rmcvey/Code.gs

## Code.gs
function responseLimit(maxResponses) {
 var form = FormApp.getActiveForm();
 var responses = form.getResponses();
 if(responses.length >= maxResponses) {
  form.setAcceptingResponses(false);
 }
}

function onOpen(e) {
  FormApp.getUi()
      .createAddonMenu()
      .addItem('Configure response limiting', 'showSidebar')
      .addToUi();
}

function onInstall(e) {
  onOpen(e);
}

function showSidebar() {
  var ui = HtmlService.createHtmlOutputFromFile('Sidebar')
      .setSandboxMode(HtmlService.SandboxMode.IFRAME)
      .setTitle('Form Limiting');
  FormApp.getUi().showSidebar(ui);
}

function saveSettings(settings) {
  PropertiesService.getDocumentProperties().setProperties(settings);
  adjustFormSubmitTrigger();
}

function getSettings() {
  var settings = PropertiesService.getDocumentProperties().getProperties();

  // Get text field items in the form and compile a list
  //   of their titles and IDs.
  var form = FormApp.getActiveForm();
  var textItems = form.getItems(FormApp.ItemType.TEXT);
  settings.textItems = [];
  for (var i = 0; i < textItems.length; i++) {
    settings.textItems.push({
      title: textItems[i].getTitle(),
      id: textItems[i].getId()
    });
  }
  return settings;
}

function adjustFormSubmitTrigger() {
  var form = FormApp.getActiveForm();
  var triggers = ScriptApp.getUserTriggers(form);
  var settings = PropertiesService.getDocumentProperties();
  var triggerNeeded = settings.getProperty('limitResponses') == 'true';

  // Create a new trigger if required; delete existing trigger
  //   if it is not needed.
  var existingTrigger = null;
  for (var i = 0; i < triggers.length; i++) {
    if (triggers[i].getEventType() == ScriptApp.EventType.ON_FORM_SUBMIT) {
      existingTrigger = triggers[i];
      break;
    }
  }
  if (triggerNeeded && !existingTrigger) {
    var trigger = ScriptApp.newTrigger('respondToFormSubmit')
        .forForm(form)
        .onFormSubmit()
        .create();
  } else if (!triggerNeeded && existingTrigger) {
    ScriptApp.deleteTrigger(existingTrigger);
  }
}

function respondToFormSubmit(e) {
  var settings = PropertiesService.getDocumentProperties();
  var authInfo = ScriptApp.getAuthorizationInfo(ScriptApp.AuthMode.FULL);

  if (settings.getProperty('limitResponses') == 'true') {
    var maxResponses = parseInt(settings.getProperty('maxResponses'), 10)
    if (!isNaN(maxResponses)) {
      responseLimit(maxResponses);
    }
  }
}

## Sidebar.html
<!DOCTYPE html>
<html>
  <head>
    <base target="_top">
    <link rel="stylesheet" href="https://ssl.gstatic.com/docs/script/css/add-ons1.css">
    <!-- The CSS package above applies Google styling to buttons and other elements. -->

    <style>
    .branding-below {
      bottom: 54px;
      top: 0;
    }

    .branding-text {
      left: 7px;
      position: relative;
      top: 3px;
    }

    .logo {
      vertical-align: middle;
    }

    .width-100 {
      width: 100%;
      box-sizing: border-box;
      -webkit-box-sizing : border-box;‌
      -moz-box-sizing : border-box;
    }

    label {
      font-weight: bold;
    }

    #creator-options,
    #respondent-options {
      background-color: #eee;
      border-color: #eee;
      border-width: 5px;
      border-style: solid;
      display: none;
    }

    #creator-email,
    #respondent-email,
    #button-bar,
    #submit-subject {
      margin-bottom: 10px;
    }

    #response-step {
      display: inline;
    }

    </style>
  </head>
  <body>
    <div class="sidebar branding-below">
      <form>
        <div class="block">
          <input type="checkbox" id="limitResponses">
          <label for="limitResponses">Limit Form Responses</label>
        </div>
        <div class="block form-group" id="creator-options">
          <label for="maxResponses">
            Maximum number of responses
          </label>
          <input type="number" class="width-100" id="maxResponses"> (default unlimited)
        </div>

        <div class="block" id="button-bar">
          <button class="action" id="save-settings">Save</button>
        </div>
      </form>
    </div>

    <div class="sidebar bottom">
      <span class="gray branding-text">Simple Response Limiter</span>
    </div>

    <script src="//ajax.googleapis.com/ajax/libs/jquery/1.9.1/jquery.min.js">
    </script>
    <script>
      /**
       * On document load, assign required handlers to each element,
       * and attempt to load any saved settings.
       */
      $(function() {
        $('#save-settings').click(saveSettingsToServer);
        $('#limitResponses').click(toggleResponseLimiting);
        $('#maxResponses').change(validateNumber);
        google.script.run
           .withSuccessHandler(loadSettings)
           .withFailureHandler(showStatus)
           .withUserObject($('#button-bar').get())
           .getSettings();
      });

