HTML Service: Communicate with Server Functions

google.script.run is an asynchronous client-side JavaScript API that allows HTML-service pages to call server-side Apps Script functions. The following example shows the most basic functionality of google.script.runcalling a function on the server from client-side JavaScript.

Code.gs

function doGet() {
  return HtmlService.createHtmlOutputFromFile('Index');
}

function doSomething() {
  Logger.log('I was called!');
}

Index.html

<!DOCTYPE html>
<html>
  <head>
    <base target="_top">
    <script>
      google.script.run.doSomething();
    </script>
  </head>
</html>

If you deploy this script as a web app and visit its URL, you won’t see anything, but if you return to the editor and choose View > Logs, you'll see that the server function doSomething() was called.

Client-side calls to server-side functions are asynchronous: after the browser requests that the server run the function doSomething(), the browser continues immediately to the next line of code without waiting for a response. This means that server function calls may not execute in the order you expect. If you make two function calls at the same time, there is no way to know which function will run first; the result may differ each time you load the page. In this situation, success handlers and failure handlers help control the flow of your code.

The google.script.run API allows 10 concurrent calls to server functions. If you make an 11th call while 10 are still running, the server function will be delayed until one of the 10 spots is freed. In practice, you should rarely have to think about this restriction, especially since most browsers already limit the number of concurrent requests to the same server at a number lower than 10. In Firefox, for example, the limit is 6. Most browsers similarly delay excess server requests until one of the existing requests has completed.

Parameters and return values

You can call a server function with parameters from the client. Similarly, a server function can return a value to the client as a parameter passed to a success handler.

Legal parameters and return values are JavaScript primitives like a Number, Boolean, String, or null, as well as JavaScript objects and arrays that are composed of primitives, objects and arrays. A form element within the page is also legal as a parameter, but it must be the function’s only parameter, and it is not legal as a return value. Requests fail if you attempt to pass a Date, Function, DOM element besides a form, or other prohibited type, including prohibited types inside objects or arrays. Objects that create circular references will also fail, and undefined fields within arrays become null.

Note that an object passed to the server becomes a copy of the original. If a server function receives an object and changes its properties, the properties on the client are not affected.

Success handlers

Because client-side code continues to the next line without waiting for a server call to complete, withSuccessHandler(function) allows you to specify a client-side callback function to run when the server responds. If the server function returns a value, the API passes the value to the new function as a parameter.

The following example displays a browser alert when the server responds. Note that this code sample requires authorization because the server-side function is accessing your Gmail account. The simplest way to authorize the script is to run the getUnreadEmails() function manually from the script editor once before you load the page. Alternately, when you deploy the web app, you can choose to execute it as the “user accessing the web app,” in which case you will be prompted for authorization when you load the app.

Code.gs

function doGet() {
  return HtmlService.createHtmlOutputFromFile('Index');
}

function getUnreadEmails() {
  return GmailApp.getInboxUnreadCount();
}

Index.html

<!DOCTYPE html>
<html>
  <head>
    <base target="_top">
    <script>
      function onSuccess(numUnread) {
        var div = document.getElementById('output');
        div.innerHTML = 'You have ' + numUnread
            + ' unread messages in your Gmail inbox.';
      }

      google.script.run.withSuccessHandler(onSuccess)
          .getUnreadEmails();
    </script>
  </head>
  <body>
    <div id="output"></div>
  </body>
</html>

Failure handlers

In case the server fails to respond or throws an error, withFailureHandler(function) allows you to specify a failure handler instead of a success handler, with the Error object (if any) passed as an argument.

By default, if you don't specify a failure handler, failures are logged to the JavaScript console. To override this, call withFailureHandler(null) or supply a failure handler that does nothing.

The syntax for failure handlers is nearly identical to success handlers, as this example shows.

Code.gs

function doGet() {
  return HtmlService.createHtmlOutputFromFile('Index');
}

function getUnreadEmails() {
  // 'got' instead of 'get' will throw an error.
  return GmailApp.gotInboxUnreadCount();
}

Index.html

<!DOCTYPE html>
<html>
  <head>
    <base target="_top">
    <script>
      function onFailure(error) {
        var div = document.getElementById('output');
        div.innerHTML = "ERROR: " + error.message;
      }

      google.script.run.withFailureHandler(onFailure)
          .getUnreadEmails();
    </script>
  </head>
  <body>
    <div id="output"></div>
  </body>
</html>

User objects

You can reuse the same success or failure handler for multiple calls to the server by calling withUserObject(object) to specify an object that will be passed to the handler as a second parameter. This “user object” — not to be confused with the User class — lets you respond to the context in which the client contacted the server. Because user objects are not sent to the server, they can be almost anything, including functions, DOM elements, and so forth, without the restrictions on parameters and return values for server calls. User objects cannot, however, be objects constructed with the new operator.

In this example, clicking either of two buttons will update that button with a value from the server while leaving the other button unchanged, even though they share one success handler. Inside the onclick handler, the keyword this refers to the button itself.

