Error handling in JavaScript: try-catch, custom errors, and error types

Handling Errors Intentionally

Unhandled errors crash your program or silently corrupt state. Explicit error handling lets you recover gracefully, log failures, or provide meaningful messages to users. Understanding JavaScript's error types and how to create custom ones is essential for production code.

try-catch-finally

try {
  const data = JSON.parse(userInput); // may throw SyntaxError
  process(data);
} catch (err) {
  if (err instanceof SyntaxError) {
    console.error("Invalid JSON:", err.message);
  } else {
    throw err; // rethrow unexpected errors — do not swallow them
  }
} finally {
  cleanup(); // always runs — close connections, release locks
}

Built-In Error Types

JavaScript has several built-in error constructors: SyntaxError, TypeError, ReferenceError, RangeError, URIError. Use instanceof to distinguish them in catch blocks.

Custom Error Classes

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

try {
  throw new ValidationError("email", "Must contain @");
} catch (err) {
  if (err instanceof ValidationError) {
    console.log(`Field ${err.field}: ${err.message}`);
  } else {
    throw err;
  }
}

Golden Rule

Never silence errors with an empty catch block. Either handle the error meaningfully, log it with context, or rethrow it. Swallowed errors are the hardest bugs to diagnose in production — they leave no trace of what went wrong or when.