How can I generate a Google Slides document from parts of other Google Slides documents?

I'm wanting to, preferably using the Google Slides API, programatically generate a new Google Slides document which pulls in some form submitted variables and various pages from other Google Slides documents.

What I'm aiming to do is create a simple HTML form which asks a series of questions such as:

• Client Name (text field)
• Project Title (text field)
• Include case studies? (yes/no)

The person generating the proposal would complete the answers above and then submit the form.

At this point, and this is what I need help with, I want to:

• Duplicate an existing Google Slides proposal template (which we already have)
• Add in the Client Name and Project Title into text fields on the first slide
• Import slides from another Google Slides document (a case studies document - which we already have) into this newly generated document (if the user answered Yes on the form)

I don't even know where to begin with this as I'm a newbie to the Google Docs API and JavaScript.

I've tried the following but it's not listing the files:

const path = require("path");

// Require the fastify framework and instantiate it
const fastify = require("fastify")({
  // set this to true for detailed logging:
  logger: false
});

// Setup our static files
fastify.register(require("fastify-static"), {
  root: path.join(__dirname, "public"),
  prefix: "/" // optional: default '/'
});

// fastify-formbody lets us parse incoming forms
fastify.register(require("fastify-formbody"));

// point-of-view is a templating manager for fastify
fastify.register(require("point-of-view"), {
  engine: {
    handlebars: require("handlebars")
  }
});

// Our main GET home page route, pulls from src/pages/index.hbs
fastify.get("/", function(request, reply) {
  // params is an object we'll pass to our handlebars template
  let params = {
    greeting: "Hello Node!"
  };
  // request.query.paramName <-- a querystring example
  reply.view("/src/pages/index.hbs", params);
});

// A POST route to handle form submissions
fastify.post("/", function(request, reply) {
  let params = {
    greeting: "Hello Form!"
  };
  // request.body.paramName <-- a form post example
  reply.view("/src/pages/index.hbs", params);
});

// Run the server and report out to the logs
fastify.listen(process.env.PORT, '0.0.0.0', function(err, address) {
  if (err) {
    fastify.log.error(err);
    process.exit(1);
  }
  console.log(`Your app is listening on ${address}`);
  fastify.log.info(`server listening on ${address}`);
});


const fs = require('fs');
const readline = require('readline');
const {google} = require('googleapis');

// If modifying these scopes, delete token.json.
const SCOPES = ['https://www.googleapis.com/auth/presentations.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';

// 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 Slides API.
  authorize(JSON.parse(content), listSlides);
});

/**
 * 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.web;
  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 retrieving 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 number of slides and elements in a sample presentation:
 * https://docs.google.com/presentation/d/1EAYk18WDjIG-zp_0vLm3CsfQh_i8eXc67Jo2O9C6Vuc/edit
 * @param {google.auth.OAuth2} auth The authenticated Google OAuth client.
 */
function listSlides(auth) {
  const slides = google.slides({version: 'v1', auth});
  slides.presentations.get({
    presentationId: '1EAYk18WDjIG-zp_0vLm3CsfQh_i8eXc67Jo2O9C6Vuc',
  }, (err, res) => {
    if (err) return console.log('The API returned an error: ' + err);
    const length = res.data.slides.length;
    console.log('The presentation contains %s slides:', length);
    res.data.slides.map((slide, i) => {
      console.log(`- Slide #${i + 1} contains ${slide.pageElements.length} elements.`);
    });
  });
}