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

Recall & Review

beginner

What does the var keyword do in JavaScript?

The var keyword declares a variable that can be used throughout its function or globally if declared outside a function.

beginner

What is the scope of a variable declared with var inside a function?

It has function scope, meaning it is accessible anywhere inside the function where it is declared.

beginner

Can you redeclare a variable using var in the same scope?

Yes, variables declared with var can be redeclared in the same scope without error.

intermediate

What happens if you declare a var variable inside a block like if or for?

The variable is not block-scoped but function-scoped, so it is accessible outside the block within the function.

intermediate

What is hoisting in relation to var variables?

Hoisting means the variable declaration is moved to the top of its scope before code runs, so you can use the variable before its declaration but its value will be undefined until assigned.

What is the scope of a variable declared with var inside a function?

A Global scope only

B Function scope

C Block scope

D No scope

Variables declared with var inside a function are accessible anywhere inside that function.

What happens if you redeclare a var variable in the same scope?

A It overwrites the previous variable

B It causes an error

C It creates a new variable with a different name

D It is ignored

Redeclaring a var variable in the same scope overwrites the previous declaration without error.

Which of these is true about var variables and hoisting?

A Both declaration and assignment are hoisted

B Only the assignment is hoisted

C Neither declaration nor assignment is hoisted

D Only the declaration is hoisted, not the assignment

With var, the declaration is hoisted to the top, but the assignment stays where it is.

If you declare a var variable inside an if block, where can you access it?

A Only outside the <code>if</code> block

B Only inside the <code>if</code> block

C Anywhere inside the function containing the <code>if</code> block

D Nowhere

var variables are function-scoped, so they are accessible anywhere inside the function, not just inside the block.

Which keyword introduced in ES6 provides block scope, unlike var?

A let

B const

C var

D function

let provides block scope, which var does not.

Explain how var variables behave with respect to scope and hoisting.

Think about where the variable is accessible and what happens if you use it before declaring.

Describe the difference between var and let in terms of scope.

Consider how variables behave inside blocks like if or for.

Practice

1. What does the var keyword do in JavaScript?

easy

A. Declares a block-scoped variable

B. Declares a constant that cannot be changed

C. Declares a variable that can be changed and redeclared

D. Declares a variable only inside loops

Solution

1. Step 1: Understand var behavior

   var declares variables that can be changed and redeclared in the same scope.

2. Step 2: Compare with other declarations

   Unlike const or let, var is function-scoped and allows redeclaration.

3. Final Answer:

   Declares a variable that can be changed and redeclared -> Option C

4. Quick Check:

   var allows redeclaration = A [OK]

Hint: Remember: var variables can be changed and redeclared easily [OK]

Common Mistakes:

• Confusing var with const or let
• Thinking var is block-scoped
• Assuming var variables cannot be redeclared

2. Which of the following is the correct syntax to declare a variable using var?

easy

A. var myVar = 10;

B. variable myVar = 10;

C. var: myVar = 10;

D. v ar myVar = 10;

Solution

1. Step 1: Recall correct var syntax

   The correct syntax is var variableName = value; with no spaces or colons.

2. Step 2: Check each option

   Only var myVar = 10; matches the correct syntax exactly.

3. Final Answer:

   var myVar = 10; -> Option A

4. Quick Check:

   Correct var syntax = A [OK]

Hint: Use 'var name = value;' format without extra symbols [OK]

Common Mistakes:

• Using 'variable' instead of 'var'
• Adding colons after var
• Adding spaces inside 'var'

3. What will be the output of this code?

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

medium

A. 5

B. Error

C. undefined

D. 10

Solution

1. Step 1: Understand redeclaration with var

   Using var, redeclaring a variable overwrites its previous value.

2. Step 2: Trace the code execution

   First x is 5, then redeclared as 10, so console.log(x) prints 10.

3. Final Answer:

   10 -> Option D

4. Quick Check:

   Redeclared var overwrites value = 10 [OK]

Hint: Redeclaring var overwrites previous value [OK]

Common Mistakes:

• Thinking redeclaration causes error
• Expecting first value to print
• Confusing var with let or const behavior

4. Find the error in this code snippet:

if (true) {
  var a = 1;
}
console.log(a);

medium

A. ReferenceError: a is not defined

B. No error, prints 1

C. TypeError: a is not a function

D. SyntaxError due to missing braces

Solution

1. Step 1: Understand var scope

   var is function-scoped, not block-scoped, so a is accessible outside the if block.

2. Step 2: Check console.log output

   Since a is declared with var, console.log(a) prints 1 without error.

3. Final Answer:

   No error, prints 1 -> Option B

4. Quick Check:

   var is function-scoped, so prints 1 [OK]

Hint: var ignores block scope, accessible outside blocks [OK]

Common Mistakes:

• Assuming var is block-scoped like let
• Expecting ReferenceError outside block
• Confusing var with const or let scope

5. Consider this code:

function test() {
  if (true) {
    var count = 5;
  }
  var count = 10;
  return count;
}
console.log(test());

What will be the output and why?

hard

A. 10 because second var redeclares and overwrites

B. Error due to redeclaration of count

C. 5 because first var inside if is used

D. undefined because var is block-scoped

Solution

1. Step 1: Analyze var declarations inside function

   Both var count declarations are in the same function scope, so the second redeclaration overwrites the first.

2. Step 2: Determine returned value

   The function returns the last assigned value, which is 10.

3. Final Answer:

   10 because second var redeclares and overwrites -> Option A

4. Quick Check:

   Function-scoped var redeclaration overwrites = 10 [OK]

Hint: Inside functions, var redeclaration overwrites previous value [OK]

Common Mistakes:

• Thinking redeclaration causes error
• Assuming block scope limits var
• Expecting first value to be returned