Retrieve SFDC Attachments using REST

/**
 * Caution: shabby script ahead.
 *
 * Description: Get files from SFDC for given record ids. Simple script, almost no error handling.
 * This is a simple REST API call to demonstrate attachment retrieval from SFDC. It is not scalable, does not handle errors properly, and is not production-ready.
 * Author:      Prashanth Krishnamurthy, @crmcog
 * License:     MIT
 **/
var fetch = require("cross-fetch");
const sec = {
  userName: "...@gmail.com",
  secPassToken: "...",
  client_id: "client_id",
  client_secret: "client_secret",
};
const recordId = ["0031J00001DkSTHQA3"]; //contact id
const authUrl =
  "https://login.salesforce.com/services/oauth2/token?grant_type=password&client_id=" +
  sec.client_id +
  "&client_secret=" +
  sec.client_secret +
  "&username=" +
  sec.userName +
  "&password=" +
  sec.secPassToken;
// authenticate
fetch(authUrl, { method: "POST" })
  .then((data) => {
    return data.json();
  })
  .then((res) => {
    console.log(
      "Authentication successful. access_token: " +
        res.access_token +
        "; instance_url:" +
        res.instance_url
    );
    //get contact
    sec.access_token = res.access_token;
    sec.instance_url = res.instance_url;
    getContactAttachments(sec, recordId);
    return res;
  })
  .catch((error) => {
    console.error("Authentication error: " + error);
  });
function getContactAttachments(sec, recordId) {
  try {
    // create sub directories to store files
    const fs = require("fs");
    const dirTimePrefix = new Date().getTime();
    const dirName = __dirname + "\\sfdc-files-" + dirTimePrefix;
    if (!fs.existsSync(dirName)) fs.mkdirSync(dirName);
    console.log("created/found directory: " + dirName);
    console.log("get attachment referencess associated with contact..");
    const getURL = sec.instance_url + "/services/data/v43.0/query?q=";
    sec.authHeader = {
      method: "GET",
      headers: {
        Authorization: "Bearer " + sec.access_token,
      },
    };
    //https://<instance_url>/services/data/v43.0/query?q=select id, LinkedEntityId,ContentDocumentId from ContentDocumentLink where LinkedEntityId = '0031J00001DkSTHQA3'
    fetch(
      getURL +
        "select id, LinkedEntityId,ContentDocumentId from ContentDocumentLink where LinkedEntityId='" +
        recordId[0] +
        "'",
      sec.authHeader
    )
      .then((data) => {
        return data.json();
      })
      .then((res) => {
        console.log(
          "Contact '" + recordId[0] + "' has " + res.totalSize + " attachments"
        );
        if (parseInt(res.totalSize, 10) > 0) {
          //get all ContentDocumentIds in the result. This will have all the attachment references from ContentDocumentLink
          var contentDocumentIds = res.records.map(
            (docId) => "ContentDocumentId = '" + docId.ContentDocumentId + "'"
          );
          //https://<instance_url>/services/data/v43.0/query?q=SELECT VersionData FROM ContentVersion WHERE (ContentDocumentId  = '0691J000004Md1DQAS' or ContentDocumentId  = '0691J000004Me8yQAC' or ContentDocumentId  = '0691J000004Me93QAC') AND IsLatest = true
          fetch(
            getURL +
              "SELECT Id, Title, FileExtension, VersionData FROM ContentVersion WHERE (" +
              contentDocumentIds.join(" OR ") +
              ") AND IsLatest = true",
            sec.authHeader
          )
            .then((dataVersion) => {
              return dataVersion.json();
            })
            .then((resVersion) => {
              console.log(
                "Validating attachments.. found " +
                  resVersion.totalSize +
                  " attachments. Fetching them now.."
              );
              if (parseInt(resVersion.totalSize, 10) > 0) {
                var contentVersions = resVersion.records.map(
                  (contentVersion) =>
                    dirName +
                    "\\" +
                    contentVersion.Title +
                    "_" +
                    contentVersion.Id +
                    "." +
                    contentVersion.FileExtension +
                    "/\\/\\" +
                    contentVersion.VersionData
                );
                contentVersions.map((contentVersion) => {
                  getAttachment(sec, contentVersion);
                });
              }
            })
            .catch((error) => {
              console.error("getVersionData fetch error: " + error);
            });
        }
      })
      .catch((error) => {
        console.error("getContactAttachments fetch error: " + error);
      });
  } catch (e) {
    console.error("getContactAttachments error: " + e.toString());
  }
}
function getAttachment(sec, contentVersion) {
  fileURL = contentVersion.split("/\\/\\");
  console.log(
    "getting attachment .." +
      sec.instance_url +
      contentVersion.split("/\\/\\")[1]
  );
  fetch(sec.instance_url + contentVersion.split("/\\/\\")[1], sec.authHeader)
    .then((dataAttach) => {
      return dataAttach.buffer();
    })
    .then((fileBuffer) => {
      var fs = require("fs");
      fs.writeFileSync(contentVersion.split("/\\/\\")[0], fileBuffer);
      console.log("file saved.. " + contentVersion.split("/\\/\\")[0]);
    })
    .catch((error) => {
      console.error("getAttachment fetch error: " + error);
    });
}