How to fix RangeError: The value of 'statusCode' is out of range. It must be >= 100 and <= 599. in Node.js

app.get('/api/user', (req, res) => {
  const statusCode = undefined; // Problem: undefined
  res.status(statusCode).json({ user: data }); // Error thrown here
});

// Bad: status might be undefined
const status = someCondition ? 200 : undefined;
res.status(status).json(data);

// Good: Always provide a valid default
const status = someCondition ? 200 : 500;
res.status(status).json(data);

// Good: Use logical OR for fallback
res.status(status || 200).json(data);

function isValidStatusCode(code) {
  return Number.isInteger(code) && code >= 100 && code <= 599;
}

app.get('/api/data', (req, res) => {
  let statusCode = calculateStatus(); // Might return invalid value

  if (!isValidStatusCode(statusCode)) {
    console.error(`Invalid status code: ${statusCode}`);
    statusCode = 500; // Fallback to safe default
  }

  res.status(statusCode).json({ data });
});

// Bad: Express treats 0 as status code
res.send(0);

// Good: Wrap numeric values in objects or arrays
res.json({ count: 0 });
res.json(0); // Also works with json()

// Bad: Old Express syntax (deprecated)
res.send(200, 'OK');

// Good: Chain status and send
res.status(200).send('OK');

// Bad: Sending multiple arguments
res.send({ id: 1 }, { id: 2 }); // First treated as status

// Good: Send single response
res.json([{ id: 1 }, { id: 2 }]);

// Robust error handler
app.use((err, req, res, next) => {
  // Extract status code safely
  let statusCode = err.statusCode || err.status || 500;

  // Validate it's in range
  if (!Number.isInteger(statusCode) || statusCode < 100 || statusCode > 599) {
    console.error(`Invalid error status code: ${statusCode}`);
    statusCode = 500;
  }

  res.status(statusCode).json({
    error: err.message,
    status: statusCode,
  });
});