AWS custom api gateway authorizer using firebase auth JWT

firebaseApiGwAuthorizer.js

const admin = require('firebase-admin');

//from your firebase project service
const serviceAccount = {
    "type": "",
    "project_id": "",
    "private_key_id": "",
    "private_key": "",
    "client_email": "",
    "client_id": "",
    "auth_uri": "",
    "token_uri": "",
    "auth_provider_x509_cert_url": ",
    "client_x509_cert_url": ""
 }

//or require('./serviceAccountKey.json');

const initializeSdk = function () {
  // Check if Firebase Admin SDK is already initialized, if not, then do it
  if (admin.apps.length == 0) {
    admin.initializeApp({
      credential: admin.credential.cert(serviceAccount),
      databaseURL: process.env.FIREBASE_DATABASE_URL // <----- LOOK !!
    });
  }
}

// Helper funtion for generating the response API Gateway requires to handle the token verification
const generateIamPolicy = (effect, resource, data) => {
  const authResponse = {};

  // Define a user object that passes user data and other user informationd decoded from the Firebase token to the Lambda API handler
  const user = {};

  // Populate the API Gateway user principalId with the Firebase user id, or 'unavailable' if not returned from Firebase
  data ? authResponse.principalId = data.user_id : 'unavailable';

  // Map values into context object passed into Lambda function, if data is present
  if (data) {
    user.email = data.email;
    user.email_verified = data.email_verified;
    user.uid = data.uid; // firebase unique id
    authResponse.context = user;
  }

  if (effect && resource) {
    var policyDocument = {};
    policyDocument.Version = '2012-10-17';
    policyDocument.Statement = [];
    var statementOne = {};
    statementOne.Action = 'execute-api:Invoke';
    statementOne.Effect = effect;
    statementOne.Resource = resource;
    policyDocument.Statement[0] = statementOne;
    authResponse.policyDocument = policyDocument;
  }

  return authResponse;
}

module.exports.firebase = function(event, context){

  if (!event.authorizationToken) {
      context.fail('Unauthorized'); // 401 status code
      return;
  }

  // If auhorizationToken is present, split on space looking for format 'Bearer <token value>'
  const tokenParts = event.authorizationToken.split(' ');
  const tokenValue = tokenParts[1];

    // Return from function if authorization header is not formatted properly
  if (!(tokenParts[0].toLowerCase() === 'bearer' && tokenValue)) {
      context.fail('Unauthorized');
      return;
  }

  initializeSdk();

  admin.auth().verifyIdToken(tokenValue)
  .then(resp=>{
    //replace '*' by event.methodArn if you want grant access to only the requested Lambda func <-- LOOK!!
    context.succeed(generateIamPolicy('Allow', '*' , resp));

  })
  .catch(err=>{
    console.log("firebase verification error",err)
    context.succeed(generateIamPolicy('Deny', event.methodArn, null));
  })


}