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.