HTML Service: Best Practices

Creating user interfaces with the HTML service follows many of the same patterns and practices as other types of web development. However, there are some aspects that are unique to the Apps Script environment or are otherwise worth highlighting. Below we'll cover some best practices you should keep in mind when developing your own HTML-service UIs.

To help demonstrate these best practices, we have created a sample web app using the HTML service called Simple Tasks. The full source code and setup instructions are available in our GitHub repository.

Separate HTML, CSS, and JavaScript

Keeping all the HTML, CSS, and JavaScript code in one file can make your project difficult to read and develop. While Apps Script does require client-side code to be placed in .html files, you can still separate your CSS and client-side JavaScript into different files for better readability. Separated files can be included back into the main HTML page using template statements such as <?!= include(myCSS); ?>.

Code.gs

function doGet(request) {
  return HtmlService.createTemplateFromFile('Page')
      .evaluate();
}

function include(filename) {
  return HtmlService.createHtmlOutputFromFile(filename)
      .getContent();
}

Page.html

<!DOCTYPE html>
<html>
  <head>
    <base target="_top">
    <?!= include('Stylesheet'); ?>
  </head>
  <body>
    <h1>Welcome</h1>
    <p>Please enjoy this helpful script.</p>
    <?!= include('JavaScript'); ?>
  </body>
</html>

Stylesheet.html

<style>
p {
  color: green;
}
</style>

JavaScript.html

<script>
window.addEventListener('load', function() {
  console.log('Page is loaded');
});
</script>

Notice that the Stylesheet and JavaScript files must contain the <style> and <script> tags respectively, because they are HTML snippets and not pure .css or .js files.

Load data asynchronously, not in templates

Templated HTML can be used to quickly build simple interfaces, but its use should be limited to ensure your UI is responsive. The code in templates is executed once when the page is loaded, and no content is sent to the client until the processing is complete. Having long-running tasks in your scriptlet code can cause your UI to appear slow.

Use scriptlet tags for quick, one-time tasks such as including other content or setting static values. All other data should be loaded using google.script.run calls. Coding in this asynchronous manner is more difficult but allows the UI to load more quickly and gives it the opportunity to present a spinner or other loading message to the user.

Don't — load in templates

<p>List of things:</p>
<? var things = getLotsOfThings(); ?>
<ul>
  <? for (var i = 0; i < things.length; i++) { ?>
    <li><?= things[i] ?></li>
  <? } ?>
</ul>

Do — load asynchronously

<p>List of things:</p>
<ul id="things">
    <li>Loading...</li>
</ul>

<script
src="//ajax.googleapis.com/ajax/libs/jquery/1.9.1/jquery.min.js">
</script>
<script>
// The code in this function runs when the page is loaded.
$(function() {
  google.script.run.withSuccessHandler(showThings)
      .getLotsOfThings();
});

function showThings(things) {
  var list = $('#things');
  list.empty();
  for (var i = 0; i < things.length; i++) {
    list.append('<li>' + things[i] + '</li>');
  }
}
</script>

In the Simple Tasks sample application, the HTML-service page Page.html loads data this way, and only uses templating to include other source code into the page.

Load resources using HTTPS

If your page is served using the newer IFRAME sandbox mode, including JavaScript or CSS files not served using HTTPS will result in errors like the one below.

Mixed Content: The page at 'https://...' was loaded over HTTPS, but requested an insecure script 'http://...'. This request has been blocked; the content must be served over HTTPS.

Most popular libraries support both HTTP and HTTPS, so switching is usually just a matter of inserting an addition 's' into the URL.

Use the HTML5 document type declaration

If your page is served using the newer IFRAME sandbox mode, make sure to include the following snippet of code at the top of you HTML file.

<!DOCTYPE html>

This document type declations tells the browser that you designed the page for modern browsers, and that it shouldn't render your page using quirks mode. Even if you don't plan to take advantage of modern HTML5 elements or JavaScript APIs, this will help ensure your page is displayed correctly.

Don't use <html>, <head>, or <body> tags

If your page is sanitized by Caja instead of the newer IFRAME sandbox mode, high-level structural tags are replaced with virtual tags and your markup is inserted into another page created by Caja. For example, an <html> tag is replaced by <caja-v-html>. Any styles or scripts you attach to these elements are preserved, but the semantic meaning changes.

Because of this behavior, we recommend that you don't use <html>, <head>, or <body> tags in your page and instead just place your styles, scripts, and other HTML content directly in the page. To set the title of your page, use the setTitle() method of HtmlOutput, since <title> tags in your page are ignored.

Load JavaScript last

Many web developers recommend loading JavaScript code at the bottom of the page to increase responsiveness, and this is even more important with the HTML service. Moving your <script> tags to the end of your page will let HTML content render before the JavaScript is processed, allowing you to present a spinner or other message to the user.

Take advantage of jQuery

jQuery is a popular JavaScript library that simplifies many common tasks in web development. We take special care to ensure that jQuery works with our Caja sanitization and encourage developers to leverage it in their applications.

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