@mdn/fred

Fred

MDN's next fr(ont)e(n)d.

Getting started

• Copy .env-dist to .env and update
• Install dependencies npm install
• Bring up the dev environment with npm run start

Commands

• npm run start
  • runs the rari server and the live-reloading development server together
  • run with NODE_ENV=production to run rari with the preview server, you'll need to have run npm run build first
• node --env-file=.env --run rari -- serve
  • runs the rari server
  • necessary for npm run dev and npm run preview
• npm run dev
  • brings up the live-reloading development server, likely what you want for doing local development
• npm run build
  • builds the production js/css/asset bundles
  • must be run at least once for npm run preview to work
• npm run preview
  • runs the preview server: using the production bundles with the rari server: useful for testing our prod rspack config

Development principles

Environment variables

See the environment variables README.

Inline JS

We need to run some JS as soon as possible at page load, to avoid layout shifts and flashes. We place this JS in entry.inline.js, and it's inlined on page load. Rspack also generates the necessary CSP hash when doing a prod build with npm run build.

Layout

See the layout README.

Media queries

See the media queries README.

Sandbox

We have a basic sandbox for testing and styling components in isolation at http://localhost:3000/sandbox

To add a component to the sandbox, add a sandbox.js file to the component, which exports a class named like MyComponentSandbox which extends the SandboxComponent exported from components/sandbox/class.js.

Components and Elements

• Components should live in the components/ folder, with reserved names which cause certain behavior, explained further below:
  • component-name/
    • global.css - (reserved): automatically added to global styles
    • element.js - (reserved): custom element, automatically imported client side, always imported server side
    • element.css - (recommended): styles for custom element's shadow dom
    • server.js - (reserved): server component, will automatically load the adjacent server.css file when used
    • server.css - (reserved): automatically added to page styles when its server component is used in that page
• global.css: components which have CSS which should be loaded on all pages should expose that through a global.css file:
  • This should be used sparingly, use it for things needed in almost all components, like colors, fonts, etc.
  • Or, when creating a custom element, use it to set the "browser default" styles on that custom element: usually as simple as just mdn-component-name { display: block; } or similar
• element.js: custom elements should be defined in components/component-name/element.js
  • The class should be exported, and named MDNComponentName
    • Acronyms should be kept all caps, to match the naming of HTMLElement class names, and added to ACRONYMS in build/plugins/generate-element-map.js to allow the correct types to be generated
  • The element should be registered with a name of mdn-component-name
  • If all this is done:
    • The element will be automatically loaded client side if it's present in the DOM at page load
      • Elements inserted client side (i.e. in a hook, or another custom element) won't be automatically loaded, and the hook should handle loading them: probably with an async import()
    • The element will be automatically loaded server side for SSR
    • The element will automatically be added to types/element-map.d.ts to provide proper types in e.g. querySelector("mdn-component-name")
• server.js: server components should be defined in components/component-name/server.js
  • The class should extend ServerComponent from components/server/index.js, and be named ComponentName
• server.css: server component styles should be placed in components/component-name/server.css
  • These will be automatically loaded server side when the adjacent ServerComponent is used
    • Therefore, these styles should be scoped to the component, usually with a wrapping class

Typing

• We use TypeScript in JSDoc annotations for typing, so we can write and directly execute JavaScript (with no transpilation step)
• We occasionally use TypeScript files directly for writing types/interface which are too complex to easily write in JSDoc
• Eventually we'll have a fully typed codebase, with no errors: while we're in active development we can ignore errors in the interest of development speed/pragmatism:
  • If we do so, we should use // @ts-expect-error so we get an error when we fix the error and don't leave unnecessary // @ts-ignore comments lying around. While we're in active development these can lack a comment, but eventually we'll require an explanatory comment on each.

Hydration errors

If our server side rendered custom elements are different to the initial state of our custom elements when rendered client side, Lit will error out during hydration, stopping the execution of our client side JS.

To avoid this, don't compute things that are server/client dependent in connectedCallback (or run functions which do this). Instead you must run these in firstUpdated (despite the warning lit will raise in development about the element scheduling an update after an update completed).

This issue is tracked upstream: https://github.com/lit/lit/issues/1434

Changelog

1.3.1 (2025-09-03)

Bug Fixes

• env: remove inline script if no runtime variables are set (#691) (771737e)

Miscellaneous

• compat-table: Port release label fix from yari (#687) (3391cc4)
• deps-dev: bump the dev group with 5 updates (#688) (bdff859)