How to Handle Errors in JavaScript? | JavaScript Interview Questions

What You'll Learn

try/catch/finally
Error types
Custom errors

Try/Catch/Finally

code.jsJavaScript

try {
  const data = JSON.parse(invalidJson);
} catch (error) {
  console.error('Parse error:', error.message);
} finally {
  console.log('Always runs');
}

Error Types

Type	Description
Error	Generic error
SyntaxError	Invalid syntax
ReferenceError	Invalid reference
TypeError	Invalid type
RangeError	Number out of range

code.jsJavaScript

new TypeError('Expected string');
new ReferenceError('x is not defined');

Custom Errors

code.jsJavaScript

class ValidationError extends Error {
  constructor(message, field) {
    super(message);
    this.name = 'ValidationError';
    this.field = field;
  }
}

function validateEmail(email) {
  if (!email.includes('@')) {
    throw new ValidationError('Invalid email', 'email');
  }
}

try {
  validateEmail('invalid');
} catch (e) {
  if (e instanceof ValidationError) {
    console.log(`${e.field}: ${e.message}`);
  }
}

Async Error Handling

code.jsJavaScript

// Promises
fetch('/api/data')
  .then(res => res.json())
  .catch(err => console.error(err));

// Async/await
async function fetchData() {
  try {
    const res = await fetch('/api/data');
    return await res.json();
  } catch (error) {
    console.error('Fetch failed:', error);
  }
}