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Lit 2.0 Upgrade Guide
Lit 2.0 is designed to work with most code written for LitElement 2.x and lit-html 1.x. There are a small number of changes required to migrate your code to Lit 2.0. The high-level changes required include:
- Updating npm packages and import paths.
- Loading
polyfill-support
script when loading the webcomponents polyfills. - Updating any custom directive implementations to use new class-based API and associated helpers
- Updating code to renamed APIs.
- Adapting to minor breaking changes, mostly in uncommon cases.
The following sections will go through each of these changes in detail.
Lit 2.0 ships with a one-stop-shop lit
package, which consolidates lit-html
and lit-element
into an easy-to-use package. Use the following commands to upgrade:
npm uninstall lit-element lit-html
npm install lit
And re-write your module imports appropriately:
From:
import {LitElement, html} from `lit-element`;
To:
import {LitElement, html} from `lit`;
Although the lit-element@^3
and lit-html@^2
packages should be largely backward-compatible, we recommend updating to the lit
package as the other packages are moving towards eventual deprecation.
The previous version of lit-element
exported all TypeScript decorators from the main module. In Lit 2.0, these have been moved to a separate module, to enable smaller bundle sizes when the decorators are unused.
From:
import {property, customElement} from `lit-element`;
To:
import {property, customElement} from `lit/decorators.js`;
Built-in lit-html directives are also now exported from the lit
package.
From:
import {repeat} from `lit-html/directives/repeat.js`;
To:
import {repeat} from `lit/directives/repeat.js`;
Lit 2.0 still supports the same browsers down to IE11. However, given the broad adoption of Web Components APIs in modern browsers, we have taken the opportunity to move all of the code required for interfacing with the webcomponents
polyfills out of the core libraries and into an opt-in support file, so that the tax for supporting older browsers is only paid when required.
In general, any time you use the webcomponents
polyfills, you should also load the lit/polyfill-support.js
support file once on the page, similar to a polyfill. For example:
<script src="node_modules/@webcomponents/webcomponentsjs/webcomponents-loader.js"></script>
<script src="node_modules/lit/platform-support.js"></script>
If using @web/test-runner
or @web/dev-server
with the legacyPlugin
for development, adding the following configuration to your web-test-runner.config.js
or web-dev-server.config.js
file will configure it to automatically inject the support file when needed:
export default {
...
plugins: [
legacyPlugin({
polyfills: {
webcomponents: true,
custom: [
{
name: 'lit-polyfill-support',
path: 'node_modules/lit/polyfill-support.js',
test: "!('attachShadow' in Element.prototype)",
module: false,
},
],
},
}),
],
};
The following advanced API's have been renamed in Lit 2.0. It should be safe to simply rename these across your codebase if used:
Previous name | New name | Notes |
---|---|---|
UpdatingElement |
ReactiveElement |
The base class underpinning LitElement . Naming now aligns with terminology we use to describe its reactive lifecycle. |
@internalProperty |
@state |
Decorator for LitElement / ReactiveElement used to denote private state that trigger updates, as opposed to public properties on the element settable by the user which use the @property decorator. |
static getStyles() |
static finalizeStyles(styles) |
Method on LitElement and ReactiveElement class used for overriding style processing. Note it now also takes an argument reflecting the static styles for the class. |
_getUpdateComplete() |
getUpdateComplete() |
Method on LitElement and ReactiveElement class used for overriding the updateComplete promise |
NodePart |
ChildPart |
Typically only used in directive code; see below. |
While the API for using directives should be 100% backward-compatible with
1.x, there is a breaking change to how custom directives are authored. The API
change improves ergonomics around making stateful directives while providing a
clear pattern for SSR-compatible directives: only render
will be called on the
server, while update
will not be.
