How to debug async code in Replit

1// Structured logging for async debugging
2async function fetchUserData(userId) {
3  console.log(`[fetchUserData] START — userId: ${userId}, time: ${Date.now()}`);
4
5  try {
6    console.log(`[fetchUserData] Step 1: Fetching profile...`);
7    const profile = await fetch(`/api/users/${userId}`);
8    console.log(`[fetchUserData] Step 1 DONE — status: ${profile.status}`);
9
10    console.log(`[fetchUserData] Step 2: Parsing JSON...`);
11    const data = await profile.json();
12    console.log(`[fetchUserData] Step 2 DONE — keys: ${Object.keys(data)}`);
13
14    console.log(`[fetchUserData] Step 3: Fetching orders...`);
15    const orders = await fetch(`/api/users/${userId}/orders`);
16    console.log(`[fetchUserData] Step 3 DONE — status: ${orders.status}`);
17
18    return data;
19  } catch (error) {
20    console.error(`[fetchUserData] ERROR: ${error.message}`);
21    throw error;
22  }
23}

1// BAD: error silently disappears
2async function saveData(data) {
3  const response = await fetch('/api/save', {
4    method: 'POST',
5    body: JSON.stringify(data)
6  });
7  return response.json();
8}
9
10// GOOD: error is caught, logged, and re-thrown
11async function saveData(data) {
12  try {
13    const response = await fetch('/api/save', {
14      method: 'POST',
15      headers: { 'Content-Type': 'application/json' },
16      body: JSON.stringify(data)
17    });
18
19    if (!response.ok) {
20      throw new Error(`HTTP ${response.status}: ${response.statusText}`);
21    }
22
23    return await response.json();
24  } catch (error) {
25    console.error('[saveData] Failed:', error.message);
26    console.error('[saveData] Stack:', error.stack);
27    throw error;
28  }
29}

1// Add to the top of your entry file (index.js / server.js)
2
3// Catch unhandled Promise rejections
4process.on('unhandledRejection', (reason, promise) => {
5  console.error('=== UNHANDLED PROMISE REJECTION ===');
6  console.error('Reason:', reason);
7  console.error('Promise:', promise);
8  console.error('Stack:', reason?.stack || 'No stack trace');
9  console.error('===================================');
10});
11
12// Catch uncaught exceptions
13process.on('uncaughtException', (error) => {
14  console.error('=== UNCAUGHT EXCEPTION ===');
15  console.error('Error:', error.message);
16  console.error('Stack:', error.stack);
17  console.error('==========================');
18  process.exit(1);
19});

1// Promise.all — one failure cancels everything (bad for debugging)
2try {
3  const [users, orders, products] = await Promise.all([
4    fetchUsers(),
5    fetchOrders(),
6    fetchProducts()
7  ]);
8} catch (error) {
9  console.error('Something failed, but what?', error.message);
10}
11
12// Promise.allSettled — see every result (good for debugging)
13const results = await Promise.allSettled([
14  fetchUsers(),
15  fetchOrders(),
16  fetchProducts()
17]);
18
19results.forEach((result, index) => {
20  const names = ['fetchUsers', 'fetchOrders', 'fetchProducts'];
21  if (result.status === 'fulfilled') {
22    console.log(`[${names[index]}] SUCCESS:`, result.value.length, 'items');
23  } else {
24    console.error(`[${names[index]}] FAILED:`, result.reason.message);
25  }
26});

1// Server-side — appears in Replit Console
2app.get('/api/data', async (req, res) => {
3  console.log('[SERVER] Request received for /api/data');
4  // This log appears in Console pane
5});
6
7// Client-side — appears in Preview DevTools
8function App() {
9  useEffect(() => {
10    console.log('[CLIENT] Component mounted');
11    // This log appears in Preview DevTools, NOT Console
12  }, []);
13}

1import asyncio
2import logging
3
4# Enable asyncio debug mode
5logging.basicConfig(level=logging.DEBUG)
6
7async def fetch_data(url):
8    logging.info(f'[fetch_data] Starting request to {url}')
9    try:
10        # Simulate async HTTP request
11        await asyncio.sleep(1)
12        logging.info(f'[fetch_data] Completed request to {url}')
13        return {'status': 'ok'}
14    except Exception as e:
15        logging.error(f'[fetch_data] Failed: {e}')
16        raise
17
18async def main():
19    logging.info('[main] Starting async operations')
20    tasks = [
21        fetch_data('https://api.example.com/users'),
22        fetch_data('https://api.example.com/orders'),
23    ]
24    results = await asyncio.gather(*tasks, return_exceptions=True)
25    for i, result in enumerate(results):
26        if isinstance(result, Exception):
27            logging.error(f'Task {i} failed: {result}')
28        else:
29            logging.info(f'Task {i} succeeded: {result}')
30
31asyncio.run(main(), debug=True)