How to fix auth/missing-uid: User ID identifier required in Firebase

import { getAuth } from 'firebase-admin/auth';

// ❌ Will fail if uid is undefined or empty
const customToken = await getAuth().createCustomToken(uid);

import { getAuth } from 'firebase-admin/auth';

async function createTokenSafely(uid: string | undefined) {
  // Validate UID exists and is not empty
  if (!uid || typeof uid !== 'string' || uid.trim() === '') {
    throw new Error('UID is required and must be a non-empty string');
  }

  try {
    const customToken = await getAuth().createCustomToken(uid);
    return customToken;
  } catch (error: any) {
    console.error('Failed to create token:', error.code, error.message);
    throw error;
  }
}

// Usage
await createTokenSafely(uid);

import { getAuth } from 'firebase-admin/auth';

// ❌ These will be undefined
const userId = user.id;              // Wrong property
const userId = user.userId;          // Wrong property
const userId = user.user_id;         // Wrong property
const userId = userRecord.uid;       // If user is undefined

// ❌ Will cause missing-uid error
const customToken = await getAuth().createCustomToken(userId);

import { getAuth } from 'firebase-admin/auth';

// ✅ Correct property name
const uid = user.uid;

// For Firebase Auth user record
const userRecord = await getAuth().getUser('some-uid');
const uid = userRecord.uid;

// For JWT decoded tokens
const decodedToken = await getAuth().verifyIdToken(idToken);
const uid = decodedToken.uid;

const customToken = await getAuth().createCustomToken(uid);

import { getAuth } from 'firebase-admin/auth';
import functions from 'firebase-functions';

// ❌ Doesn't validate uid parameter exists
exports.myFunction = functions.https.onRequest(async (req, res) => {
  const uid = req.body.uid;  // Could be undefined

  const token = await getAuth().createCustomToken(uid);  // Fails!
  res.json({ token });
});

import { getAuth } from 'firebase-admin/auth';
import functions from 'firebase-functions';

exports.myFunction = functions.https.onRequest(async (req, res) => {
  const uid = req.body?.uid;

  // Validate UID parameter
  if (!uid || typeof uid !== 'string') {
    return res.status(400).json({
      error: 'Missing or invalid uid parameter',
      code: 'MISSING_UID'
    });
  }

  try {
    const token = await getAuth().createCustomToken(uid);
    res.json({ token });
  } catch (error: any) {
    console.error('Error:', error.code);
    res.status(500).json({ error: error.message });
  }
});

import { getAuth } from 'firebase-admin/auth';

async function updateUserClaims(user: any) {  // ❌ any type
  // UID might be missing and TypeScript won't catch it
  await getAuth().setCustomUserClaims(user.uid, { role: 'admin' });
}

import { getAuth } from 'firebase-admin/auth';

interface UserData {
  uid: string;  // ✅ Required property
  email?: string;
  displayName?: string;
}

async function updateUserClaims(user: UserData, claims: Record<string, any>) {
  // TypeScript ensures uid is present
  if (!user.uid) {
    throw new Error('User UID is required');
  }

  await getAuth().setCustomUserClaims(user.uid, claims);
}

// Usage
await updateUserClaims({ uid: 'user-123' }, { role: 'admin' });

{
  "compilerOptions": {
    "strict": true,
    "strictNullChecks": true,
    "noImplicitAny": true
  }
}

import { getAuth } from 'firebase-admin/auth';

async function createTokenWithDebug(uid: any) {
  console.log('UID value:', uid);
  console.log('UID type:', typeof uid);
  console.log('UID length:', uid?.length || 'N/A');
  console.log('Is UID truthy:', !!uid);

  // Additional checks
  if (uid === undefined) {
    console.error('ERROR: uid is undefined');
    throw new Error('auth/missing-uid: User ID identifier required');
  }

  if (uid === null) {
    console.error('ERROR: uid is null');
    throw new Error('auth/missing-uid: User ID identifier required');
  }

  if (uid === '') {
    console.error('ERROR: uid is empty string');
    throw new Error('auth/missing-uid: User ID identifier required');
  }

  if (typeof uid !== 'string') {
    console.error('ERROR: uid is not a string, got:', typeof uid);
    throw new Error('auth/missing-uid: User ID identifier required');
  }

  console.log('Creating custom token for UID:', uid);
  const customToken = await getAuth().createCustomToken(uid);
  return customToken;
}

# Install Firebase CLI
npm install -g firebase-tools

# Start emulator
firebase emulators:start --only auth

# In another terminal, initialize Firebase with emulator
export FIREBASE_AUTH_EMULATOR_HOST=localhost:9099

import { getAuth } from 'firebase-admin/auth';

// When FIREBASE_AUTH_EMULATOR_HOST env var is set,
// Firebase Admin SDK connects to local emulator instead of production

async function testCreateCustomToken() {
  try {
    // This will use the emulator
    const token = await getAuth().createCustomToken('test-user-123');
    console.log('Success:', token);
  } catch (error: any) {
    console.error('Error:', error.code, error.message);
  }
}

await testCreateCustomToken();