Node.js 8 Cloud Function to write to a Sheets document

index.js

// Copyright 2018 Google LLC.
// SPDX-License-Identifier: Apache-2.0
const {google} = require("googleapis");
const {Storage} = require("@google-cloud/storage");

exports.function2sheet = async (req, res) => {
  var fileName = req.name;
  const auth = await google.auth.getClient({
    scopes: ['https://www.googleapis.com/auth/spreadsheets',
             'https://www.googleapis.com/auth/devstorage.read_only']
  });
  const sheetsAPI = google.sheets({version: 'v4', auth});
  const sheetName = fileName.slice(0,-4);
  const sheetId = await addEmptySheet(sheetsAPI, sheetName);
  const the
  = await readCSVContent(sheetsAPI, req, sheetName);
  await populateSheet(sheetsAPI, theData, sheetName); // wrap into "await Promise.all()" if making calls returning multiple promises
};

function readCSVContent(sheetsAPI, file, sheetName) {
  return new Promise((resolve, reject) => {
    const storage = new Storage();
    let fileContents = new Buffer('');
    let rows = [];
    storage.bucket(file.bucket).file(file.name).createReadStream()
    .on('error', function(err) {
      reject('The Storage API returned an error: ' + err);
    })
    .on('data', function(chunk) {
      fileContents = Buffer.concat([fileContents, chunk]);
    })  
    .on('end', function() {
      let content = fileContents.toString('utf8');
      let lines = content.split(/\r\n|\r|\n/);
      lines.forEach(function(line) {
          rows.push ( line.split(',') );
      });
      console.log("CSV content read (" + rows.length + " rows) from " + file.name);
      resolve(rows);
    });
  });
}

function addEmptySheet(sheetsAPI, sheetName) {
  return new Promise((resolve, reject) => {
    const addEmptySheetParams = {
      spreadsheetId: process.env.SPREADSHEET_ID,
      resource: {
        requests: [{
            addSheet: {
              properties: {
                title: sheetName,
                index: 1,
                gridProperties: {
                  rowCount: 2000,
                  columnCount: 26,
                  frozenRowCount: 1
                }
              }
            }
        }]}
    };
    sheetsAPI.spreadsheets.batchUpdate(addEmptySheetParams, function( err, response ) {
      if (err) {
        reject("The Sheets API returned an error: " + err);
      } else {
        const sheetId = response.data.replies[0].addSheet.properties.sheetId;
        console.log("Created empty sheet: " + sheetId);
        resolve(sheetId);
      }
    });
  });
}

function populateSheet(sheetsAPI, theData, sheetName) {
  return new Promise((resolve, reject) => {
    const editRange = sheetName+"!A1:Z";
    const initTrixHeadersRequest = {
      spreadsheetId: process.env.SPREADSHEET_ID,
      range: editRange,
      resource : {
        range: editRange,
        majorDimension: "ROWS",
        values: theData
      },
      valueInputOption: "RAW"
    }
    sheetsAPI.spreadsheets.values.update(initTrixHeadersRequest, function(err, response) {
      if (err) {
        reject ("The Sheets API returned an error: " + err);
      } else {
        console.log(sheetName + " sheet populated with " + theData.length + " rows" );
        resolve();
      }
    });  
  });
}