How To Fix Nodemon App Crashed Error Waiting for File Changes Before Starting

Understanding the Role of Nodemon

Nodemon is an invaluable tool for Node.js developers. It automatically restarts the Node.js process when files are changed, acting like live reload for backend JavaScript projects.

To use it, simply run:

nodemon app.js

Nodemon then monitors for any changes and restarts the app automatically. But if it crashes, you’ll need to troubleshoot the root cause. Let’s explore some steps.

Step 1: Check for a Missing Main Script

The first thing to check is that your command actually specifies a main script for nodemon to run:

nodemon app.js

Not passing a file or script here will cause nodemon to crash:

nodemon

Always provide a startup script argument so nodemon knows what to run!

Step 2: Fix Any Syntax Errors in Scripts

Another simple source of crashes is syntax errors in your JavaScript files like:

// Missing closing brace
const port = 8080 

// Unresolved variable
const url = endpoin 

These will crash nodemon before it can finish launching since the scripts cannot be parsed.

Fix syntax problems like missing commas, brackets, semicolons etc before anything else.

Step 3: Check for Infinite Loops Crashing Process

Infinite loops will completely hang or crash nodemon:

// Infinite loop!
while (true) {
  console.log('running forever!');
}

These endless loops overwhelm the process so it cannot start monitoring files. Remove any loops that may crash start up.

Step 4: Increase nodemon Timeout Threshold

nodemon’s crash restart timeout threshold determines how long to wait before restarting a crashed process. You can extend this to avoid restarts on short spikes:

nodemon --exitcrash=60000 app.js

Try bumping --exitcrash from default 1000ms to higher values like 60000 (1 minute) to handle temporary crashes.

Step 5: Verify nodemon Can Access Files

Filesystem permission issues can also crash nodemon before it starts by restricting access:

nodemon app.js
# Error: ENOENT: no such file or directory

Ensure nodemon has access to monitor and require files and modules. Adjust permissions or run as admin if needed.

Step 6: Check for Port or Network Conflicts

A process already occupying the application’s port will cause immediate crashes:

const port = 8080;

app.listen(port, () => {
  console.log('App listening on port ${port}'); 
});

# Crash! Port 8080 in use

Choose a different unused port if needed. Also verify firewalls are not blocking network access.

Step 7: Test Without Watch Mode

nodemon’s file watching capabilities may be triggering the crash. Try standard node instead:

node app.js

If this runs fine, an issue with nodemon’s watch mode is likely. Narrow down what file change or monitor event causes crashes.

Step 8: Disable node_modules Monitoring

nodemon recursively watching large node_modules folders can sometimes lead to crashes. Exclude module tracking:

nodemon --ignore node_modules app.js

Ignoring these heavy directories may resolve flaky behavior. Monitor source files only.

Step 9: Reset Cache and Delete tmp Folder

A corrupt cache or temporary data may be crashing nodemon:

nodemon --reset-cache app.js 

rm -rf /tmp/nodemon*

Reset nodemon’s cache and remove any tmp files to eliminate potential bad data causing crashes on start.

Step 10: Reinstall Node.js and nodemon

If crashes persist, completely removing and reinstalling nodemon and Node.js itself often resolves deeper issues:

npm uninstall nodemon -g
npm cache clean

# Reinstall Node.js then
npm install nodemon -g 

Stale installations can cause “waiting for file changes” errors. A fresh install fixes crashes.

Summary of Resolutions

• Pass a valid startup script
• Fix syntax errors preventing launch
• Remove infinite loops overwhelming process
• Increase timeout threshold
• Confirm nodemon has filesystem access
• Handle port conflicts
• Test without watch mode to isolate issue
• Ignore heavy node_modules folder
• Reset cache and delete tmp files
• Fresh install of Node.js and nodemon

Trying these troubleshooting steps will help narrow down what is causing the nodemon crash, and get it running properly again.

Key Takeaways for Fixing Nodemon Crashes

To recap, focus on:

• Passing a valid startup script
• Fixing syntax errors and infinite loops
• Adjusting timeout thresholds
• Ensuring filesystem permissions
• Checking ports and network access
• Testing without watch mode
• Cleaning cache files and tmp data
• Reinstalling Node.js/nodemon completely

Targeting these specific issue areas will help resolve the vague “app crashed” error. Never let a nodemon crash slow you down again!

Conclusion

While frustrating, a nodemon “app crashed” error almost always results from fixable issues like incorrect configurations, code errors, or software corruption.

With the right troubleshooting technique, you can isolate the culprit and get nodemon running properly again. Use this guide to confidently diagnose and resolve those cryptic nodemon crashes so you can keep development moving. Happy coding!