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Compiler to prepare Lit templates at build time

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A compiler for optimizing Lit templates.

Warning @lit-labs/compiler is part of the Lit Labs set of packages – it is published in order to get feedback on the design and may receive breaking changes. RFC:

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@lit-labs/compiler exports a TypeScript Transformer that can be run over your JavaScript or TypeScript files to optimize away the lit-html prepare render phase. For template heavy applications this can result in a quicker first render.


This transformer can be used anywhere TypeScript transformers are accepted, which is dependent on your build setup.

Below is an example using Rollup with the plugin @rollup/plugin-typescript:

// File: rollup.config.js
import typescript from '@rollup/plugin-typescript';
import {compileLitTemplates} from '@lit-labs/compiler';

export default {
  // ...
  plugins: [
      transformers: {
        before: [compileLitTemplates()],
    // other rollup plugins

See an example of the transformer in use in this project's test for source-maps validity in this rollup config file.


What are the tradeoffs for using the compiler?

  1. Running the compiler requires a build step that can accept a TypeScript transformer.

  2. The very first template render is faster (sometimes up to 45% faster for template heavy pages), but currently the output file is about 5% larger (gzipped).

How do I know optimizations have been applied?

Given your original source code containing the html tag function to declare templates:

const hi = (name) => html`<h1>Hello ${name}!</h1>`;

This code should have been emitted at the end of your build without the html tag function. E.g. the above authored example is transformed into something like:

const b = (s) => s;
const lit_template_1 = {h: b`<h1>Hello <?></h1>`, parts: [{type: 2, index: 1}]};
const hi = (name) => ({_$litType$: lit_template_1, values: [name]});

What templates are optimized by the compiler?

In order for a template to be optimized by the compiler, it must be:

  1. A well-formed template that wouldn't raise runtime diagnostics in development builds of lit-html. For example, templates with expressions in invalid locations will not be compiled.
  2. Use html imported directly from the module lit or lit-html. Re-exports of html from other modules are not supported. The following imports are supported:
    1. import {html} from 'lit'; Usage: html`...`
    2. import {html as litHtml} from 'lit'; Usage: litHtml`...`
    3. import * as litModule from 'lit' Usage: litModule.html`...`
  3. Cannot contain dynamic bindings within the raw text elements: textarea, title, style, and script. This is due to these elements containing raw text nodes as children & the limitation that raw text nodes cannot be placed as adjacent children in HTML markup.

Does the compiler work on JavaScript files?

Because JavaScript is a subset of TypeScript, the TypeScript transform has been implemented and tested such that it handles JavaScript.

You will need to run the compiler transformer over your JavaScript files.

Future work

  • Investigate if it's possible to reduce the file size increase on the compilers output.
  • Expand compilation so the complete optimization of a lit application can also tree-shake the relevant lit-html runtime that is no longer needed.
  • Explore more optimizations than just the prepare phase.
  • Provide different ways of consuming and using the compiler.


Last updated on 14 Mar 2024

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