Concept | Previous API | New API |
---|---|---|
Code idiom | Function that takes directive arguments, and returns function that takes part and returns value |
Class that extends Directive with update & render methods which accept directive arguments |
Declarative rendering | Pass value to part.setValue()
|
Return value from render() method |
DOM manipulation | Implement in directive function | Implement in update() method |
State | Stored in WeakMap keyed on part
|
Stored in class instance fields |
Part validation |
instanceof check on part in every render |
part.type check in constructor |
Async updates |
part.setValue(v); part.commit();
|
Extend AsyncDirective instead of Directive and call this.setValue(v)
|
Below is an example of a lit-html 1.x directive, and how to migrate it to the new API:
1.x Directive API:
import {html, directive, Part, NodePart} from 'lit-html';
// State stored in WeakMap
const previousState: WeakMap<Part, number> = new WeakMap();
// Functional-based directive API
export const renderCounter = directive((initialValue: number) => (part: Part) => {
// When necessary, validate part type each render using `instanceof`
if (!(part instanceof NodePart)) {
throw new Error('renderCounter only supports NodePart');
}
// Retrieve value from previous state
let value = previousState.get(part);
// Update state
if (value === undefined) {
value = initialValue;
} else {
value++;
}
// Store state
previousState.set(part, value);
// Update part with new rendering
part.setValue(html`<p>${value}</p>`);
});
2.0 Directive API:
import {html} from 'lit';
import {directive, Directive, Part, PartInfo, PartType} from 'lit/directive.js';
// Class-based directive API
export class RenderCounter extends Directive {
// State stored in class field
value: number | undefined;
constructor(partInfo: PartInfo) {
super(partInfo);
// When necessary, validate part in constructor using `part.type`
if (partInfo.type !== PartType.CHILD) {
throw new Error('renderCounter only supports child expressions');
}
}
// Optional: override update to perform any direct DOM manipulation
update(part: Part, [initialValue]: DirectiveParameters<this>) {
/* Any imperative updates to DOM/parts would go here */
return this.render(initialValue);
}
// Do SSR-compatible rendering (arguments are passed from call site)
render(initialValue: number) {
// Previous state available on class field
if (this.value === undefined) {
this.value = initialValue;
} else {
this.value++;
}
return html`<p>${this.value}</p>`;
}
}
export const renderCounter = directive(RenderCounter);
For completeness, the following is a list of minor but notable breaking changes that you may need to adapt your code to. We expect these changes to affect relatively few users.
- The
update
andrender
callbacks will only be called when the element is connected to the document. If an element is disconnected while an update is pending, or if an update is requested while the element is disconnected, update callbacks will be called if/when the element is re-connected. - For simplicity,
requestUpdate
no longer returns a Promise. Instead await theupdateComplete
Promise. - Errors that occur during the update cycle were previously squelched to allow subsequent updates to proceed normally. Now errors are re-fired asynchronously so they can be detected. Errors can be observed via an
unhandledrejection
event handler on window. - Creation of
shadowRoot
viacreateRenderRoot
and support for applyingstatic styles
to theshadowRoot
has moved fromLitElement
toReactiveElement
. - The
createRenderRoot
method is now called just before the first update rather than in the constructor. Element code can not assume therenderRoot
exists before the elementhasUpdated
. This change was made for compatibility with server-side rendering. -
ReactiveElement
'sinitialize
method has been removed. This work is now done in the element constructor. - The static
render
method on theLitElement
base class has been removed. This was primarily used for implementing ShadyDOM integration, and was not intended as a user-overridable method. ShadyDOM integration is now achieved via thepolyfill-support
module. - When a property declaration is
reflect: true
and itstoAttribute
function returnsundefined
the attribute is now removed where previously it was left unchanged (#872). - The dirty check in
attributeChangedCallback
has been removed. While technically breaking, in practice it should very rarely be (#699). - LitElement's
adoptStyles
method has been removed. Styling is now adopted increateRenderRoot
. This method may be overridden to customize this behavior. - Removed
requestUpdateInternal
. TherequestUpdate
method is now identical to this method and should be used instead. - The type of the
css
function has been changed toCSSResultGroup
and is now the same asLitElement.styles
. This avoids the need to cast thestyles
property toany
when a subclass setsstyles
to an Array and its super class set a single value (or visa versa).
-
render()
no longer clears the container it's rendered to on first render. It now appends to the container by default. - Expressions in comments are not rendered or updated.
- Template caching happens per callsite, not per template-tag/callsize pair. This means some rare forms of highly dynamic template tags are no longer supported.
- Arrays and other iterables passed to attribute bindings are not specially handled. Arrays will be rendered with their default toString representation. This means that
html`<div class=${['a', 'b']}>
will render<div class="a,b">
instead of<div class="a b">
. To get the old behavior, usearray.join(' ')
. - The
templateFactory
option ofRenderOptions
has been removed. -
TemplateProcessor
has been removed. - Symbols are not converted to a string before mutating DOM, so passing a Symbol to an attribute or text binding will result in an exception.
lit-html 1.4.0 adds a number of forward-compatible modules to assist migration from 1.x to 2.x, so that many of the changes to prepare for Lit 2.0 can be done incrementally in the 1.x codebase before upgrading the entire app.
-
lit-html/directive.js
- Class-based directive API -
lit-html/async-directive.js
- Class based async directive API (notedisconnected
andreconnected
callbacks will not be called in 1.x) -
lit-html/directive-helpers.js
- Provides forward-compatible implementation ofisTemplateResult
In addition, imports from lit-html 1.x and lit-element 2.x will continue to work after upgrading to the Lit 2.0 versions (lit-html 2.x and lit-element 3.x). However, once all components are upgraded in an app, we recommend switching imports to the lit
package as described above, as the separate packages may be deprecated in future major releases.