Code.gs

function doGet() {
  return HtmlService.createHtmlOutputFromFile('Index');
}

function getEmail() {
  return Session.getActiveUser().getEmail();
}

Index.html

<!DOCTYPE html>
<html>
  <head>
    <base target="_top">
    <script>
      function updateButton(email, button) {
        button.value = 'Clicked by ' + email;
      }
    </script>
  </head>
  <body>
    <input type="button" value="Not Clicked"
      onclick="google.script.run
          .withSuccessHandler(updateButton)
          .withUserObject(this)
          .getEmail()" />
    <input type="button" value="Not Clicked"
      onclick="google.script.run
          .withSuccessHandler(updateButton)
          .withUserObject(this)
          .getEmail()" />
  </body>
</html>

Forms

If you call a server function with a form element as a parameter, the form becomes a single object with field names as keys and field values as values. The values are all converted to strings, except for the contents of file-input fields, which become Blob objects.

This example processes a form, including a file-input field, without reloading the page; it uploads the file to Google Drive and then prints the URL for the file in the client-side page. Inside the onsubmit handler, the keyword this refers to the form itself. Note that upon loading all forms in the page have the default submit action disabled by preventFormSubmit. This prevents the page from redirecting to an inaccurate URL in the event of an exception.

Code.gs

function doGet() {
  return HtmlService.createHtmlOutputFromFile('Index');
}

function processForm(formObject) {
  var formBlob = formObject.myFile;
  var driveFile = DriveApp.createFile(formBlob);
  return driveFile.getUrl();
}

Index.html

<!DOCTYPE html>
<html>
  <head>
    <base target="_top">
    <script>
      // Prevent forms from submitting.
      function preventFormSubmit() {
        var forms = document.querySelectorAll('form');
        for (var i = 0; i < forms.length; i++) {
          forms[i].addEventListener('submit', function(event) {
            event.preventDefault();
          });
        }
      }
      window.addEventListener('load', preventFormSubmit);

      function handleFormSubmit(formObject) {
        google.script.run.withSuccessHandler(updateUrl).processForm(formObject);
      }
      function updateUrl(url) {
        var div = document.getElementById('output');
        div.innerHTML = '<a href="' + url + '">Got it!</a>';
      }
    </script>
  </head>
  <body>
    <form id="myForm" onsubmit="handleFormSubmit(this)">
      <input name="myFile" type="file" />
      <input type="submit" value="Submit" />
    </form>
    <div id="output"></div>
 </body>
</html>

Script runners

You can think of google.script.run as a builder for a “script runner.” If you add a success handler, failure handler, or user object to a script runner, you aren't changing the existing runner; instead, you get back a new script runner with new behavior.

You can use any combination and any order of withSuccessHandler(), withFailureHandler(), and withUserObject(). You can also call any of the modifying functions on a script runner that already has a value set. The new value simply overrides the previous value.

This example sets a common failure handler for all three server calls, but two separate success handlers:

var myRunner = google.script.run.withFailureHandler(onFailure);
var myRunner1 = myRunner.withSuccessHandler(onSuccess);
var myRunner2 = myRunner.withSuccessHandler(onDifferentSuccess);

myRunner1.doSomething();
myRunner1.doSomethingElse();
myRunner2.doSomething();

Private functions

Server functions whose names end with an underscore are considered private. These functions cannot be called by google.script and their names are never sent to the client. You can thus use them to hide implementation details that need to be kept secret on the server. google.script is also unable to see functions within libraries.

In this example, the function getBankBalance() is available in the client code; a user who inspects your source code can discover its name even if you don't call it. However, the function deepSecret_() is completely invisible to the client.

Code.gs

function doGet() {
  return HtmlService.createHtmlOutputFromFile('Index');
}

function getBankBalance() {
  var email = Session.getActiveUser().getEmail()
  return deepSecret_(email);
}

function deepSecret_(email) {
 // Do some secret calculations
 return email + ' has $1,000,000 in the bank.';
}

Index.html

<!DOCTYPE html>
<html>
  <head>
    <base target="_top">
    <script>
      function onSuccess(balance) {
        var div = document.getElementById('output');
        div.innerHTML = balance;
      }

      google.script.run.withSuccessHandler(onSuccess)
          .getBankBalance();
    </script>
  </head>
  <body>
    <div id="output">No result yet...</div>
  </body>
</html>

Resizing dialogs in G Suite applications

Custom dialog boxes in Google Docs, Sheets, or Forms can be resized by calling the google.script.host methods setWidth(width) or setHeight(height) in client-side code. (To set the initial size of a dialog, use the HtmlOutput methods setWidth(width) and setHeight(height).) Note that dialogs do not re-center in the parent window when resized, and it is not possible to resize sidebars.

Closing dialogs and sidebars in G Suite

If you use the HTML service to display a dialog box or sidebar in Google Docs, Sheets, or Forms, you cannot close the interface by calling window.close(). Instead, you must call google.script.host.close(). For an example, see the section on serving HTML as a G Suite user interface.

Moving browser focus in G Suite

To switch focus in the user's browser from a dialog or sidebar back to the Google Docs, Sheets, or Forms editor, simply call the method google.script.host.editor.focus(). This method is particularly useful in combination with the Document service methods Document.setCursor(position) and Document.setSelection(range).

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