javascript - Nodejs And Google spreadsheet Integration ( Cannot read property 'length' of undefined )

Question

I have started using nodejs ( and have a beginner level understanding of javascript ) .

Currently following google Spreadsheet API guide to building a small application. https://developers.google.com/sheets/api/quickstart/nodejs

While I was able to establish the connectivity with my google sheet and able to fetch the data I want to achieve it in a specific way as below.

Put the data fetching code in one file ( fetchdatafromgooglesheet.js getdata() function to fetch the data and return it to main function as rows ) and later use the data in my main file ( start.js ) and then perform the actions based on the values.

However I m not able to achieve this sharing the code of both files. Please help .

start.js

const googlesheet = require('./fetchdatafromgooglesheet');

var rows=googlesheet.getData();

console.log(rows.length);

fetchdatafromgooglesheet

const fs = require('fs');
const readline = require('readline');
const {google} = require('googleapis');
//var rows=null;

// If modifying these scopes, delete token.json.
const SCOPES = ['https://www.googleapis.com/auth/spreadsheets.readonly'];
// The file token.json stores the user's access and refresh tokens, and is
// created automatically when the authorization flow completes for the first
// time.
const TOKEN_PATH = 'token.json';
var rows;

const getData= ()=>
{
  
// Load client secrets from a local file.
fs.readFile('credentials.json', (err, content) => {
  if (err) return console.log('Error loading client secret file:', err);
  // Authorize a client with credentials, then call the Google Sheets API.
  authorize(JSON.parse(content), listMajors);
});
return rows;
}

/**
 * Create an OAuth2 client with the given credentials, and then execute the
 * given callback function.
 * @param {Object} credentials The authorization client credentials.
 * @param {function} callback The callback to call with the authorized client.
 */
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);
  });
}

/**
 * Get and store new token after prompting for user authorization, and then
 * execute the given callback with the authorized OAuth2 client.
 * @param {google.auth.OAuth2} oAuth2Client The OAuth2 client to get token for.
 * @param {getEventsCallback} callback The callback for the authorized client.
 */
function getNewToken(oAuth2Client, callback) {
  const authUrl = oAuth2Client.generateAuthUrl({
    access_type: 'offline',
    scope: SCOPES,
  });
  console.log('Authorize this app by visiting this url:', authUrl);
  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 console.error('Error while trying to retrieve access token', 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);
    });
  });
}

/**
 * Prints the names and majors of students in a sample spreadsheet:
 * @see https://docs.google.com/spreadsheets/d/1BxiMVs0XRA5nFMdKvBdBZjgmUUqptlbs74OgvE2upms/edit
 * @param {google.auth.OAuth2} auth The authenticated Google OAuth client.
 */
function listMajors(auth) {
  const sheets = google.sheets({version: 'v4', auth});
  sheets.spreadsheets.values.get({
    spreadsheetId: '1MMx_OcwmQApgKJUVdU0ySC_lUx1FB6-81AfV3E',
    range: 'Sheet2!A2:F',
  }, (err, res) => {
    if (err) return console.log('The API returned an error: ' + err);
    var fetchedData = res.data.values;
  rows=fetchedData;
  });
}

module.exports={getData};

Getting below error while execution.

D:study
odejsgooglesheetstart.js:5
console.log(rows.length);
                 ^

TypeError: Cannot read property 'length' of undefined
    at Object.<anonymous> (D:study
odejsgooglesheetstart.js:5:18)
    at Module._compile (internal/modules/cjs/loader.js:1138:30)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:1158:10)
    at Module.load (internal/modules/cjs/loader.js:986:32)
    at Function.Module._load (internal/modules/cjs/loader.js:879:14)
    at Function.executeUserEntryPoint [as runMain] (internal/modules/run_main.js:71:12)
    at internal/main/run_main_module.js:17:47

After some research and following up with suggestions given by others I have modified the code as below it gets the job done but I think its not the right way of doing things. Please let me know (here is the modified code )

function fetchData () {
    
    var rows=googlesheet.getData();
    if (rows==undefined && rows==null) {
        setTimeout(fetchData, 300); 
    } else {
      //setTimeout(fetchData, 300); // try again in 300 milliseconds
      console.log(rows.length);
    }
  }
  
fetchData();

Answer 1

Since rows is undefined, you can't access the length attribute. Better to do an undefined & null check before.

const googlesheet = require('./fetchdatafromgooglesheet');
rows=googlesheet.getData();
if(rows!==undefined && rows!==null) console.log(rows.length);

Update:

1. Since getData() is an async method, you have to wait for it to fetch the result using await keyword.

   rows = await googlesheet.getData();
   

2. But since you can use await only inside async functions, you have to make use of Promise.then() method as async method always returns a Promise.

   const googlesheet = require('./fetchdatafromgooglesheet');
   googlesheet.getData()
          .then((rows) => { console.log(rows.length) });
   

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/async_function