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}.