Throwing errors in Javascript - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output when throwing a string error?

Consider this JavaScript code that throws a string as an error. What will be printed to the console?

Javascript

try {
  throw "Oops! Something went wrong.";
} catch (e) {
  console.log(e);
}

A Error: Oops! Something went wrong.

B Oops! Something went wrong.

C undefined

D SyntaxError

💡 Hint

When you throw a string, the catch block receives that string directly.

Throwing a string means the catch block gets that string as the error value. So logging it prints the string itself.

❓ Predict Output

intermediate

What error is thrown by this code?

What error will be thrown by this code snippet?

Javascript

function test() {
  throw new TypeError('Wrong type!');
}
try {
  test();
} catch (e) {
  console.log(e.name);
}

A Error

B SyntaxError

C ReferenceError

D TypeError

💡 Hint

Look at the type of error created with new.

The code throws a TypeError object, so e.name is "TypeError".

❓ Predict Output

advanced

What happens if you throw an object without message property?

What will be the output of this code?

Javascript

try {
  throw {code: 404};
} catch (e) {
  console.log(e.message);
}

A undefined

B 404

C Error: 404

D TypeError

💡 Hint

The thrown object does not have a message property.

Since the thrown object only has a code property and no message, e.message is undefined.

❓ Predict Output

advanced

What is the output when throwing a custom error class?

What will this code print?

Javascript

class CustomError extends Error {
  constructor(msg) {
    super(msg);
    this.name = 'CustomError';
  }
}

try {
  throw new CustomError('Custom problem');
} catch (e) {
  console.log(e.name + ': ' + e.message);
}

A CustomError: Custom problem

B TypeError: Custom problem

C CustomError

D Error: Custom problem

💡 Hint

The custom error sets its name property explicitly.

The error's name is "CustomError" and message is "Custom problem", so the output combines both.

❓ Predict Output

expert

What is the output when throwing inside a Promise executor?

What will this code print to the console?

Javascript

new Promise((resolve, reject) => {
  throw new Error('Promise failed');
}).catch(e => {
  console.log(e.message);
});

A Uncaught Error

B undefined

C Promise failed

D SyntaxError

💡 Hint

Throwing inside a Promise executor triggers the rejection.

Throwing an error inside the Promise executor causes the Promise to reject with that error, so the catch handler receives it and logs its message.

Practice

1. What does the throw statement do in JavaScript?

easy

A. Stops the program and creates an error.

B. Catches an error and handles it.

C. Declares a new variable.

D. Runs a function asynchronously.

Solution

1. Step 1: Understand the purpose of throw

   The throw statement is used to create an error and stop normal program execution.

2. Step 2: Differentiate from error handling

   Unlike try...catch which handles errors, throw actually creates the error.

3. Final Answer:

   Stops the program and creates an error. -> Option A

4. Quick Check:

   throw creates error = A [OK]

Hint: Remember: throw creates errors, try...catch handles them [OK]

Common Mistakes:

• Confusing throw with try...catch
• Thinking throw handles errors
• Believing throw declares variables

2. Which of the following is the correct syntax to throw a new error with the message "Invalid input"?

easy

A. throw 'Invalid input';

B. throw error('Invalid input');

C. throw new Error('Invalid input');

D. throw new error('Invalid input');

Solution

1. Step 1: Recall the correct Error constructor usage

   In JavaScript, to create an error object, use new Error('message').

2. Step 2: Check syntax correctness

   Only throw new Error('Invalid input'); uses throw new Error('Invalid input'); which is correct syntax.

3. Final Answer:

   throw new Error('Invalid input'); -> Option C

4. Quick Check:

   Correct syntax uses new Error = C [OK]

Hint: Always use 'throw new Error' with capital E [OK]

Common Mistakes:

• Missing 'new' keyword
• Using lowercase 'error' instead of 'Error'
• Omitting parentheses

3. What will be the output of the following code?

try {
  throw new Error('Oops!');
} catch (e) {
  console.log(e.message);
}

medium

A. Oops!

B. Error: Oops!

C. undefined

D. No output

Solution

1. Step 1: Understand the throw and catch behavior

   The code throws an error with message 'Oops!'. The catch block catches it and logs e.message.

2. Step 2: Determine what e.message contains

   e.message is the string 'Oops!'. So console.log(e.message) prints 'Oops!'.

3. Final Answer:

   Oops! -> Option A

4. Quick Check:

   Logging error message prints 'Oops!' = A [OK]

Hint: Catch error and log e.message to see the message [OK]

Common Mistakes:

• Printing whole error object instead of message
• Expecting 'Error: Oops!' instead of just message
• Thinking no output occurs

4. Identify the error in this code snippet:

try {
  throw new Error('Fail');
} catch e {
  console.log(e.message);
}

medium

A. Incorrect use of throw keyword.

B. No error; code runs fine.

C. Error message string is invalid.

D. Missing parentheses around catch parameter.

Solution

1. Step 1: Check catch syntax

   In JavaScript, the catch block must have parentheses around the error parameter, like catch (e).

2. Step 2: Identify the syntax error

   The code uses catch e without parentheses, which causes a syntax error.

3. Final Answer:

   Missing parentheses around catch parameter. -> Option D

4. Quick Check:

   Catch parameter needs parentheses = B [OK]

Hint: Always write catch with parentheses: catch (e) [OK]

Common Mistakes:

• Omitting parentheses in catch
• Misplacing throw statement
• Confusing error message with syntax error

5. You want to throw an error only if a user input is empty or null. Which code correctly throws an error in this case?

hard

A. if (input) throw new Error('Input required');

B. if (!input) throw new Error('Input required');

C. if (input == null) throw new error('Input required');

D. if (input === '') throw Error('Input required');

Solution

1. Step 1: Understand falsy values for empty or null input

   In JavaScript, !input is true if input is null, undefined, empty string, 0, or false.

2. Step 2: Check which option correctly throws error for empty or null

   if (!input) throw new Error('Input required'); uses if (!input) throw new Error('Input required'); which covers empty string and null correctly with proper syntax.

3. Final Answer:

   if (!input) throw new Error('Input required'); -> Option B

4. Quick Check:

   Use !input to check empty/null and throw Error = D [OK]

Hint: Use if (!input) to catch empty or null inputs [OK]

Common Mistakes:

• Using lowercase 'error' instead of 'Error'
• Throwing error only for empty string but missing null
• Throwing error when input is valid