Try–catch block in Javascript - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of try–catch with thrown error

What will be printed to the console when this code runs?

Javascript

try {
  console.log('Start');
  throw new Error('Oops!');
  console.log('End');
} catch (e) {
  console.log('Caught:', e.message);
}

A Caught: Oops!

B Start\nEnd\nCaught: Oops!

C Start\nCaught: Oops!

D Start\nEnd

💡 Hint

Remember that code after throw inside try does not run.

The code prints 'Start' first. Then the error is thrown, so 'End' is skipped. The catch block runs and prints 'Caught: Oops!'.

❓ Predict Output

intermediate

Value of variable after try–catch

What is the value of variable x after this code runs?

Javascript

let x = 0;
try {
  x = 5;
  throw 'error';
  x = 10;
} catch (e) {
  x = 20;
}

A 20

B 5

C 10

D 0

💡 Hint

Check which assignments run before and after the throw.

x is set to 5, then error is thrown, so x=10 is skipped. Catch block sets x to 20.

❓ Predict Output

advanced

Output with nested try–catch and finally

What will this code print to the console?

Javascript

try {
  console.log('Outer try');
  try {
    console.log('Inner try');
    throw 'error';
  } catch (e) {
    console.log('Inner catch');
    throw 'new error';
  } finally {
    console.log('Inner finally');
  }
} catch (e) {
  console.log('Outer catch');
} finally {
  console.log('Outer finally');
}

A Outer try\nInner try\nInner catch\nOuter catch\nOuter finally

B Outer try\nInner try\nInner catch\nOuter catch\nInner finally\nOuter finally

C Outer try\nInner try\nInner finally\nInner catch\nOuter catch\nOuter finally

D Outer try\nInner try\nInner catch\nInner finally\nOuter catch\nOuter finally

💡 Hint

Remember finally always runs after try or catch blocks.

The inner try throws 'error', caught by inner catch which throws 'new error'. Inner finally runs next. Outer catch catches 'new error'. Outer finally runs last.

❓ Predict Output

advanced

Error type caught in catch block

What will be the output of this code?

Javascript

try {
  JSON.parse('invalid json');
} catch (e) {
  console.log(e instanceof SyntaxError);
  console.log(e instanceof Error);
  console.log(e.name);
}

A false\ntrue\nSyntaxError

B true\ntrue\nSyntaxError

C true\nfalse\nError

D false\nfalse\nError

💡 Hint

Check the type of error JSON.parse throws on invalid input.

JSON.parse throws a SyntaxError, which is also an Error. So instanceof SyntaxError and instanceof Error are both true. The name is 'SyntaxError'.

🧠 Conceptual

expert

Behavior of try–catch with asynchronous code

Consider this code snippet. Which statement is true about the output?

Javascript

try {
  setTimeout(() => {
    throw new Error('Async error');
  }, 0);
} catch (e) {
  console.log('Caught:', e.message);
}
console.log('End');

A Prints only 'End' and the error is uncaught

B Prints 'Caught: Async error' then 'End'

C Throws SyntaxError before printing anything

D Prints 'End' then 'Caught: Async error'

💡 Hint

Remember how try–catch works with asynchronous callbacks.

The throw inside setTimeout runs asynchronously, outside the try block's scope. So catch does not catch it. 'End' prints first. The error is uncaught and will appear as an unhandled exception.

Practice

1.

What is the main purpose of a try block in JavaScript?

easy

A. To run code that might cause an error

B. To declare variables

C. To create loops

D. To define functions

Solution

1. Step 1: Understand the role of try

   The try block is used to run code that might throw an error during execution.

2. Step 2: Differentiate from other code blocks

   Other options like declaring variables, loops, or functions are unrelated to error handling.

3. Final Answer:

   To run code that might cause an error -> Option A

4. Quick Check:

   try runs risky code [OK]

Hint: Try block runs risky code safely [OK]

Common Mistakes:

• Thinking try declares variables
• Confusing try with loops
• Assuming try defines functions

2.

