Google Firestore - Deploy Error: Function load error: Code in file index.js can't be loaded

Question

I am trying to deploy my functions to firebase, I'm getting the following error:

Function load error: Code in file index.js can't be loaded. Did you list all required modules in the package.json dependencies?

Detailed stack trace: Error: Cannot find module 'express'
at Function.Module._resolveFilename (module.js:469:15)
at Function.Module._load (module.js:417:25)
at Module.require (module.js:497:17)
at require (internal/module.js:20:19)
at Object. (/user_code/index.js:6:15)
at Module._compile (module.js:570:32)
at Object.Module._extensions..js (module.js:579:10)
at Module.load (module.js:487:32)
at tryModuleLoad (module.js:446:12)
at Function.Module._load (module.js:438:3)

Firebase:

"firebase-admin": "~5.8.1"

"firebase-functions": "^0.8.1"

This issue is already raised here : https://github.com/firebase/firebase-tools/issues/515 (But for 'firebase-admin' module)

Based on inputs from there, I tried doing this: npm install --save-exact [email protected] npm install --save-exact [email protected]. but I ran into the following error :

Error: Error occurred while parsing your function triggers. Please ensure you have the latest firebase-functions SDK by running "npm i --save firebase-functions@latest" inside your functions folder.

Node version, v8.4.0.

Index.js

const functions = require('firebase-functions'); 
const admin = require('firebase-admin'); 
admin.initializeApp(functions.config().firebase); 
var express = require('express'); 
var bodyParser = require('body-parser'); 

const users = require('../functions/controllers/users.controller'); 
var app = express(); 
app.use(bodyParser.urlencoded({ extended: true })) 
app.use(bodyParser.json()); 
app.post('/users', users.createUsers); 
exports.myappusers = functions.https.onRequest(app);

users.controller.js

const admin = require('firebase-admin');
const db = admin.firestore();

exports.createUsers = function(req, res){
    if(!req.body.name){
        return res.status(404).send({message: 'User name cannot be empty'})
    }
    else{
        db.collection('user-test')
        .add({
            name: req.body.name
        })
        .then((writeResult) => {
            return res.json({ result: `User ID: ${writeResult.id} added.`});
        })
        .catch((err)=>{
            return res.status(500).send(err);
        })
    }
}

Answer 1

After breaking my head for a day, I did the obvious. I installed Express and did a deploy but this time I got the same error for body-parser, _Error: Cannot find module 'body-parser'_ Installed body-parser as well. Deployed again, voila!! deployment was successful.

But I'm not really sure why I had to explicitly install them even though both express and body-parser module were present in the node-modules folder.

Summary:

npm install --save --save-exact [email protected]    
npm install --save --save-exact [email protected]    
firebase deploy --only functions