Error Handling in JavaScript

Error Handling in JavaScript

Errors are inevitable in any real application. Users enter unexpected input, network requests fail, APIs return unexpected data. Robust JavaScript code anticipates these failures and handles them gracefully, providing useful feedback instead of crashing silently or with confusing messages.

try / catch / finally

function parseJSON(jsonString) {
  try {
    const data = JSON.parse(jsonString);
    return data;
  } catch (error) {
    console.error('Invalid JSON:', error.message);
    return null;
  } finally {
    console.log('parseJSON completed');
  }
}

console.log(parseJSON('{"name":"Alice"}')); // { name: 'Alice' }
console.log(parseJSON('invalid json'));        // null

The Error Object

try {
  null.property; // TypeError
} catch (error) {
  console.log(error.name);    // 'TypeError'
  console.log(error.message); // "Cannot read properties of null"
  console.log(error.stack);   // full stack trace
}

Throwing Custom Errors

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

function validateAge(age) {
  if (typeof age !== 'number') {
    throw new ValidationError('age', 'Age must be a number');
  }
  if (age < 0 || age > 150) {
    throw new ValidationError('age', 'Age must be between 0 and 150');
  }
  return true;
}

try {
  validateAge(-5);
} catch (error) {
  if (error instanceof ValidationError) {
    console.error(`Validation failed on field '${error.field}': ${error.message}`);
  } else {
    throw error; // re-throw unexpected errors
  }
}