign the object

B You can neither reassign nor change properties

C You can reassign the object but not change properties

D You cannot change properties of a <code>const</code> object

const prevents reassignment but object properties can be changed.

Which keyword declares a variable that cannot be reassigned?

A var

B const

C let

D static

const declares variables that cannot be reassigned.

What will this code output?

const name = 'Alice';
console.log(name);

A Error

B undefined

C Alice

D null

The code outputs the string 'Alice' because name is initialized with that value.

Explain how const works for variable declaration in JavaScript.

Think about what you can and cannot do after declaring with const.

Describe the difference between const and let in JavaScript.

Focus on reassignment and scope.

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