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site</a></li></ul></li></ul></div></div><main id="page-content"><div id="site-toc--side" class="site-toc"><header class="site-toc__title">Contents</header><ul class="section-nav"><li class="toc-entry nav-item"><a class="nav-link" href="#dart-js">dart:js</a></li><li class="toc-entry nav-item"><a class="nav-link" href="#package-js">package:js</a><ul class="nav"><li class="toc-entry nav-item"><a class="nav-link" href="#staticinterop">@staticInterop</a></li></ul></li><li class="toc-entry nav-item"><a class="nav-link" href="#dart-js_util">dart:js_<wbr>util</a></li></ul></div><article><div class="content"><div id="site-content-title"><h1>Past JS interop</h1></div><div id="site-toc--inline" class="site-toc"><header class="site-toc__title">Contents</header><ul class="section-nav"><li class="toc-entry"><a href="#dart-js">dart:js</a></li><li class="toc-entry"><a href="#package-js">package:js</a><ul><li class="toc-entry"><a href="#staticinterop">@staticInterop</a></li></ul></li><li class="toc-entry"><a href="#dart-js_util">dart:js_util</a></li></ul></div><aside class="alert alert-warning"><div class="alert-header"><i class="material-symbols" aria-hidden="true">warning</i> <span>Warning</span></div><div class="alert-content"><p>None of these legacy interop libraries are supported when compiling to <a href="/web/wasm">Wasm</a>.</p></div></aside><p>This page addresses previous iterations of JS interop for Dart that are considered legacy. They are not deprecated yet, but will likely be in the future. Therefore, prefer using <a href="https://api.dart.dev/dart-js_interop/dart-js_interop-library.html"><code>dart:js_interop</code></a> going forwards and migrate usages of old interop libraries when possible. While <a href="https://api.dart.dev/dart-html/dart-html-library.html"><code>dart:html</code></a> and other web libraries are closely related, they're covered in the <a href="/interop/js-interop/package-web"><code>package:web</code></a> page.</p><div class="header-wrapper"><h2 id="dart-js"><code>dart:js</code></h2><a class="heading-link" href="#dart-js" aria-label="Link to 'dart:js' section">#</a></div><p><a href="https://api.dart.dev/dart-js/dart-js-library.html"><code>dart:js</code></a> exposed a concrete <a href="https://api.dart.dev/dart-js/JsObject-class.html"><code>object wrapper</code></a> to interop with JS objects. This wrapper contained String-based methods to dynamically get, set, and call properties on the wrapped JS object. It was less performant due to wrapping costs and ergonomically more difficult to use. For example, you did not get code-completion as you couldn't declare interop members and instead relied on Strings. Many of the functionalities exposed in <code>dart:js</code> like <a href="https://api.dart.dev/dart-js_util/allowInterop.html"><code>allowInterop</code></a> were later re-exposed through other interop libraries.</p><p>This library has been legacy ever since <code>package:js</code> and <code>dart:js_util</code> were released. It will likely be the first to be deprecated.</p><div class="header-wrapper"><h2 id="package-js"><code>package:js</code></h2><a class="heading-link" href="#package-js" aria-label="Link to 'package:js' section">#</a></div><p><a href="https://pub.dev/packages/js"><code>package:js</code></a> introduced functionality to declare interop types and members. It allowed users to write interop classes instead of interop extension types. At runtime, these classes were erased to a type that is similar to <code>dart:js_interop</code>'s <a href="https://api.dart.dev/dart-js_interop/JSObject-extension-type.html"><code>JSObject</code></a>.</p><div class="code-block-wrapper language-dart"><div class="code-block-body"><span class="code-block-language" title="Language dart">dart</span><pre class="shiki dash-light" tabindex="0"><code><span class="line"><span style="color:#D43324">@JS</span><span style="color:#222222">()</span></span> <span class="line"><span style="color:#D43324">class</span><span style="color:#0468D7"> JSType</span><span style="color:#222222"> {}</span></span></code></pre></div></div><p>Users of <code>package:js</code> will find the syntax and semantics of <code>dart:js_interop</code> familiar. You may be able to migrate to <code>dart:js_interop</code> by replacing the class definition with an extension type and have it work in many cases.</p><p>There are significant differences, however:</p><ul><li><code>package:js</code> types could not be used to interop with browser APIs. <code>dart:js_interop</code> types can.</li><li><code>package:js</code> allowed dynamic dispatch. This meant that if you casted the <code>package:js</code> type to <code>dynamic</code> and called an interop member on it, it would forward to the right member. This is no longer possible with <code>dart:js_interop</code>.</li><li><code>package:js</code>' <a href="https://github.com/dart-lang/sdk/blob/main/sdk/lib/js/_js_annotations.dart#L11"><code>@JS</code></a> has no soundness guarantees as return types of <code>external</code> members were not checked. <code>dart:js_interop</code> is sound.</li><li><code>package:js</code> types could not rename instance members or have non-<code>external</code> members.