writeToFile.js

/**
 * Writes fileContents to a file with the given fileName in the specified folder. If a file with the
 * same name already exists, it will either overwrite it or append to it, based on the value of the append parameter.
 *
 * @param {string} fileName - The name of the file to write to.
 * @param {string} fileContents - The contents to write to the file.
 * @param {string} folderIdOrUrl - The ID or URL of the folder to write the file to.
 * @param {boolean} [append=false] - Whether to append the fileContents to an existing file with the same name.
 * @returns {string} The ID of the created or modified file.
 */
function writeToFile(fileName, fileContents, folderIdOrUrl, append = false) {
  let folder = DriveApp.getFolderById(getIdFromUrl(folderIdOrUrl));
  let files = folder.getFilesByName(fileName);
  if (isJSON(fileContents)) {
    fileContents = JSON.stringify(fileContents,"",1)
  }
  if (files.hasNext()) {
    // If file already exists, overwrite it or append to it with new contents
    let file = files.next();
    let newContents = fileContents;
    if (append) {
      let existingContents = file.getBlob().getDataAsString();
      if (existingContents) { //  && !fileContents.endsWith("\n")
        newContents = "\n" + fileContents;
      }
      file.setContent(existingContents + newContents);
    } else {
      file.setContent(newContents);
    }
    return file.getId();
  } else {
    // If file doesn't exist, create a new file with given contents
    let file = folder.createFile(fileName, fileContents);
    return file.getId();
  }
}

function writeToFile_test() {
  let content = "hello world"
  let parentFolder = DRIVE_FOLDER_ROOT_URL
  let subFolderId = getOrCreateSubfolderId('mySubfolder',parentFolder)
  let filename = "hello.txt"
  let newFileId = writeToFile(filename,content,subFolderId,true)
  console.log({newFileId}) // { newFileId: '1nU082-um9Pv8hUjMAybCY1wXuI-C0R8E' }
}