Quick Answer
Error messages tell you what went wrong, but you need to know how to read them. This guide covers the most common errors vibecoders encounter—from npm failures to server errors to JavaScript undefined errors—with plain-English explanations and fixes.

How to read an error message

Most error messages have three parts:

1. Error Type/Name    →  ModuleNotFoundError
2. Error Message      →  No module named 'requests'
3. Location/Context   →  File "app.py", line 3

Focus on the message first, then check the location. Stack traces (the long list of function calls) show you the path—start from the top.

npm / Node.js errors

npm ERR! ENOENT

npm ERR! code ENOENT
npm ERR! syscall open
npm ERR! path /your/project/package.json
npm ERR! errno -2
npm ERR! enoent ENOENT: no such file or directory

What it means: “ENOENT” = “Error NO ENTry” = file not found. npm can’t find package.json.

Fixes:

  • Are you in the right folder? Run ls or dir to check
  • Run npm init -y to create a package.json
  • You might be one folder too deep or too shallow

npm ERR! ERESOLVE unable to resolve dependency tree

npm ERR! ERESOLVE unable to resolve dependency tree
npm ERR! peer dep missing: react@^17.0.0

What it means: Two packages want different versions of the same dependency.

Fixes:

bash
# Try forcing it (usually works)
npm install --legacy-peer-deps

# Or use the force flag
npm install --force

# Nuclear option: delete and reinstall
rm -rf node_modules package-lock.json
npm install

npm ERR! code EACCES (permission denied)

npm ERR! code EACCES
npm ERR! syscall mkdir
npm ERR! path /usr/local/lib/node_modules

What it means: npm is trying to install globally but doesn’t have permission.

Fixes:

  • Don’t use sudo npm install (creates more problems)
  • Install locally instead: npm install package-name (no -g)
  • Fix npm permissions: npm docs on permissions

Module not found: Can’t resolve ‘package-name’

Module not found: Can't resolve 'axios'

What it means: You’re importing a package that isn’t installed.

Fixes:

bash
npm install axios
# Then restart your dev server

Error: listen EADDRINUSE: address already in use :::3000

What it means: Something is already running on port 3000.

Fixes:

bash
# Find what's using the port (Mac/Linux)
lsof -i :3000

# Kill it
kill -9 <PID>

# Or just use a different port
PORT=3001 npm run dev

JavaScript errors

TypeError: Cannot read properties of undefined (reading ‘x’)

javascript
TypeError: Cannot read properties of undefined (reading 'name')
    at UserProfile (UserProfile.js:15)

What it means: You tried to access .name on something that’s undefined.

Common causes:

javascript
// The data hasn't loaded yet
const user = undefined;
console.log(user.name);  // Crash!

// The API response doesn't have what you expected
const data = { results: [] };
console.log(data.user.name);  // data.user is undefined

Fixes:

javascript
// Optional chaining
console.log(user?.name);  // Returns undefined instead of crashing

// Check before accessing
if (user) {
  console.log(user.name);
}

// Default value
console.log(user?.name || 'Unknown');

ReferenceError: x is not defined

javascript
ReferenceError: userName is not defined

What it means: You’re using a variable that doesn’t exist.

Common causes:

  • Typo in variable name (userName vs username)
  • Variable declared in a different scope
  • Forgot to import something

Fix: Check spelling, check imports, check if variable is in scope.

SyntaxError: Unexpected token

javascript
SyntaxError: Unexpected token ')'
SyntaxError: Unexpected end of input

What it means: The code has a syntax error—missing bracket, extra comma, etc.

Fixes:

  • Look at the line number
  • Check for missing } or )
  • Check for extra commas in objects/arrays
  • Use an editor with syntax highlighting

Uncaught (in promise) Error / Unhandled promise rejection

What it means: An async operation failed and you didn’t handle the error.

javascript
// This will show unhandled rejection if the fetch fails
fetch('/api/data').then(r => r.json())

// Fix: add .catch() or try/catch
fetch('/api/data')
  .then(r => r.json())
  .catch(err => console.error('Fetch failed:', err));

// Or with async/await
try {
  const response = await fetch('/api/data');
  const data = await response.json();
} catch (err) {
  console.error('Fetch failed:', err);
}

Python errors

ModuleNotFoundError: No module named ‘x’

python
ModuleNotFoundError: No module named 'requests'

What it means: The package isn’t installed.

Fixes:

bash
pip install requests

# Or if you're using a virtual environment
python -m pip install requests

IndentationError: unexpected indent

What it means: Python is very strict about indentation. Your spaces/tabs are inconsistent.

