register.js

function getHeaders(sheet) {
  var last_col = sheet.getLastColumn();
  var range = sheet.getRange(1, 1, 1, last_col);
  var headers = range.getValues();
  return headers[0];
}
function getColumnNumByHeader(sheet, header) {
  var n = getHeaders(sheet).indexOf(header);
  if (n < 0) {
    throw Error("ヘッダーが見つからなかった");
  }
  return n + 1;
}
function getRandomInt(min, max) {
  min = Math.ceil(min);
  max = Math.floor(max);
  return Math.floor(min + (max - min) * Math.random());
}
function generateCode() {
  var CHARSET = "34679abcdefghijkmnpqrstuvwxyzACEFGHJKMLNPRTUVWXY";
  var code = "";
  for(var i = 0; i < 6; i++) {
    code +=CHARSET[getRandomInt(0, CHARSET.length)];
  }
  return code;
}
function getProps(value, key) {
  return value[key][0];
}
function keyValue(key, value)  {
  return "【" + key + "】\n" + value + "\n";
}
function onFormSubmit(e) {
  console.log(Object.keys(e.namedValues));
  var namedValues = e.namedValues;
  var keys = Object.keys(namedValues);

  var name = namedValues['お名前（本名フルネーム）'][0];
  var email = namedValues['メールアドレス'][0];
  
  var code = generateCode();

  var spreadsheet = SpreadsheetApp.getActiveSpreadsheet();
  var sheet = spreadsheet.getActiveSheet();

  var answer = [];
  getHeaders(sheet).forEach(function(v, i) {
    if(keys.indexOf(v) >= 0) {
      answer.push({key: v, value: getProps(namedValues, v)});
    }
  });
  answer.push({key: "あなたの紹介コード", value: code});
  console.log(answer);
  var answerWithoutEmpty = answer.filter(function(v) {return v.value !== ""});
  console.log(answerWithoutEmpty);
  var confirm = answerWithoutEmpty.map(function(v) {return keyValue(v.key, v.value)});

  var dataRange     = sheet.getDataRange();
  var lastRow = dataRange.getLastRow();
  var codeColumn    = dataRange.getCell(lastRow, getColumnNumByHeader(sheet, "発行された紹介コード"));
  codeColumn.setValue(code);
 
  var mailSetting = spreadsheet.getSheetByName("メール設定");
  var subject = mailSetting.getRange("B1").getValue();
  var greeting = mailSetting.getRange("B2").getValue();
  var mailSettingData = mailSetting.getDataRange();

  var body = greeting + '\n' + confirm.join("\n");
  
  MailApp.sendEmail({
    to: email,
    subject: subject,
    body: body
  });
}