@lit-labs/compiler

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: https://github.com/lit/rfcs/pull/21

Give feedback: https://github.com/lit/lit/discussions/4117

Overview

@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.

Usage

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: [
    typescript({
      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.

FAQ

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.