      /**
       * Callback function that populates the notification options using
       * previously saved values.
       *
       * @param {Object} settings The saved settings from the client.
       */
      function loadSettings(settings) {
        $('#maxResponses').val(settings.maxResponses);

        $('#submit-subject').val(!settings.responseSubject ?
           'Thank you for filling out our form!' :
           settings.responseSubject);

        $('#submit-notice').val(!settings.responseText ?
           'Thank you for responding to our form!' :
           settings.responseText);


        if (settings.limitResponses === 'true') {
          $('#limitResponses').prop('checked', true);
          $('#creator-options').show();
        }
      }

      /**
       * Toggles the visibility of the form creator notification options.
       */
      function toggleResponseLimiting() {
        $('#status').remove();
        if ($('#limitResponses').is(':checked')) {
          $('#creator-options').show();
        } else {
          $('#creator-options').hide();
        }
      }

      /**
       * Ensures that the entered step is a number between 1
       * and 99999, inclusive.
       */
      function validateNumber() {
        var value = $('#maxResponses').val();
        if (value < 1) {
          $('#maxResponses').val(1);
        } else if (value > 99999) {
          $('#maxResponses').val(99999);
        }
      }

      /**
       * Collects the options specified in the add-on sidebar and sends them to
       * be saved as Properties on the server.
       */
      function saveSettingsToServer() {
        this.disabled = true;
        $('#status').remove();
        var limitResponses = $('#limitResponses').is(':checked');
        var maxResponses = $('#maxResponses').val();
        var settings = {
          'limitResponses': limitResponses,
          'maxResponses': maxResponses
        };

        // Save the settings on the server
        google.script.run
            .withSuccessHandler(
              function(msg, element) {
                showStatus('Saved settings', $('#button-bar'));
                element.disabled = false;
              })
            .withFailureHandler(
              function(msg, element) {
                showStatus(msg, $('#button-bar'));
                element.disabled = false;
              })
            .withUserObject(this)
            .saveSettings(settings);
      }

      /**
       * Inserts a div that contains an status message after a given element.
       *
       * @param {String} msg The status message to display.
       * @param {Object} element The element after which to display the Status.
       */
      function showStatus(msg, element) {
         var div = $('<div>')
             .attr('id', 'status')
             .attr('class','error')
             .text(msg);
        $(element).after(div);
      }
    </script>
  </body>
</html>
	function responseLimit(maxResponses) {
	var form = FormApp.getActiveForm();
	var responses = form.getResponses();
	if(responses.length >= maxResponses) {
	form.setAcceptingResponses(false);
	}
	}

	function onOpen(e) {
	FormApp.getUi()
	.createAddonMenu()
	.addItem('Configure response limiting', 'showSidebar')
	.addToUi();
	}

	function onInstall(e) {
	onOpen(e);
	}

	function showSidebar() {
	var ui = HtmlService.createHtmlOutputFromFile('Sidebar')
	.setSandboxMode(HtmlService.SandboxMode.IFRAME)
	.setTitle('Form Limiting');
	FormApp.getUi().showSidebar(ui);
	}

	function saveSettings(settings) {
	PropertiesService.getDocumentProperties().setProperties(settings);
	adjustFormSubmitTrigger();
	}

	function getSettings() {
	var settings = PropertiesService.getDocumentProperties().getProperties();

	// Get text field items in the form and compile a list
	// of their titles and IDs.
	var form = FormApp.getActiveForm();
	var textItems = form.getItems(FormApp.ItemType.TEXT);
	settings.textItems = [];
	for (var i = 0; i < textItems.length; i++) {
	settings.textItems.push({
	title: textItems[i].getTitle(),
	id: textItems[i].getId()
	});
	}
	return settings;
	}

	function adjustFormSubmitTrigger() {
	var form = FormApp.getActiveForm();
	var triggers = ScriptApp.getUserTriggers(form);
	var settings = PropertiesService.getDocumentProperties();
	var triggerNeeded = settings.getProperty('limitResponses') == 'true';

	// Create a new trigger if required; delete existing trigger
	// if it is not needed.
	var existingTrigger = null;
	for (var i = 0; i < triggers.length; i++) {
	if (triggers[i].getEventType() == ScriptApp.EventType.ON_FORM_SUBMIT) {
	existingTrigger = triggers[i];
	break;
	}
	}
	if (triggerNeeded && !existingTrigger) {
	var trigger = ScriptApp.newTrigger('respondToFormSubmit')
	.forForm(form)
	.onFormSubmit()
	.create();
	} else if (!triggerNeeded && existingTrigger) {
	ScriptApp.deleteTrigger(existingTrigger);
	}
	}

	function respondToFormSubmit(e) {
	var settings = PropertiesService.getDocumentProperties();
	var authInfo = ScriptApp.getAuthorizationInfo(ScriptApp.AuthMode.FULL);

	if (settings.getProperty('limitResponses') == 'true') {
	var maxResponses = parseInt(settings.getProperty('maxResponses'), 10)
	if (!isNaN(maxResponses)) {
	responseLimit(maxResponses);
	}
	}
	}