Fixes:

  • Use consistent indentation (4 spaces is standard)
  • Don’t mix tabs and spaces
  • Your editor might have “convert tabs to spaces” setting

TypeError: ‘NoneType’ object is not subscriptable

python
result = some_function()
print(result['key'])  # Crash if result is None

What it means: You’re trying to access ['key'] on None.

Fix:

python
if result is not None:
    print(result['key'])

HTTP errors (browser/server)

400 Bad Request

What it means: Your request is malformed. The server can’t understand it.

Common causes:

  • Invalid JSON in request body
  • Missing required fields
  • Wrong data types

Fix: Check your request format, validate JSON, check API documentation.

401 Unauthorized

What it means: You’re not authenticated. No API key, or it’s wrong/expired.

Fix: Check your API key or token. Check that it’s being sent correctly (header, bearer token, etc.).

403 Forbidden

What it means: You’re authenticated but don’t have permission for this action.

Fix: Check user permissions, API scopes, or if you’re accessing the right resource.

404 Not Found

What it means: The URL doesn’t exist.

Common causes:

  • Typo in URL
  • Resource was deleted
  • Wrong API endpoint

Fix: Check the URL, check API documentation, verify the resource exists.

500 Internal Server Error

What it means: The server crashed. It’s not your fault (usually).

What to do:

  • Check server logs (if it’s your server)
  • Try again later (if it’s someone else’s server)
  • The error is on the backend, not your frontend code

502 Bad Gateway

What it means: A server in the middle (proxy, load balancer) couldn’t reach the actual server.

Common causes:

  • Server is down
  • Server is overloaded
  • Deployment is in progress

Fix: Usually wait and retry. If it’s your server, check if it’s running.

503 Service Unavailable

What it means: Server is temporarily unable to handle requests. Often during maintenance or overload.

Fix: Wait and retry.

CORS error

Access to fetch at 'https://api.example.com' from origin 'http://localhost:3000' 
has been blocked by CORS policy

What it means: The browser is blocking a request to a different domain for security reasons.

See: What is CORS? for full explanation and fixes.

Git errors

fatal: not a git repository

What it means: You’re not in a Git repository.

Fixes:

bash
# Initialize a new repo
git init

# Or navigate to the right folder
cd your-project

error: failed to push some refs

error: failed to push some refs to 'github.com/user/repo.git'
hint: Updates were rejected because the remote contains work that you do not have locally.

What it means: Someone else pushed changes (or you pushed from another machine). Your local is behind.

Fix:

bash
git pull origin main
# Resolve any conflicts if needed
git push origin main

CONFLICT: Merge conflict in file.js

What it means: Two changes touched the same lines, and Git can’t automatically combine them.

Fix: Open the file, look for <<<<<<<, =======, >>>>>>> markers. Keep the code you want, delete the markers, save, then:

bash
git add file.js
git commit -m "Resolved merge conflict"

React / Next.js errors

Hydration mismatch

Error: Hydration failed because the initial UI does not match what was rendered on the server.

What it means: Server-rendered HTML differs from what React renders on the client.

Common causes:

  • Using Date.now() or Math.random() during render
  • Browser extensions modifying the page
  • Conditional rendering based on window or document

Fix: Make sure server and client render the same thing initially. Use useEffect for client-only code.

Invalid hook call

Invalid hook call. Hooks can only be called inside of the body of a function component.

What it means: You’re using a hook (useState, useEffect) wrong.

Common causes:

  • Hook inside a regular function (not a component)
  • Hook inside a condition or loop
  • Multiple React versions

Fix: Hooks must be at the top level of a function component, not nested.

Database errors

Connection refused (PostgreSQL, MySQL)

ECONNREFUSED 127.0.0.1:5432

What it means: Can’t connect to the database.

Common causes:

  • Database isn’t running
  • Wrong host/port
  • Firewall blocking connection

Fix: Start the database, check connection string, check if you’re connecting to the right place.

Relation/table does not exist

relation "users" does not exist

What it means: You’re querying a table that doesn’t exist.

Fix: Run migrations, check table name spelling, check you’re connected to the right database.

The debugging mindset

  1. Read the error - The actual message, not just “it’s broken”
  2. Find the location - Which file, which line?
  3. Understand what it’s saying - Look up unfamiliar error types
  4. Form a hypothesis - “I think the API isn’t returning what I expect”
  5. Test it - Add a console.log, check the data
  6. Fix and verify - Does the error go away?

When you’re stuck, search for the exact error message. Someone else has had this problem.

Further reading