Running JavaScript in browser console - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of console.log with variable scope

What will be printed in the browser console after running this code snippet?

Javascript

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

A 20\n20

B 10\n20

C 20\n10

D 10\n10

💡 Hint

Remember that variables declared inside a function have local scope.

Inside the function, x is 20 because of local declaration. Outside, x remains 10.

❓ Predict Output

intermediate

Console output of asynchronous code

What is the order of outputs in the browser console after running this code?

Javascript

console.log('Start');
setTimeout(() => console.log('Timeout'), 0);
console.log('End');

A Start\nEnd\nTimeout

B Timeout\nStart\nEnd

C Start\nTimeout\nEnd

D End\nStart\nTimeout

💡 Hint

setTimeout callbacks run after the current call stack is empty.

Synchronous logs run first, then the setTimeout callback runs.

🔧 Debug

advanced

Identify the error in console usage

Which option will cause an error when run in the browser console?

Javascript

console.log('Hello');

A console.log('Hello'

B console.log('Hello')

C console.log('Hello');

D console.log('Hello');;

💡 Hint

Check for missing parentheses or syntax errors.

Option A is missing a closing parenthesis, causing SyntaxError.

🧠 Conceptual

advanced

Console methods for different outputs

Which console method prints a message only if a condition is false?

A console.info()

B console.assert()

C console.warn()

D console.error()

💡 Hint

This method is used for debugging assumptions.

console.assert() prints message only if the first argument is false.

❓ Predict Output

expert

Output of console.log with object mutation

What will be the output in the console after running this code?

Javascript

const obj = {a: 1};
console.log(obj);
obj.a = 2;
console.log(obj);

A {a: 1}\n{a: 2}

B {a: 1}\n{a: 1}

C SyntaxError

D {a: 2}\n{a: 2}

💡 Hint

Objects logged show their state at the time of logging.

First log shows {a:1}, after mutation second log shows {a:2}.

Practice

1. What is the primary purpose of the browser console in JavaScript?

easy

A. To quickly run and test JavaScript code

B. To design website layouts

C. To write HTML code

D. To manage browser bookmarks

Solution

1. Step 1: Understand the browser console role

   The browser console is a tool inside web browsers used mainly for running and testing JavaScript code quickly. The other options relate to tasks like design, HTML, or bookmarks, which are not the console's main use.

2. Final Answer:

   To quickly run and test JavaScript code -> Option A

3. Quick Check:

   Console = run/test JS code [OK]

Hint: Console runs JavaScript code fast in browser [OK]

Common Mistakes:

• Confusing console with HTML editor
• Thinking console manages bookmarks
• Assuming console designs layouts

2. Which keyboard shortcut opens the browser console in most browsers?

easy

A. Ctrl + S

B. F12

C. Ctrl + P

D. Alt + F4

Solution

1. Step 1: Recall common browser shortcuts

   F12 is the standard shortcut to open developer tools, including the console, in most browsers. Ctrl + S saves files, Ctrl + P prints, and Alt + F4 closes windows, so they don't open the console.

2. Final Answer:

   F12 -> Option B

3. Quick Check:

   Open console = F12 [OK]

Hint: Press F12 to open console quickly [OK]

Common Mistakes:

• Using Ctrl + S thinking it opens console
• Confusing print shortcut Ctrl + P
• Mistaking Alt + F4 for console shortcut

3. What will be the output in the browser console after running this code?

console.log(2 + 3 * 4);

medium

A. 20

B. Undefined

C. 14

D. 24

Solution

1. Step 1: Apply operator precedence in JavaScript

   Multiplication (*) has higher priority than addition (+), so 3 * 4 = 12 is calculated first, then 2 + 12 = 14, which is the final result logged to the console.

2. Final Answer:

   14 -> Option C

3. Quick Check:

   2 + (3 * 4) = 14 [OK]

Hint: Multiply before add in expressions [OK]

Common Mistakes:

• Adding before multiplying
• Expecting string output
• Confusing output with undefined

4. You typed console.log('Hello World' in the browser console and got an error. What is the problem?

medium

A. Wrong quotes used

B. Missing semicolon

C. console.log is not a function

D. Missing closing parenthesis

Solution

1. Step 1: Check the syntax of the code

   The code has an opening parenthesis after console.log but no closing parenthesis at the end. JavaScript syntax rules require matching parentheses; quotes are correct, console.log is valid, and semicolon is optional.

2. Final Answer:

   Missing closing parenthesis -> Option D

3. Quick Check:

   Syntax error = missing ')' [OK]

Hint: Check matching parentheses in console code [OK]

Common Mistakes:

• Ignoring missing parenthesis error
• Blaming quotes instead of parentheses
• Thinking semicolon is mandatory here

5. You want to quickly check the value of variable z in the browser console after running this code:

let x = 10;
let y = 5;
let z = x + y;

Which command will show the value of z immediately?

medium

A. console.log(z);

B. print(z);

C. echo(z);

D. show(z);

Solution

1. Step 1: Recall how to display values in browser console

   console.log() is the standard JavaScript function to print values to the console. print(), echo(), and show() are not valid JavaScript functions in the browser console.

2. Final Answer:

   console.log(z); -> Option A

3. Quick Check:

   Show value = console.log() [OK]

Hint: Use console.log() to display values [OK]

Common Mistakes:

• Using print() instead of console.log()
• Trying echo() which is not JavaScript
• Assuming show() works in console