Catching runtime errors in Javascript - Mini Project: Build & Apply

Catching runtime errors

📖 Scenario: Imagine you are building a simple calculator app that divides two numbers. Sometimes users might enter zero as the divisor, which causes an error. You want to catch this error and show a friendly message instead of crashing.

🎯 Goal: You will create a program that safely divides two numbers and catches any runtime errors using try and catch. If an error happens, it will show a clear message.

📋 What You'll Learn

Create two variables called numerator and denominator with exact values

Create a variable called result and set it to null

Use a try block to divide numerator by denominator and assign to result

Use a catch block to set result to the string 'Error: Cannot divide by zero'

Print the value of result

💡 Why This Matters

🌍 Real World

Handling errors prevents apps from crashing and helps show friendly messages to users.

💼 Career

Error handling is a key skill for developers to build reliable and user-friendly software.

1

Create the numbers to divide

Create a variable called numerator and set it to 10. Create another variable called denominator and set it to 0.

Javascript

// Your code here

Hint

Use const to create the variables with the exact numbers.

2

Prepare a variable to hold the result

Create a variable called result and set it to null.

Javascript

const numerator = 10;
const denominator = 0;
// Your code here

Hint

Use let because result will change later.

3

Use try and catch to divide safely

Write a try block where you divide numerator by denominator and assign it to result. Add a catch block that sets result to the string 'Error: Cannot divide by zero'.

Javascript

const numerator = 10;
const denominator = 0;
let result = null;
// Your code here

Hint

Use throw new Error() inside try when denominator is zero. Catch the error and set result to the error message.

4

Show the result

Write console.log(result) to print the value of result.

Javascript

const numerator = 10;
const denominator = 0;
let result = null;
try {
    if (denominator === 0) {
        throw new Error('Cannot divide by zero');
    }
    result = numerator / denominator;
} catch (error) {
    result = 'Error: Cannot divide by zero';
}
// Your code here

Hint

Use console.log(result) to show the final output.

Practice

1.

What is the main purpose of using try and catch in JavaScript?

easy

A. To create new variables

B. To speed up the program execution

C. To handle errors and prevent the program from crashing

D. To repeat code multiple times

Solution

1. Step 1: Understand the role of try block

   The try block runs code that might cause an error.

2. Step 2: Understand the role of catch block

   The catch block handles any error from the try block to keep the program running smoothly.

3. Final Answer:

   To handle errors and prevent the program from crashing -> Option C

4. Quick Check:

   try-catch = error handling [OK]

Hint: Try-catch blocks catch errors to keep programs running [OK]

Common Mistakes:

• Thinking try-catch speeds up code
• Using try-catch to create variables
• Confusing try-catch with loops

2.

Which of the following is the correct syntax to catch errors in JavaScript?

try {
  // code
} catch (error) {
  // handle error
}

easy

A. try { } catch (error) { }

B. try { } catch error() { }

C. try (error) { } catch { }

D. try { } catch error { }

Solution

1. Step 1: Check the try block syntax

   The try block must be followed by catch with parentheses around the error variable.

2. Step 2: Verify the catch block syntax

   The correct syntax is catch (error) { } with parentheses around the error parameter.

3. Final Answer:

   try { } catch (error) { } -> Option A

4. Quick Check:

   catch needs parentheses [OK]

Hint: Catch block always uses parentheses for error [OK]

Common Mistakes:

• Omitting parentheses in catch
• Using parentheses after try
• Adding parentheses after catch keyword

3.

What will be the output of the following code?

try {
  let result = 10 / 0;
  console.log(result);
} catch (e) {
  console.log('Error caught');
}

medium

A. Infinity

B. Error caught

C. 0

D. undefined

Solution

1. Step 1: Understand division by zero in JavaScript

   Dividing 10 by 0 does not cause an error; it results in Infinity.

2. Step 2: Check if catch block runs

   Since no error occurs, the catch block is skipped and Infinity is logged.

3. Final Answer:

   Infinity -> Option A

4. Quick Check:

   10/0 = Infinity, no error [OK]

Hint: Division by zero returns Infinity, no error thrown [OK]

Common Mistakes:

• Assuming division by zero throws error
• Expecting catch block to run
• Confusing 0 with Infinity

4.

Find the error in this code and choose the correct fix:

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

medium

A. Declare x before try block

B. Change catch e to catch (e)

C. Add finally block after catch

D. Remove the catch block

Solution

1. Step 1: Identify syntax error in catch block

   The catch block must have parentheses around the error variable, so catch e is invalid syntax.

2. Step 2: Correct the catch syntax

   Changing it to catch (e) fixes the syntax and allows error handling.

3. Final Answer:

   Change catch e to catch (e) -> Option B

4. Quick Check:

   Catch needs parentheses around error [OK]

Hint: Catch error variable must be in parentheses [OK]

Common Mistakes:

• Omitting parentheses in catch
• Adding finally when not needed
• Ignoring undeclared variable error

5.

You want to safely parse JSON data that might be invalid. Which code snippet correctly catches parsing errors?

const jsonString = '{"name": "Alice", age: 25}';
try {
  const data = JSON.parse(jsonString);
  console.log(data.name);
} catch (error) {
  console.log('Invalid JSON:', error.message);
}

hard

A. You must use if-else instead of try-catch for JSON parsing

B. This code will crash because JSON.parse never throws errors

C. The catch block should be after console.log(data.name)

D. This code correctly catches JSON parsing errors and logs a message

Solution

1. Step 1: Understand JSON.parse behavior

   JSON.parse throws an error if the JSON string is invalid, like missing quotes around keys.

2. Step 2: Check try-catch usage

   The try block runs JSON.parse, and the catch block handles any parsing error to avoid crashing.

3. Final Answer:

   This code correctly catches JSON parsing errors and logs a message -> Option D

4. Quick Check:

   try-catch handles JSON.parse errors [OK]

Hint: Use try-catch to handle JSON.parse errors safely [OK]

Common Mistakes:

• Assuming JSON.parse never throws errors
• Placing catch block incorrectly
• Using if-else instead of try-catch for errors