Error Handling

try / catch / finally

try {
  const data = JSON.parse("invalid json");
} catch (error) {
  console.error(error.message); // "Unexpected token i in JSON..."
  console.error(error.name);    // "SyntaxError"
  console.error(error.stack);   // full stack trace
} finally {
  console.log("always runs");   // cleanup code
}

Error Types

Error Type	When
SyntaxError	Invalid syntax
ReferenceError	Variable not found
TypeError	Wrong type operation
RangeError	Value out of range
URIError	Bad URI functions
EvalError	Error in eval()

Throwing Errors

function divide(a, b) {
  if (b === 0) throw new Error("Cannot divide by zero");
  return a / b;
}

// You can throw anything
throw "error string";
throw 404;
throw { message: "not found", code: 404 };
throw new TypeError("Expected a number");

Custom Errors

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

try {
  throw new ValidationError("email", "Invalid email format");
} catch (error) {
  if (error instanceof ValidationError) {
    console.log(`${error.field}: ${error.message}`);
  }
}

Async Error Handling

// Promise
fetch("/api")
  .then((res) => res.json())
  .catch((err) => console.error(err));

// Async/Await
async function getData() {
  try {
    const res = await fetch("/api");
    return await res.json();
  } catch (err) {
    console.error("Request failed:", err);
  }
}

// Global handlers
window.addEventListener("unhandledrejection", (event) => {
  console.error("Unhandled promise rejection:", event.reason);
});

Key Takeaways

• Always wrap risky code in try/catch.
• Create custom error classes for domain-specific errors.
• Use finally for cleanup that must always run.
• Handle async errors with .catch() or try/catch in async functions.