Finally block in Javascript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of the finally block in JavaScript?

The finally block contains code that always runs after the try and catch blocks, no matter if an error occurred or not.

beginner

When does the code inside a finally block execute?

It executes after the try block finishes, whether an error was thrown or not, and after the catch block if an error was caught.

intermediate

Can the finally block change the return value of a function?

Yes, if the finally block contains a return statement, it overrides any previous return from try or catch.

advanced

What happens if an error is thrown inside the finally block?

The error thrown in finally will replace any previous error or return value, and it will propagate out of the whole try-catch-finally structure.

beginner

Write a simple example of a try-catch-finally block in JavaScript.

try { console.log('Try block runs'); throw new Error('Oops'); } catch (e) { console.log('Catch block runs:', e.message); } finally { console.log('Finally block always runs'); }

When does the finally block run in a try-catch-finally statement?

A Only if an error occurs

B Only if no error occurs

C Always, after try and catch blocks

D Never

The finally block always runs after the try and catch blocks, regardless of errors.

What happens if there is a return statement inside both try and finally blocks?

A The return in <code>finally</code> overrides the one in <code>try</code>

B An error occurs

C Both returns are ignored

D The return in <code>try</code> is used

The finally block's return overrides any previous return from try or catch.

If an error is thrown inside the finally block, what happens?

A The error replaces any previous error or return

B The error is ignored

C The program continues without error

D The error is caught automatically

An error thrown in finally replaces any previous error or return value and propagates out.

Which of these is a correct use of finally?

A To catch errors thrown in <code>try</code>

B To clean up resources like closing files or connections

C To declare variables

D To skip code execution

finally is often used to clean up resources regardless of errors.

Can a finally block exist without a catch block?

A Only in strict mode

B No, <code>catch</code> is required

C Only in older JavaScript versions

D Yes, <code>finally</code> can follow <code>try</code> directly

A finally block can follow a try block without a catch block.

Explain the role of the finally block in JavaScript error handling and when it executes.

Think about what code you want to run no matter what happens in try or catch.

Describe what happens if a return statement is present in both the try and finally blocks.

Consider which return value the function actually sends back.

Practice

1.

What is the main purpose of the finally block in JavaScript?

easy

A. To define variables used in try block

B. To run code that always executes after try and catch blocks

C. To catch errors thrown in the try block

D. To skip the catch block if no error occurs

Solution

1. Step 1: Understand try-catch-finally structure

   The try block runs code that might throw errors, catch handles those errors.

2. Step 2: Role of finally block

   The finally block runs code after try and catch, no matter if an error occurred or not.

3. Final Answer:

   To run code that always executes after try and catch blocks -> Option B

4. Quick Check:

   finally always runs = C [OK]

Hint: finally always runs after try/catch, no matter what [OK]

Common Mistakes:

• Thinking finally only runs if there is an error
• Confusing catch and finally blocks
• Believing finally can catch errors

2.

Which of the following is the correct syntax to add a finally block after a try and catch block?

try {
  // code
} catch (e) {
  // handle error
} ??? {
  // cleanup code
}

easy

A. cleanup

B. final

C. finally

D. end

Solution

1. Step 1: Recall JavaScript error handling syntax

   The correct keyword after catch is 'finally' to run code always.

2. Step 2: Check options for correct keyword

   Only 'finally' is valid JavaScript syntax here.

3. Final Answer:

   finally -> Option C

4. Quick Check:

   finally keyword syntax = D [OK]

Hint: finally is the exact keyword after catch for cleanup [OK]

Common Mistakes:

• Using 'final' or 'end' instead of 'finally'
• Omitting the finally block keyword
• Placing finally before catch

3.

What will be the output of this code?

function test() {
  try {
    console.log('Try block');
    return 'Return from try';
  } catch (e) {
    console.log('Catch block');
  } finally {
    console.log('Finally block');
  }
}
console.log(test());

medium

A. Finally block\nReturn from try

B. Try block\nCatch block\nFinally block\nReturn from try

C. Try block\nReturn from try

D. Try block\nFinally block\nReturn from try

Solution

1. Step 1: Trace the try block execution

   It logs 'Try block' and then hits return 'Return from try'.

2. Step 2: Understand finally runs after return

   Even after return, finally runs and logs 'Finally block'. Then function returns.

3. Final Answer:

   Try block\nFinally block\nReturn from try -> Option D

4. Quick Check:

   finally runs after return = A [OK]

Hint: finally runs even after return in try [OK]

Common Mistakes:

• Thinking finally does not run after return
• Expecting catch block to run without error
• Missing the order of console logs

4.

Find the error in this code snippet:

try {
  throw new Error('Oops');
} catch {
  console.log('Caught error');
} finally {
  console.log('Cleanup');
}

medium

A. No error, code runs fine

B. finally block cannot be used without catch parameter

C. throw statement is incorrect

D. Missing error parameter in catch block

Solution

1. Step 1: Check catch block syntax

   Optional catch binding (ES2019+) allows catch {} without parameter. Valid in modern JavaScript.

2. Step 2: Verify execution

   Throws error, catch logs 'Caught error', finally logs 'Cleanup'. No errors.

3. Final Answer:

   No error, code runs fine -> Option A

4. Quick Check:

   catch {} valid = D [OK]

Hint: catch {} valid in modern JS with optional binding [OK]

Common Mistakes:

• Thinking catch always requires parameter
• Believing syntax error without catch param
• Confusing with pre-ES2019 JS

5.

Consider this code:

let result = '';
function test() {
  try {
    result += 'Try';
    throw new Error('Error!');
  } catch (e) {
    result += 'Catch';
    return result;
  } finally {
    result += 'Finally';
  }
}
test();
console.log(result);

What will be printed to the console?

hard

A. TryCatchFinally

B. TryCatch

C. TryFinally

D. Try

Solution

1. Step 1: Trace try block execution

   result becomes 'Try', then error is thrown.

2. Step 2: Catch block runs and appends 'Catch', then returns result ('TryCatch')

   Before returning, finally runs and appends 'Finally' to result.

3. Step 3: Check console.log output

   console.log prints result after finally appended, so 'TryCatchFinally'.

4. Final Answer:

   TryCatchFinally -> Option A

5. Quick Check:

   finally runs before return completes = B [OK]

Hint: finally runs before return completes, modifies variables [OK]

Common Mistakes:

• Assuming return stops finally from running
• Expecting console.log to print before finally modifies result
• Ignoring that finally runs even after return