Google Apps Script snippet which uses viewer settings on Google Sheet to control who views

code.gs

var DOC_ID = '1K5IJLwK7qBUfLAG-tic9hOK3L8x4qUIQgd3gryHNE6Y';

function doGet() {
  var doc =SpreadsheetApp.openById(DOC_ID);
  var auth_user = doc.getViewers();
  var auth = auth_user.map(function(user) {
    return String(user);
  });
  var user = Session.getActiveUser().getEmail();
  if (auth.indexOf(user) > -1){
    return HtmlService.createHtmlOutput("you are allowed to view this").setSandboxMode(HtmlService.SandboxMode.IFRAME);
  } else {
    return HtmlService.createHtmlOutput(user+"you are <b>not</b> allowed to view this").setSandboxMode(HtmlService.SandboxMode.IFRAME);
  }
}