Try–catch block in Javascript

Introduction

A try-catch block helps your program handle errors without stopping. It lets you try some code and catch problems if they happen.

When reading a file that might not exist.

When asking for data from the internet that might fail.

When converting user input that might be wrong.

When running code that depends on other code working right.

Syntax

Javascript

try {
  // code that might cause an error
} catch (error) {
  // code to handle the error
}

The try block runs code that might fail.

The catch block runs only if there is an error in try.

Examples

This tries to divide by zero, but JavaScript does not throw an error here, so catch is not used.

Javascript

try {
  let result = 10 / 0;
  console.log(result);
} catch (error) {
  console.log('Error caught:', error.message);
}

This tries to read bad JSON text and catches the error to show a message.

Javascript

try {
  JSON.parse('bad json');
} catch (error) {
  console.log('Parsing failed:', error.message);
}

Sample Program

This program tries to read a broken JSON string. The catch block runs because the JSON is invalid, and it shows a friendly message.

Javascript

try {
  let userInput = '{"name": "Alice"'; // missing closing brace
  let user = JSON.parse(userInput);
  console.log('User name:', user.name);
} catch (error) {
  console.log('Oops! Invalid JSON:', error.message);
}

Important Notes

Only errors inside the try block are caught.

You can use error.message to get a simple description of the problem.

Try-catch blocks help keep your program running even if something goes wrong.

Summary

Use try to run code that might fail.

Use catch to handle errors and keep your program safe.

This makes your programs more friendly and reliable.

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