bacom

Base for Web Components (bacom)

Basic library for writing lightweight web components. Suitable for low-level web components in UI libraries. Ensures standard behaviour of custom elements with shadow DOM efficiently.

• Tiny size - 2.19 kB minified, 1.1 kB gzipped, 963 B brotlied.
• Consumable as ESM, UMD and CJS modules.
• Zero dependencies.
• Written in TypeScript.
• Templates and stylesheets compilable from .html and .css, .less or .scss sources.
• SSR and Tests in Node.js feasible using @prantlf/dom-lite.

Synopsis

greetme.ts:

// import decorators from `bacom`
import { comp, prop, elem, event } from 'bacom'
// import a function returning HTMLTemplateElement
import template from './template.html'
// import a function returning CSSStylesheet or HTMLStyleElement
import style from './style.css'

// register a custom element with a stylesheet and a template content
@comp({ tag: 'greet-me', styles: [style], template })
export class GreetMeElement extends HTMLElement {
  // reflects a property to an attribute and back, watches for changes
  @prop({ type: 'string', reflect: true })
  public name: string

  // pins an element with the ID `display-name` in the rendered content
  @elem()
  private displayName: HTMLElement

  // listens to the `click` event bubbled to the host
  @event()
  onClick(): void {
    this.classList.toggle('flash')
  }

  // updates the element content whenever a reflected property changes
  attributeChangedCallback(name: string, oldValue: string, newValue: string): void {
    this.displayName.textContent = newValue
  }
}

template.html:

Hello, <span id=display-name></span>!

style.css:

:host {
  --greetme-font-size: 1rem;
  font-size: var(--greetme-font-size);
}

test.html:

<greet-me name=John></greet-me>
<!-- Renders "Hello, John!" -->
<script type=module src=dist/greetme.esm.min.js></script>

Installation

You can install this package using your favourite Node.js package manager:

npm i -D bacom
yarn add -D bacom
pnpm i -D bacom

If you do not want to bundle this package in your build output, you can load it separately on your web page before your script bundle:

<script src=https://unpkg.com/bacom@1.0.0/dist/index.umd.min.js></script>
<script src=yours/index.js></script>

All named exports are available in the global object bacom.

Features

The following features are implemented:

• Registering of the custom element with the provided tag name.
• Optionally synchronising values of a properties and attributes (reflection).
• Rendering the shadow DOM content from a template.
• Applying common styles by constructible stylesheets.
• Setting an child element to a property using an ID or an selector.
• Listening to an event on the host element or on a child element.
• Building with plugins for esbuild, rollup and webpack to transform css, less, scss and html files to functions returning CSSStylesheet and HTMLTemplateElement, including memoization for the best performance.

Build

If you want to keep stylesheets and templates in separate .css or .less or .scss and .html files instead of keeping the in strings in the script code, you will need bundler plugins.

Note: If you compile your stylesheets with SASS using one of the bacom/tools/sass/* plugins, you have to include the dependency on sass in your project yourself:

npm i -D sass
yarn add -D sass
pnpm i -D sass

Similarly, if you compile your stylesheets with LESS using one of the bacom/tools/less/* plugins, you have to include the dependency on less in your project yourself:

npm i -D less
yarn add -D less
pnpm i -D less

Plugins for rollup:

• bacom/tools/less/rollup: compiles LESS sources to a string with CSS; parameters (include, exclude, minify, options)
• bacom/tools/sass/rollup: compiles SASS sources to a string with CSS; parameters (include, exclude, minify, options)
• bacom/tools/style/rollup: compiles a CSS source to a string; parameters (include, exclude, minify)
• bacom/tools/templ/rollup: compiles a HTML source to a string; parameters (include, exclude, minify)

import typescript from '@rollup/plugin-typescript'
import { nodeResolve } from '@rollup/plugin-node-resolve'
import { minify } from 'rollup-plugin-swc-minify'
import sourcemaps from 'rollup-plugin-sourcemaps'
import style from 'bacom/tools/style/rollup'
import templ from 'bacom/tools/templ/rollup'

export default {
  input: 'src/index.ts',
  output: [{
    file: 'dist/index.min.js', format: 'iife', sourcemap: true
  }],
  plugins: [
    nodeResolve(), style({ minify: true }), templ({ minify: true }),
    typescript(), sourcemaps(), minify()
  ]
}

Plugin parameters:

Name	Type	Description
include	string[]	wildcard expressions matching file names to process
exclude	string[]	wildcard expressions matching file names to ignore
minify	boolean	minify the output stylesheet
options	object	the options parameter for the less compiler or the sass compiler

Plugins for esbuild:

• bacom/tools/less/esbuild: compiles LESS sources to a string with CSS; parameters (include, exclude, minify, options)
• bacom/tools/sass/esbuild: compiles SASS sources to a string with CSS; parameters (include, exclude, minify, options)
• bacom/tools/style/esbuild: compiles a CSS source to a string; parameters (include, exclude, minify)
• bacom/tools/templ/esbuild: compiles a HTML source to a string; parameters (include, exclude, minify)

const { build } = require('esbuild')
const style = require('bacom/tools/style/esbuild')
const templ = require('bacom/tools/templ/esbuild')

const targets = [
  {
    entryPoints: ['src/index.ts'],
    outfile: 'dist/index.min.js',
    format: 'iife', sourcemap: true, bundle: true, minify: true,
    plugins: [style({ minify: true }), templ({ minify: true })]
  }
]

Promise.all(targets.map(build)).catch(() => process.exitCode = 1)

Plugin parameters:

Name	Type	Description
include	string	regular expression matching file names to process
exclude	string	regular expression matching file names to ignore
minify	boolean	minify the output stylesheet
options	object	the options parameter for the less compiler or the sass compiler

Plugins for webpack:

• bacom/tools/less/webpack: compiles LESS sources to a string with CSS; parameters (minify, options)
• bacom/tools/sass/webpack: compiles SASS sources to a string with CSS; parameters (minify, options)
• bacom/tools/style/webpack: compiles a CSS source to a string; parameters (minify)
• bacom/tools/templ/webpack: compiles a HTML source to a string; parameters (minify)

const path = require('path')
const nodeExternals = require('webpack-node-externals')

export default [
  {
    entry: './src/index.ts',
    output: {
      filename: 'index.min.js',
      path: path.resolve(__dirname, 'dist'),
      module: true,
      library: { type: 'iife' }
    },
    mode: 'production',
    devtool: 'source-map',
    module: {
      rules: [
        { test: /\.ts$/, use: 'ts-loader' },
        { test: /\.css$/, use: 'bacom/tools/style/webpack' },
        { test: /\.t?html$/, use: 'bacom/tools/templ/webpack' }
      ]
    }
  }
]

Plugin parameters:

Name	Type	Description
minify	boolean	minify the output stylesheet (true by default if mode === 'production')
options	object	the options parameter for the less compiler or the sass compiler

Custom Rendering

If you do not pass a template as a parameter to the comp decorator, you can implement the render method to populate the shadow DOM yourself. You will still be able to use the decorators elem and event and refer to the Shadow DOM content.

import { comp, prop, elem } from 'bacom'

@comp({ tag: 'greet-me' })
export class GreetMeElement extends HTMLElement {
  @prop({ type: 'string' })
  public name: string

  @elem()
  private displayName: HTMLElement

  render(): void {
    this.shadowRoot = 'Hello, <span id=display-name></span>!'
  }

  attributeChangedCallback(name: string, oldValue: string, newValue: string): void {
    this.displayName.textContent = newValue
  }
}

If you implement the render method, it will be called even if you pass the template parameter to the comp decorator. You can use it to update the Shadow DOM content.

SSR

import '@prantlf/dom-lite/global'
import './components/greetme'

const fragment = document.createDocumentFragment()
fragment.innerHTML = '<greet-me name=John></greet-me>'
const output = fragment.getInnerHTML({ includeShadowRoots: true })
// The output will contain:
// <greet-me name=John>
//   <template shadowroot="open">Hello, <span id="display-name">John</span>!</template>
// </greet-me>

Test

import '@prantlf/dom-lite/global'
import './components/greetme'
import suite from 'tehanu'
import assert from 'assert'

const test = suite('greet-me')

test('greets with the specified name', () => {
  const el = document.createElement('greet-me')
  el.name = 'John'
  assert.equal(el.shadowRoot.innerHTML, 'Hello, <span id="display-name">John</span>!')
})

Contributing

In lieu of a formal styleguide, take care to maintain the existing coding style. Test your code using npm test.

License

Copyright (c) 2021-2024 Ferdinand Prantl

Licensed under the MIT license.