Global scope in Javascript - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this code involving global and local variables?

Consider the following JavaScript code:

var x = 10;
function test() {
  var x = 20;
  console.log(x);
}
test();
console.log(x);

What will be printed to the console?

Javascript

var x = 10;
function test() {
  var x = 20;
  console.log(x);
}
test();
console.log(x);

A 20\n10

B 10\n20

C 20\n20

D 10\n10

💡 Hint

Remember that variables declared inside a function have local scope and do not affect the global variable.

The function test has a local variable x set to 20, so console.log(x) inside the function prints 20. Outside the function, the global x is still 10, so console.log(x) prints 10.

❓ Predict Output

intermediate

What happens when you omit var/let/const in a function?

Look at this code:

function setValue() {
  y = 5;
}
setValue();
console.log(y);

What will be the output?

Javascript

function setValue() {
  y = 5;
}
setValue();
console.log(y);

A TypeError

B ReferenceError: y is not defined

C undefined

D 5

💡 Hint

Variables assigned without var, let, or const inside functions become global.

Because y is assigned without declaration inside the function, it becomes a global variable. So after calling setValue(), y exists globally with value 5.

🧠 Conceptual

advanced

Which statement about global scope is true?

Choose the correct statement about global scope in JavaScript.

A Variables declared with <code>let</code> at the top level become properties of the global object.

B Variables declared with <code>var</code> at the top level become properties of the global object.

C Using <code>const</code> inside a function creates a global variable.

D Functions declared inside blocks are always global.

💡 Hint

Think about how var and let differ in global scope.

Variables declared with var at the top level become properties of the global object (like window in browsers). Variables declared with let or const do not become global object properties. Functions declared inside blocks are block-scoped, not global. const inside functions is local to that function.

❓ Predict Output

advanced

What is the output when modifying a global variable inside a function?

Consider this code:

let count = 0;
function increment() {
  count++;
}
increment();
console.log(count);

What will be printed?

Javascript

let count = 0;
function increment() {
  count++;
}
increment();
console.log(count);

A ReferenceError

B NaN

C 1

D 0

💡 Hint

Variables declared with let at global scope can be accessed and changed inside functions.

The variable count is declared globally with let. The function increment increases it by 1. So after calling increment(), count becomes 1.

❓ Predict Output

expert

What is the output of this code with nested scopes and global variable?

Analyze this code snippet:

var a = 1;
function outer() {
  var a = 2;
  function inner() {
    a = 3;
  }
  inner();
  console.log(a);
}
outer();
console.log(a);

What will be printed to the console?

Javascript

var a = 1;
function outer() {
  var a = 2;
  function inner() {
    a = 3;
  }
  inner();
  console.log(a);
}
outer();
console.log(a);

A 3\n1

B 3\n3

C 2\n3

D 2\n1

💡 Hint

Remember that inner changes the a in outer's scope, not the global a.

The variable a inside outer is initially 2. The inner function changes this a to 3. So console.log(a) inside outer prints 3. The global a remains 1, so the last console.log(a) prints 1.

Practice

1. What does it mean when a variable is in the global scope in JavaScript?

easy

A. The variable can be accessed anywhere in the code.

B. The variable can only be used inside a function.

C. The variable is hidden from all parts of the code.

D. The variable is deleted after a function finishes.

Solution

1. Step 1: Understand global scope meaning

   Global scope means a variable is declared outside any function and is accessible everywhere in the code.

2. Step 2: Compare options with definition

   Only The variable can be accessed anywhere in the code. correctly states that the variable can be accessed anywhere.

3. Final Answer:

   The variable can be accessed anywhere in the code. -> Option A

4. Quick Check:

   Global scope = accessible everywhere [OK]

Hint: Global variables are outside functions, accessible everywhere [OK]

Common Mistakes:

• Thinking global variables are only inside functions
• Confusing global with local scope
• Believing global variables disappear after function ends

2. Which of the following is the correct way to declare a global variable in JavaScript?

easy

A. Inside a function using let

B. Outside any function using var

C. Inside a block using const

D. Inside a function using const

Solution

1. Step 1: Identify where to declare global variables

   Global variables must be declared outside any function or block.

2. Step 2: Check declaration keywords

   Using var outside functions creates a global variable. Options A, B, and D declare inside functions or blocks, so they are not global.

3. Final Answer:

   Outside any function using var -> Option B

4. Quick Check:

   Global variable = declared outside functions [OK]

Hint: Declare global variables outside functions with var [OK]

Common Mistakes:

• Declaring variables inside functions and expecting global scope
• Using let or const inside blocks thinking they are global
• Confusing block scope with global scope

3. What will be the output of this code?

var x = 10;
function show() {
  console.log(x);
}
show();

medium

A. 10

B. undefined

C. ReferenceError

D. null

Solution

1. Step 1: Identify variable scope

   Variable x is declared globally with var and has value 10.

2. Step 2: Check function access

   The function show logs x. Since x is global, it is accessible inside the function.

3. Final Answer:

   10 -> Option A

4. Quick Check:

   Global variable accessed inside function = 10 [OK]

Hint: Global variables are accessible inside functions [OK]

Common Mistakes:

• Expecting undefined because variable is outside function
• Thinking console.log will cause ReferenceError
• Confusing local and global scope

4. Find the error in this code related to global scope:

function test() {
  var a = 5;
}
console.log(a);

medium

A. No error, prints 5

B. TypeError: a is undefined

C. ReferenceError: a is not defined

D. SyntaxError: var inside function

Solution

1. Step 1: Check variable scope

   Variable a is declared inside function test with var, so it is local to that function.

2. Step 2: Access variable outside function

   Trying to log a outside the function causes a ReferenceError because a is not in global scope.

3. Final Answer:

   ReferenceError: a is not defined -> Option C

4. Quick Check:

   Local variable accessed globally = ReferenceError [OK]

Hint: Variables inside functions are not global [OK]

Common Mistakes:

• Assuming var inside function is global
• Expecting output 5 instead of error
• Confusing ReferenceError with SyntaxError

5. You want to create a global counter variable that increases every time a function runs. Which code correctly uses global scope to do this?

hard

A. let counter = 0;
function count() {
let counter = 0;
counter++;
console.log(counter);
}

B. function count() {
var counter = 0;
counter++;
console.log(counter);
}

C. function count() {
let counter = 0;
counter++;
console.log(counter);
}

D. var counter = 0;
function count() {
counter++;
console.log(counter);
}

Solution

1. Step 1: Identify global variable declaration

   var counter = 0;
   function count() {
     counter++;
     console.log(counter);
   }

   declares counter outside the function, making it global and accessible inside the function.

2. Step 2: Check counter increment behavior

   Inside count, counter++ increases the global variable, so the count persists across calls.

3. Final Answer:

   var counter = 0; function count() { counter++; console.log(counter); } -> Option D

4. Quick Check:

   Global counter increments inside function =

   var counter = 0;
   function count() {
     counter++;
     console.log(counter);
   }

   [OK]

Hint: Declare counter outside function to keep it global [OK]

Common Mistakes:

• Declaring counter inside function resets it each call
• Using let inside function hides global variable
• Expecting local counter to remember previous value