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mergeDocumentDescriptors

functionAdvancedserverbrowserlive demo

Merges several { html, body } attribute descriptors into React props for the html and body elements: classes combined, other attributes last-wins.

import { mergeDocumentDescriptors, DocumentElementDescriptor, DocumentElementAttrs } from '@salla.sa/twilight-theme-engine/utils';

In plain words

Classes and attributes on <html> and <body> (lang, dir, class="page-cart") often come from several places: the layout, the page, the theme. A descriptor is a small object saying what one of them wants: { html: { lang: 'ar' }, body: { class: 'page-cart' } }.

mergeDocumentDescriptors(a, b, …) combines any number of them into { htmlAttrs, bodyAttrs }, objects you spread onto the elements, as in <body {...bodyAttrs}>. It only calculates; it changes nothing on the page.

Signature

function mergeDocumentDescriptors(...descriptors: DocumentElementDescriptor[]): {
  bodyAttrs: Record<string, string>;   // React names: className
  htmlAttrs: Record<string, string>;
}

interface DocumentElementDescriptor {
  body?: DocumentElementAttrs;
  html?: DocumentElementAttrs;
}

interface DocumentElementAttrs {
  class?: string | string[];                       // combined across descriptors
  [attr: string]: string | string[] | undefined;   // the last non-empty string wins
}

Try it live

Two descriptors, a layout and a page, merged into props for the html and body elements.Try this: repeat a class in both: it appears once. Empty the page lang: an empty string is skipped, so the layout value stays.
Storefront canvas · en · LTR
htmlAttrs = {
  "lang": "en",
  "dir": "rtl"
}

bodyAttrs = {
  "data-page": "cart",
  "className": "font-sans antialiased page-cart"
}
Controls
What a theme writes
import { mergeDocumentDescriptors } from '@salla.sa/twilight-theme-engine/utils';

const { bodyAttrs, htmlAttrs } = mergeDocumentDescriptors(
  { html: { lang: 'ar', dir: 'rtl' }, body: { class: 'font-sans antialiased' } },
  { html: { lang: 'en' }, body: { class: 'page-cart antialiased', 'data-page': 'cart' } }
);

export function Shell({ children }: { children: React.ReactNode }) {
  return (
    <html {...htmlAttrs}>
      <body {...bodyAttrs}>{children}</body>
    </html>
  );
}

Example

app/routes/__root.tsx (excerpt)
import { HeadContent, Outlet, Scripts } from '@tanstack/react-router';
import { TwilightProvider } from '@salla.sa/twilight-theme-engine';
import { getTwilightContext } from '@salla.sa/twilight-theme-engine/tanstack';
import { mergeDocumentDescriptors } from '@salla.sa/twilight-theme-engine/utils';

function RootComponent() {
  const ctx = getTwilightContext();
  const { htmlAttrs, bodyAttrs } = mergeDocumentDescriptors(
    { html: { lang: ctx.locale, dir: ctx.dir }, body: { class: 'font-sans antialiased' } },
    { body: { class: 'theme-lookbook', 'data-theme': 'lookbook' } }
  );

  return (
    <html {...htmlAttrs} suppressHydrationWarning>
      <head>
        <HeadContent />
      </head>
      <body {...bodyAttrs} suppressHydrationWarning>
        <TwilightProvider>
          <Outlet />
        </TwilightProvider>
        <Scripts />
      </body>
    </html>
  );
}

How it behaves

  • class values from every descriptor are split on whitespace and combined without duplicates, in first-seen order. For every other attribute the last descriptor with a non-empty string wins.

  • The output uses React names: class becomes className, and nothing else is renamed.

  • It is toReactProps(mergeElementAttrs(…)) for each element. mergeAttrs from @salla.sa/twilight-theme-engine/tanstack and from /nextjs is this same function.

  • Pure: it works in a server render and never touches the DOM. For attributes that follow a component, use useDocumentClass(), which merges through DocumentClassProvider and updates the real elements.

Gotchas

  • An empty string never clears an earlier value: { html: { dir: '' } } after { html: { dir: 'rtl' } } leaves rtl. Leave the attribute out of the earlier descriptor instead.

  • Arrays count only for class. The type accepts string[] for any attribute, but on other attributes an array is dropped without a warning.

  • TwilightProvider writes lang, dir and theme classes to the same <html> and <body> after hydration, through DocumentClassProvider. Its classes are added to yours, but for lang and dir its value replaces the one you rendered.

  • docs/20-document-class.md lists DocumentElementDescriptor and DocumentElementAttrs under @salla.sa/twilight-theme-engine. The package root exports neither; import them from @salla.sa/twilight-theme-engine/utils.

Related

useDocumentClass

Adds classes and attributes to the page <html> and <body> while the calling component is mounted, merged with everyone else.

DocumentClassProvider

Collects every useDocumentClass() request in its tree and applies the merged classes and attributes to the real body and html elements.

normalizeClasses, mergeElementAttrs, toReactProps

The three steps of every html and body attribute merge: split class lists, merge attribute objects in order, rename class to className.

Source and docs