index.js

const functions = require('firebase-functions');
const admin = require('firebase-admin');
const nodemailer = require('nodemailer');
const cors = require('cors')({origin: true});
const rp = require('request-promise');
const jsonDiff = require('json-diff');
//const jsondiffpatch = require('jsondiffpatch').create();
//const deepDiff = require('deep-diff').diff;
admin.initializeApp(functions.config().firebase);

/**
* Here we're using Gmail to send 
*/
let transporter = nodemailer.createTransport({
    service: 'gmail',
    auth: {
        user: '[email protected]',
        pass: 'xxxxxx'//put above email password
    }
});

exports.sendMail = functions.remoteConfig.onUpdate(versionMetadata => {
	// getting dest email by query string
        const dest = '[email protected]';

return admin.credential.applicationDefault().getAccessToken()
    .then(accessTokenObj => {
      return accessTokenObj.access_token;
    })
    .then(accessToken => {
      const currentVersion = versionMetadata.versionNumber;
      const templatePromises = [];
      templatePromises.push(getTemplate(currentVersion, accessToken));
      templatePromises.push(getTemplate(currentVersion - 1, accessToken));

      return Promise.all(templatePromises);
    })
    .then(results => {
      const currentTemplate = results[0];
      const previousTemplate = results[1];

      const diff = jsonDiff.diffString(previousTemplate, currentTemplate);
      const logicalDiff = parseDiff(diff);
      //const diff = jsondiffpatch.diff(previousTemplate, currentTemplate);
      //const diff = deepDiff(previousTemplate, currentTemplate);

      const mailOptions = {
            from: '[email protected]', // Something like: Jane Doe <[email protected]>
            to: dest,
            subject: '[Firebase] Remote Config Added, Updated or Deleted', // email subject
            text: 'change is ' + diff ,
	    html: `<p style="font-size: 16px;">Hey, below RemoteConfig have been changed, <br />${diff}<br /><br /> ${logicalDiff}</p>
            ` // email content in HTML
        };
        console.log(diff);
  
        // returning result
        return transporter.sendMail(mailOptions, (erro, info) => {
            if(erro){
                return console.log(erro.toString());//res.send(erro.toString());
            }
            return console.log('Sent');//res.send('Sended');
        });

      //return null;
    }).catch(error => {
      console.error(error);
      return null;
    });
});

function getTemplate(version, accessToken) {
  const options = {
    uri: 'https://firebaseremoteconfig.googleapis.com/v1/projects/<your-own-firebase-project-id>/remoteConfig',
    qs: {
      versionNumber: version
    },
    headers: {
        Authorization: 'Bearer ' + accessToken
    },
    json: true // Automatically parses the JSON string in the response
  };
  return rp(options).then(resp => {
    return Promise.resolve(resp);
  }).catch(err => {
    console.error(err);
    return Promise.resolve(null);
  });
}

function getIndicesOf(searchStr, str, caseSensitive) {
    var searchStrLen = searchStr.length;
    if (searchStrLen == 0) {
        return [];
    }
    var startIndex = 0, index, indices = [];
    if (!caseSensitive) {
        str = str.toLowerCase();
        searchStr = searchStr.toLowerCase();
    }
    while ((index = str.indexOf(searchStr, startIndex)) > -1) {
        indices.push(index);
        startIndex = index + searchStrLen;
    }
    return indices;
}

function parseDiff(str) {
  const
    process = string => {
        const lines = string.match(/\[3\dm.*?\[39m/g).map(s => s.slice(4, -4));
        let colon = lines[0].indexOf(':'),
            i = 0,
            key = '';

        if (lines[0][0] === '-' && lines[1][0] === '+' && lines[0].slice(1, colon) === lines[1].slice(1, colon)) {
            return [
                'UPDATE A KEY',
                `key name : ${lines[0].slice(1, colon).trim()}`,
                `old value: ${JSON.parse(JSON.stringify(lines[i].slice(colon + 1)))}`,
                `new value: ${JSON.parse(JSON.stringify(lines[i].slice(colon + 1)))}`
            ];
        }

        while (lines[i].slice(colon + 1).trim() === '{') {
            key += (key && '.') + lines[i].slice(1, colon).trim();
            colon = lines[++i].indexOf(':');
        }

        key += (key && '.') + lines[i].slice(1, colon).trim();

        return [
            lines[0][0] === '-' ? 'DELETE A KEY' : 'ADD A KEY',
            `key name: ${key}`,
            `value   : ${JSON.parse(JSON.stringify(lines[i].slice(colon + 1)))}`
        ];
    },
    data = [
      str
    ];
    return data.map(process);
}