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LitElement 3.0 & lit-html 2.0: Early Preview Release

Preview the next major versions of LitElement and lit-html.

Lit Team

Today we’re publishing the first preview releases of the next major versions of our flagship libraries, LitElement and lit-html.

These releases include most of the breaking changes we intend to make, and most of the functionality we want to carry over from the previous versions. They are not yet feature complete or fully API stable. Notably, they don’t yet support legacy browsers like IE11, or browsers requiring the web components polyfill.

We’ve been very happy with the current versions of our libraries—they’re fast, small, and stable (yay!)—and in some ways we don’t have very many pressing needs to make breaking changes. We don’t make breaking changes lightly. But there are some compelling reasons for changes that we think will improve the user-experience of components and applications built with LitElement.

  1. Performance

    We have found that some of our browser-bug workaround code and customization abstractions prevent optimizations that we would like to do.

  2. Size

    That same code, and our extensive public API, costs bytes. We always want to find ways to make the libraries smaller.

  3. Features & API cleanup

    Some features are difficult to add in a cost-effective way with the current architecture, or can’t really be improved without breaking changes.

  4. Server-side-rendering

    lit-html has an extremely flexible and customizable API, and in some ways is more of a template-system construction kit than a single template-system. But this flexibility complicates SSR, which needs to make assumptions about how the server-rendered HTML maps to templates. SSR would only work well with the default, uncustomized lit-html, so limiting customization makes SSR more reliable. Very few developers used the customization APIs anyway.

We also want to make the directive API SSR compatible by limiting access to the DOM.

These are new major versions, so there are some breaking changes, but we want to minimize disruption to our users as much as possible. We’ve limited the breaking changes so that they don’t affect most users, or only require a mechanical change (like changing an import).

Please see the READMEs (LitElement, lit-html) and CHANGELOGs (LitElement, lit-html) for the most detailed list of changes.

The most important changes:

  • Customizing the syntax of lit-html is no longer directly supported. The templateFactory and TemplateProcessor APIs have been removed.

  • The public API has been minimized in order to facilitate better minification and future evolution.

  • The lit-html directive API has changed to be class-based and to persist directive instances. Directives should be easier to write and easier to make SSR compatible.

  • The LitElement decorators are no longer exported from the main module—they have to be imported individually or from a new lit-element/decorators.js module. This means smaller app sizes for non-decorator-users and opens the door to new decorators implementing the current TC39 JavaScript proposal when those arrive.

  • lit-html no longer uses instanceof or module-level WeakMaps to detect special objects like template results and directives, which should improve the compatibility of multiple copies of lit-html in a single app. We still recommend de-duping npm packages, but more cases will work now.

  • Safari 12 has a critical template literal bug, which is no longer worked around in lit-html. If you support Safari 12 you will have to compile template literals to their ES5 equivalent. Note that babel-preset-env already does this for the broken versions of Safari.

There are smaller changes listed in the change logs. Overall we hope these versions are drop in replacements for most users, or only require updating decorator imports.

Run:

And/or:

We’re in the process of moving the next versions of LitElement and lit-html into a mono-repo. Please file issues on the current lit-html repo, using a prefix of [lit-html] or [lit-element]. We expect that, as with any pre-release, there will be common issues we will have to fix. Please search for your issue first. Issues for the next major versions will have the label lit-next.

For the next preview release we will be focusing on browser and polyfill support, especially IE11.