Which of the following is the correct syntax to start a try-catch block in JavaScript?

?

easy

A. try: { /* code */ } catch: { /* handle error */ }

B. try { /* code */ } catch (error) { /* handle error */ }

C. try (/* code */) catch (error) { /* handle error */ }

D. try { /* code */ } catch error { /* handle error */ }

Solution

1. Step 1: Recall correct try-catch syntax

   The correct syntax uses curly braces and parentheses for catch: try { ... } catch (error) { ... }.

2. Step 2: Identify syntax errors in other options

   Options B, C, and D misuse colons, parentheses, or omit parentheses around the error parameter.

3. Final Answer:

   try { /* code */ } catch (error) { /* handle error */ } -> Option B

4. Quick Check:

   Curly braces and catch(error) needed [OK]

Hint: Catch needs parentheses around error [OK]

Common Mistakes:

• Using colons instead of braces
• Omitting parentheses in catch
• Using parentheses after try

3.

What will be the output of the following code?

try {
  console.log('Start');
  throw new Error('Oops');
  console.log('End');
} catch (e) {
  console.log('Caught:', e.message);
}

medium

A. Start\nEnd

B. Start

C. Caught: Oops

D. Start\nCaught: Oops

Solution

1. Step 1: Trace code inside try block

   The first console logs 'Start'. Then an error is thrown, so the next console.log('End') is skipped.

2. Step 2: Catch block runs after error

   The catch block catches the error and logs 'Caught: Oops'.

3. Final Answer:

   Start\nCaught: Oops -> Option D

4. Quick Check:

   Error stops try, catch logs message [OK]

Hint: Error stops try; catch runs next [OK]

Common Mistakes:

• Thinking code after throw runs
• Ignoring catch block output
• Confusing error message with error object

4.

Find the error in this code snippet:

try {
  console.log('Hello');
} catch e {
  console.log('Error:', e);
}

medium

A. Missing parentheses around e in catch

B. Missing semicolon after console.log

C. Try block cannot have console.log

D. Catch block must come before try

Solution

1. Step 1: Check catch syntax

   The catch block must have parentheses around the error variable: catch (e).

2. Step 2: Verify other parts

   Semicolons are optional in JavaScript; console.log is allowed in try; catch must come after try.

3. Final Answer:

   Missing parentheses around e in catch -> Option A

4. Quick Check:

   Catch needs (error) [OK]

Hint: Catch always needs parentheses around error [OK]

Common Mistakes:

• Omitting parentheses in catch
• Thinking semicolons are mandatory here
• Reversing try and catch order

5.

You want to safely parse a JSON string that might be invalid. Which code correctly uses try-catch to handle errors and return null if parsing fails?

?

hard

A. try {
JSON.parse(str);
} catch {
return null;
}

B. try {
return JSON.parse(str);
} catch e {
return null;
}

C. try {
return JSON.parse(str);
} catch (e) {
return null;
}

D. try {
return JSON.parse(str);
} catch (e) {
throw null;
}

Solution

1. Step 1: Understand JSON parsing risks

   Parsing invalid JSON throws an error, so we use try-catch to handle it safely.

2. Step 2: Check correct catch syntax and behavior

   try { return JSON.parse(str); } catch (e) { return null; } uses correct catch syntax with (e) and returns null on error, which is the goal.

3. Step 3: Identify errors in other options

   try { JSON.parse(str); } catch { return null; } misses return before JSON.parse(str), so returns undefined on success. try { return JSON.parse(str); } catch e { return null; } has syntax error (catch e needs catch (e)). try { return JSON.parse(str); } catch (e) { throw null; } throws null instead of returning it.

4. Final Answer:

   try { return JSON.parse(str); } catch (e) { return null; } -> Option C

5. Quick Check:

   Catch error with (e), return null on failure [OK]

Hint: Catch error with (e), return null on failure [OK]

Common Mistakes:

• Omitting parentheses in catch
• Throwing instead of returning null
• Assuming catch without parameter always works