Embedding Dart in HTML

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Written by Sigmund Cherem, Vijay Menon, and Seth Ladd
October 2011 (updated December 2014)

Dart apps compile to JavaScript to run across modern desktop and mobile browsers. Dart apps can also run inside a Dart virtual machine (VM), which can be embedded into web browsers.

This article covers how to integrate Dart apps into web pages, when first compiled to JavaScript or when run in the Dart VM.

Quick start

  1. Compile your Dart app to JavaScript with dart2js. If your Dart file is app.dart then name your JavaScript version app.dart.js.
  2. Load your Dart app with a <script type="application/dart"> tag (only one per HTML page).
  3. Follow that tag with a <script> tag for the dart.js file.

Here is a minimal example, which works in all browsers (even if they don’t support the Dart VM):

<!DOCTYPE html>
<html>
  <head>
    <title>Simple Dart App</title>
  </head>
  <body>
    <h1>Hello, Dart!</h1>
    <script type="application/dart" src="app.dart"></script>
    <script type="text/javascript" src="packages/browser/dart.js"></script>
  </body>
</html>

Dart MIME type and the script tag

HTML script tags provide a type attribute to define the language of the script. For Dart, this attribute has the value application/dart.

Here is an example:

<script type="application/dart" src="app.dart"></script>

Dart doesn’t support inline scripts (scripts defined inside the HTML page). While an inline script technically works in Dartium (a build of Chromium with the Dart VM), the Dart-to-JavaScript compiler (dart2js) doesn’t work with inline scripts.

The Dart app must have a visible top-level function called main(). The browser invokes main() when the DOM content is loaded (but see caveat in the next section).

You should use only one <script type="application/dart"> inside the HTML page. The dart2js compiler produces JavaScript that assumes it is the only Dart app on the page.

Dart semantics in HTML

The precise semantics of Dart execution on an HTML page are evolving and likely to change by the time Dart is natively supported in a production browser.

Currently, your Dart app’s main() function is run after DOM content is loaded, but the exact timing is undefined (e.g., with respect to the load event or deferred JavaScript). In the future, we may invoke main() synchronously during HTML parsing instead of waiting for DOM content to load.

As such, we recommend the following:

  1. Restrict your application to a single Dart script per document.
  2. Place your Dart script at the end of your document’s body (followed only by the dart.js script if you choose to use that).
  3. Do not use async or defer attributes on your Dart script tag at this point.
  4. Do not rely upon ordering between your Dart script and any JavaScript scripts that may execute after it (e.g., that appear after it or are marked async or defer).
  5. Do not rely upon script injection of Dart code.

The dart.js script

Use the dart.js script, part of the browser package, to run your app in browsers that don’t have a Dart VM. The dart.js script allows you to use the same HTML page for both Dartium and other browsers.

If no Dart VM is detected in the browser, the dart.js script swaps out the application/dart script for a text/javascript script that points to the JavaScript version of the Dart app.

The dart.js script tag must come after the Dart script tag.

For example:

<script type="application/dart" src="awesome_app.dart"></script>
<script type="text/javascript" src="packages/browser/dart.js"></script>

To get a copy of the dart.js file, you can use the pub package manager. Here’s how:

  1. Add the following to your application’s pubspec.yaml, replacing the version range with the appropriate version:

    dependencies:
      browser: ">=0.10.0 <0.11.0"
    
  2. Run pub get.

  3. Use a relative script tag to point to the installed version:

    <script src="packages/browser/dart.js"></script>
    

Fundamental differences from JavaScript

Embedding Dart code is different from embedding JavaScript in a few ways.

A single script tag

Each HTML document can have at most one Dart script tag, or only one script tag that references a Dart app that is compiled to JavaScript. This fundamentally differs from the way that JavaScript is embedded in HTML—in JavaScript, you can have multiple script tags per document.

In JavaScript, additional script tags are used to import third party libraries (e.g., jQuery). In Dart, this is not necessary: import is part of the language.

Web components and HTML imports are an important caveat to the one script per document rule. For example, the Polymer Dart framework uses HTML imports to allow developers to include HTML components into a Dart application. Dart scripts in imported HTML documents are injected into the main application. See the Polymer Dart documentation for more details.

Execution timing

Unlike in JavaScript, top-level Dart constructs (such as interfaces, classes, and functions) are declarative. Each Dart application (defined via a script tag) provides an explicit main() entry point that is invoked by the browser when it is ready to run.

Currently, the Dart app’s main() function is invoked after the page’s DOM is loaded.

No inline event listeners

We disallow inline event listeners of the form:

<div onclick="foo()">Click this text.</div>

With JavaScript, programmers can embed inline event listener code directly onto HTML nodes. However, this is typically discouraged in modern JavaScript applications. HTML pages generally render more quickly if listeners are added programmatically afterwards. Modern security guidelines also discourage inline code. As such, we propose to not allow inline Dart listeners.

No script injection of Dart code

We do not currently support or recommend dynamically injecting a <script> tag that loads Dart code. Recent browser security trends, like Content Security Policy, actively prevent this practice.