How to monitor app logs in Replit

Replit has three distinct places where output appears. The Console tab shows output from the Run button — it displays structured entries with command, stdout, stderr, duration, and status metadata. The Shell is an interactive terminal for ad-hoc commands. Deployment logs appear in the Deployments pane under the Logs tab and show output from your production app. Client-side JavaScript logs (browser console.log) only appear in the Preview DevTools, not in the Console. Understanding which pane to check saves significant debugging time.

Instead of using console.log for everything, implement structured log levels: debug for development details, info for normal operations, warn for recoverable issues, and error for failures. Create a simple logger utility that prefixes each message with the level and timestamp. This makes it easy to filter logs by severity and find the important messages quickly.

1// src/utils/logger.js
2const LOG_LEVELS = { debug: 0, info: 1, warn: 2, error: 3 };
3const CURRENT_LEVEL = LOG_LEVELS[process.env.LOG_LEVEL || 'info'];
4
5function log(level, message, data = {}) {
6  if (LOG_LEVELS[level] < CURRENT_LEVEL) return;
7  const entry = {
8    timestamp: new Date().toISOString(),
9    level: level.toUpperCase(),
10    message,
11    ...data,
12  };
13  const output = JSON.stringify(entry);
14  if (level === 'error') console.error(output);
15  else if (level === 'warn') console.warn(output);
16  else console.log(output);
17}
18
19module.exports = {
20  debug: (msg, data) => log('debug', msg, data),
21  info: (msg, data) => log('info', msg, data),
22  warn: (msg, data) => log('warn', msg, data),
23  error: (msg, data) => log('error', msg, data),
24};

For web applications, logging every incoming HTTP request helps you understand traffic patterns and identify slow endpoints. Add middleware that logs the method, path, status code, and response time for each request. This is the most valuable logging you can add to a web app — it gives you a complete picture of what your server is doing.

1// src/middleware/requestLogger.js
2const logger = require('../utils/logger');
3
4function requestLogger(req, res, next) {
5  const start = Date.now();
6  
7  res.on('finish', () => {
8    const duration = Date.now() - start;
9    const logData = {
10      method: req.method,
11      path: req.originalUrl,
12      status: res.statusCode,
13      duration_ms: duration,
14      ip: req.ip,
15    };
16    
17    if (res.statusCode >= 500) {
18      logger.error('Server error', logData);
19    } else if (res.statusCode >= 400) {
20      logger.warn('Client error', logData);
21    } else {
22      logger.info('Request completed', logData);
23    }
24  });
25  
26  next();
27}
28
29module.exports = requestLogger;

Import the logger and request logging middleware into your main application file. The request logger middleware should be added early in the middleware chain so it captures all requests. Use the logger throughout your application to replace raw console.log statements with structured, leveled output.

1// index.js
2const express = require('express');
3const logger = require('./src/utils/logger');
4const requestLogger = require('./src/middleware/requestLogger');
5
6const app = express();
7const PORT = process.env.PORT || 3000;
8
9// Add request logging middleware
10app.use(requestLogger);
11app.use(express.json());
12
13app.get('/health', (req, res) => {
14  res.json({ status: 'ok' });
15});
16
17app.get('/api/data', (req, res) => {
18  logger.info('Fetching data', { userId: req.query.userId });
19  // ... fetch data ...
20  res.json({ data: [] });
21});
22
23app.listen(PORT, '0.0.0.0', () => {
24  logger.info('Server started', { port: PORT, env: process.env.NODE_ENV });
25});

Use the [run.env] section in .replit to set LOG_LEVEL=debug during development so you see all log output. For production deployments, set LOG_LEVEL=info or LOG_LEVEL=warn in the Deployments pane under Settings to reduce log volume and focus on important events. The REPLIT_DEPLOYMENT environment variable (set to '1' in production) can also be used to detect the environment automatically.

1# .replit file
2run = "node index.js"
3
4[run.env]
5LOG_LEVEL = "debug"
6NODE_ENV = "development"

After deploying your app, monitor production logs by opening the Deployments pane from the left sidebar and clicking the Logs tab. These logs show real-time output from your deployed application, including startup messages, request logs, warnings, and errors. The logs update in real time as traffic hits your deployed app. This is where you investigate production issues — check for error-level entries, slow request durations, and unexpected status codes.