Respond with JSON object from Express endpoint using Gmail API - node.js

Question

I am trying to return the value of the data object from the function listLabels in my code below. I would like to have it be accessed from an Express endpoint, e.g. (localhost:3001).
I have been able to console log the object and I have been able to send myself emails. Any help in returning the value when accessing the Express endpoint would be greatly appreciated.
const express = require('express')
const app = express()
const fs = require('fs')
const readline = require('readline')
const {google} = require('googleapis')
app.get('/', function(req, res) {
res.setHeader('Content-Type', 'application/json')
let data = getLabels()
console.log('data: ' + data)
res.json(data)
})
app.get('/hello', function (req, res) {
res.send('hello there')
})
app.listen(3001, () => console.log('server up on 3001'))
const SCOPES = ['https://mail.google.com/',
'https://www.googleapis.com/auth/gmail.modify',
'https://www.googleapis.com/auth/gmail.compose',
'https://www.googleapis.com/auth/gmail.send'
];
const TOKEN_PATH = 'credentials.json';
function getLabels() {
fs.readFile('client_secret.json', (err, content) => {
if (err) return console.log('Error loading client secret file:', err);
authorize(JSON.parse(content), listLabels);
});
}
function authorize(credentials, callback) {
const {client_secret, client_id, redirect_uris} = credentials.installed;
const oAuth2Client = new google.auth.OAuth2(
client_id, client_secret, redirect_uris[0]);
// Check if we have previously stored a token.
fs.readFile(TOKEN_PATH, (err, token) => {
if (err) return getNewToken(oAuth2Client, callback);
oAuth2Client.setCredentials(JSON.parse(token));
callback(oAuth2Client);
});
}
function getNewToken(oAuth2Client, callback) {
const authUrl = oAuth2Client.generateAuthUrl({
access_type: 'offline',
scope: SCOPES,
});
console.log('Authorize this app by visiting this url:', authUrl);
fs.writeFile('getCredentials.txt', authUrl, (err) => {
if (err) throw err;
console.log('getCredentials.txt saved!')
})
const rl = readline.createInterface({
input: process.stdin,
output: process.stdout,
});
rl.question('Enter the code from that page here: ', (code) => {
rl.close();
oAuth2Client.getToken(code, (err, token) => {
if (err) return callback(err);
oAuth2Client.setCredentials(token);
// Store the token to disk for later program executions
fs.writeFile(TOKEN_PATH, JSON.stringify(token), (err) => {
if (err) return console.error(err);
console.log('Token stored to', TOKEN_PATH);
});
callback(oAuth2Client);
});
});
}
function listLabels(auth) {
const gmail = google.gmail({version: 'v1', auth});
console.log('here')
gmail.users.messages.list({
userId: 'me',
maxResults: 10
}, (err, {data}) => {
if (err) return console.log('The API returned an error: ' + err);
console.log('data in listlabels ' + JSON.stringify(data))
//return data
let dataForExpress
messages.forEach(function (message, index) {
gmail.users.messages.get({
userId: 'me',
id: data.messages[index].id,
format: 'raw'
}, (err, {data}) => {
if (err) return console.log('Error: ' + err)
console.log(data)
dataForExpress.push(data)
});
})
return dataForExpress
})
}

Answer 1

The data you are expecting is coming back in an asynchronous fashion, so you will need to handle it asynchronously using either callbacks or async await.
Here is an example using callbacks.
const express = require('express')
const app = express()
const fs = require('fs')
const readline = require('readline')
const {google} = require('googleapis')
app.get('/', function(req, res) {
res.setHeader('Content-Type', 'application/json')
// let data = getLabels()
getLabels((labels) => {
console.log(labels)
res.json(labels)
})
//console.log('data: ' + data)
//res.json(data)
})
app.get('/hello', function (req, res) {
res.send('hello there')
})
app.listen(3001, () => console.log('server up on 3001'))
const SCOPES = ['https://mail.google.com/',
'https://www.googleapis.com/auth/gmail.modify',
'https://www.googleapis.com/auth/gmail.compose',
'https://www.googleapis.com/auth/gmail.send'
];
const TOKEN_PATH = 'credentials.json';
function getLabels(callback) {
fs.readFile('client_secret.json', (err, content) => {
if (err) return console.log('Error loading client secret file:', err);
// so here, instead of passing it listLabels directly, we will pass it just a regular callback, and call listlabels inside that callback. This way we can choose what to do with listlabels return value.
authorize(JSON.parse(content), (auth) => {
listLabels(auth, (listOfLabels) => {
console.log(listOfLabels) // we should have the list here
callback(listOfLabels)
}) //match the original arguments
});
});
}
function authorize(credentials, callback) {
const {client_secret, client_id, redirect_uris} = credentials.installed;
const oAuth2Client = new google.auth.OAuth2(
client_id, client_secret, redirect_uris[0]);
// Check if we have previously stored a token.
fs.readFile(TOKEN_PATH, (err, token) => {
if (err) return getNewToken(oAuth2Client, callback);
oAuth2Client.setCredentials(JSON.parse(token));
callback(oAuth2Client);
});
}
function getNewToken(oAuth2Client, callback) {
const authUrl = oAuth2Client.generateAuthUrl({
access_type: 'offline',
scope: SCOPES,
});
console.log('Authorize this app by visiting this url:', authUrl);
fs.writeFile('getCredentials.txt', authUrl, (err) => {
if (err) throw err;
console.log('getCredentials.txt saved!')
})
const rl = readline.createInterface({
input: process.stdin,
output: process.stdout,
});
rl.question('Enter the code from that page here: ', (code) => {
rl.close();
oAuth2Client.getToken(code, (err, token) => {
if (err) return callback(err);
oAuth2Client.setCredentials(token);
// Store the token to disk for later program executions
fs.writeFile(TOKEN_PATH, JSON.stringify(token), (err) => {
if (err) return console.error(err);
console.log('Token stored to', TOKEN_PATH);
});
callback(oAuth2Client);
});
});
}
function listLabels(auth, callback) {
const gmail = google.gmail({version: 'v1', auth});
console.log('here')
gmail.users.messages.list({
userId: 'me',
maxResults: 10
}, (err, {data}) => {
if (err) return console.log('The API returned an error: ' + err);
console.log('data in listlabels ' + JSON.stringify(data))
//return data
let dataForExpress
let messages = data.messages
messages.forEach(function (message, index) {
console.log("Inside foreach, message and index :", message, index)
gmail.users.messages.get({
userId: 'me',
id: message.id,
format: 'raw'
}, (err, {data}) => {
if (err) return console.log('Error: ' + err)
console.log(data)
dataForExpress.push(data)
if dataForExpress.length == messages.length { // we done
callback(dataForExpress)
}
});
})
})
}