index.js

const functions = require('firebase-functions');
const os = require('os');
const fs = require('fs');
const path = require('path');
const admin = require('firebase-admin');
admin.initializeApp();

exports.testItOut = functions.firestore
    .document('blarg/{docId}')
    .onUpdate((change, context) => {
        console.log( "Inside #testItOut" );
        const storage = admin.storage()
        let bucketName = 'signatures';
        let fileName = 'temp.txt';
        const tempFilePath = path.join(os.tmpdir(), fileName);
        console.log( `Writing out to ${tempFilePath}` );
        fs.writeFileSync(tempFilePath, "something!" );

        return storage
            .bucket( bucketName )
            .upload( tempFilePath )
            .then( () => fs.unlinkSync(tempFilePath) )
            .catch(err => console.error('ERROR inside upload: ', err) );
    });