</li><li><code>package:js</code> types could subtype and be a supertype of non-interop classes. This was often used for mocks. With <code>dart:js_interop</code>, mocking is done by substituting the JS object instead. See the <a href="/interop/js-interop/mock">tutorial on mocking</a>.</li><li><a href="https://github.com/dart-lang/sdk/blob/main/sdk/lib/js/_js_annotations.dart#L40"><code>@anonymous</code></a> types were a way to declare an interop type with an object literal constructor. <code>dart:js_interop</code> doesn't distinguish types this way and any <code>external</code> named-argument constructor is an object literal constructor.</li></ul><div class="header-wrapper"><h3 id="staticinterop"><code>@staticInterop</code></h3><a class="heading-link" href="#staticinterop" aria-label="Link to '@staticInterop' section">#</a></div><p>Along with <code>@JS</code> and <code>@anonymous</code>, <code>package:js</code> later exposed <a href="https://github.com/dart-lang/sdk/blob/main/sdk/lib/js/_js_annotations.dart#L48"><code>@staticInterop</code></a>, which was a prototype of interop extension types. It is as expressive and restrictive as <code>dart:js_interop</code> and was meant to be a transitory syntax until extension types were available.</p><p><code>@staticInterop</code> types were implicitly erased to <code>JSObject</code>. It required users to declare all instance members in extensions so that only static semantics could be used, and had stronger soundness guarantees. Users could use it to interact with browser APIs, and it also allowed things like renaming and non-<code>external</code> members. Like interop extension types, it didn't have support for dynamic dispatch.</p><p><code>@staticInterop</code> classes can almost always be migrated to an interop extension type by just changing the class to an extension type and removing the annotations.</p><p><code>dart:js_interop</code> exposed <code>@staticInterop</code> (and <code>@anonymous</code>, but only if <code>@staticInterop</code> is also used) to support static interop semantics until extension types were added to the language. All such types should now be migrated to extension types.</p><div class="header-wrapper"><h2 id="dart-js_util"><code>dart:js_util</code></h2><a class="heading-link" href="#dart-js_util" aria-label="Link to 'dart:js_util' section">#</a></div><p><a href="https://api.dart.dev/dart-js_util/dart-js_util-library.html"><code>dart:js_util</code></a> supplied a number of utility functions that could not be declared in an <code>package:js</code> type or were necessary for values to be passed back and forth. This included members like:</p><ul><li><code>allowInterop</code> (which is now <a href="https://api.dart.dev/dart-js_interop/FunctionToJSExportedDartFunction/toJS.html"><code>Function.toJS</code></a>)</li><li><code>getProperty</code>/<code>setProperty</code>/<code>callMethod</code>/<code>callConstructor</code> (which are now in <a href="https://api.dart.dev/dart-js_interop_unsafe/dart-js_interop_unsafe-library.html"><code>dart:js_interop_unsafe</code></a>)</li><li>Various JS operators</li><li>Type-checking helpers</li><li>Mocking support</li><li>And more.</li></ul><p><code>dart:js_interop</code> and <code>dart:js_interop_unsafe</code> contain these helpers now with possibly alternate syntax.</p><p id="page-github-links"><span>Unless stated otherwise, the documentation on this site reflects Dart 3.5.4. Page last updated on 2024-11-27.</span> <a href="https://github.com/dart-lang/site-www/tree/main/src/content/interop/js-interop/past-js-interop.md" target="_blank" rel="noopener">View source</a> <span>or </span><a href="https://github.com/dart-lang/site-www/issues/new?template=1_page_issue.yml&page-url=https://dart.dev/interop/js-interop/past-js-interop/&page-source=https://github.com/dart-lang/site-www/tree/main/src/content/interop/js-interop/past-js-interop.md" title="Report an issue with this page" target="_blank" rel="noopener">report an issue</a>.</p></div></article></main><footer id="page-footer"><div class="footer-section footer-main"><a href="/" class="brand" title="Dart"><img src="/assets/img/logo/logo-white-text.svg" alt="Dart" width="164"></a><div class="footer-social-links"><a href="https://medium.com/dartlang" target="_blank" rel="noopener" title="Medium blog"><svg><use href="/assets/img/social/medium.svg#medium"></use></svg> </a><a href="https://github.com/dart-lang" target="_blank" rel="noopener" title="GitHub"><svg><use href="/assets/img/social/github.svg#github"></use></svg> </a><a href="https://twitter.com/dart_lang" target="_blank" rel="noopener" title="X (Twitter)"><svg><use href="/assets/img/social/x.svg#x"></use></svg></a></div></div><div class="footer-section footer-tray"><div class="footer-licenses">Except as otherwise noted, this site is licensed under a <a href="https://creativecommons.org/licenses/by/4.0/">Creative Commons Attribution 4.0 International License</a>, and code samples are licensed under the <a href="https://opensource.org/licenses/BSD-3-Clause">3-Clause BSD License</a>.</div><div class="footer-utility-links"><ul><li><a href="/terms" title="Terms of use">Terms</a></li><li><a href="https://policies.google.com/privacy" target="_blank" rel="noopener" title="Privacy policy">Privacy</a></li><li><a href="/security" title="Security philosophy and practices">Security</a></li></ul></div></div></footer></body></html>