Troubleshooting Common Errors

Quick fixes for errors you'll encounter

Terminal Errors

"command not found: node"

Problem: Node.js isn't installed or not in PATH.

Fix:

# Check if installed
which node

# If not, install
# Mac: https://nodejs.org or
brew install node

# Windows: https://nodejs.org

"command not found: npm"

Problem: npm comes with Node. Same fix as above.

"EACCES: permission denied"

Problem: npm trying to write where it shouldn't.

Fix:

# Mac/Linux: Use sudo (not ideal)
sudo npm install -g package

# Better: Fix npm permissions
# https://docs.npmjs.com/resolving-eacces-permissions-errors

"ENOENT: no such file or directory"

Problem: File or folder doesn't exist.

Fix: Check your path. Common causes:

• Typo in filename
• Wrong directory
• File deleted

React Errors

"Cannot read property 'X' of undefined"

Problem: Accessing property on undefined variable.

Fix:

// Bad
user.name  // crashes if user is undefined

// Good
user?.name  // returns undefined safely

"Each child should have a unique key prop"

Problem: List items missing keys.

Fix:

// Bad
{items.map(item => <Item item={item} />)}

// Good
{items.map(item => <Item key={item.id} item={item} />)}

"Too many re-renders"

Problem: Infinite render loop, usually from setState in render.

Fix:

// Bad - updates state on every render
function Component() {
  const [count, setCount] = useState(0);
  setCount(count + 1);  // Infinite loop!
}

// Good - update in event handler
function Component() {
  const [count, setCount] = useState(0);
  return <button onClick={() => setCount(count + 1)}>+</button>;
}

"React Hook must be called at the top level"

Problem: Hook inside condition or loop.

Fix:

// Bad
if (loggedIn) {
  const [data] = useState();  // Error!
}

// Good
const [data] = useState();  // Always at top

TypeScript Errors

"Type 'X' is not assignable to type 'Y'"

Problem: Wrong type being used.

Fix: Check what type is expected and provide it.

// If expecting string[]
const items: string[] = ['a', 'b'];  // not [1, 2]

"Property 'X' does not exist on type 'Y'"

Problem: Using property that's not defined.

Fix:

// Add to interface
interface User {
  name: string;
  email: string;  // Add missing property
}

"Object is possibly 'undefined'"

Problem: TypeScript protecting you from null access.

Fix:

// Option 1: Optional chaining
user?.name

// Option 2: Check first
if (user) {
  console.log(user.name);
}

// Option 3: Non-null assertion (only if you're sure)
user!.name

Firebase Errors

"Firebase: Error (auth/wrong-password)"

Problem: Incorrect password.

Fix: Check password. Consider adding password reset.

"Firebase: Error (auth/user-not-found)"

Problem: No account with this email.

Fix: Register first or check email spelling.

"Firebase: Error (auth/email-already-in-use)"

Problem: Account already exists.

Fix: Login instead of register.

"Missing or insufficient permissions"

Problem: Firestore security rules blocking access.

Fix:

1. Check you're logged in
2. Check userId matches document owner
3. Verify security rules are correct

"Firebase: No Firebase App '[DEFAULT]'"

Problem: Firebase not initialized before use.

Fix: Make sure initializeApp() runs before accessing Firebase services.

Git Errors

"fatal: not a git repository"

Problem: Not in a Git-initialized folder.

Fix:

# Initialize Git
git init

# Or, you're in wrong folder
cd correct-folder

"error: failed to push some refs"

Problem: Remote has changes you don't have.

Fix:

# Pull first
git pull

# Then push
git push

"CONFLICT (content): Merge conflict"

Problem: Same lines changed in different places.

Fix:

1. Open conflicting files
2. Look for <<<<<<< markers
3. Decide which version to keep
4. Remove markers
5. Commit the resolution

Build Errors

"Module not found"

Problem: Package not installed or wrong import path.

Fix:

# Install missing package
npm install package-name

# Check import path
import X from './correct/path'  # Not '../wrong/path'

"SyntaxError: Unexpected token"

Problem: Invalid syntax somewhere.

Fix: Check the line number in the error. Common causes:

• Missing comma
• Extra bracket
• Typo in keyword

Browser Errors

"CORS error"

Problem: Browser blocking cross-origin request.

Fix: This is a server-side fix. With Firebase, shouldn't happen for normal use.

"Failed to fetch"

Problem: Network request failed.

Fix:

• Check internet connection
• Check if server/Firebase is up
• Check URL is correct

Debug Steps

When something breaks:

1. Read the error — It often tells you what's wrong
2. Check the line number — Go to that file and line
3. Add console.log — See what values actually are
4. Search the error — Someone else had this problem
5. Simplify — Remove code until it works, then add back
6. Ask AI — Paste the error and relevant code

Console.log Debugging

function brokenFunction(data) {
  console.log('1. Function called');
  console.log('2. Data received:', data);

  const result = process(data);
  console.log('3. After process:', result);

  return result;
}

Check which log appears last → problem is after that.

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Keep this handy when errors appear!