Highlighting

Highlighting is off by default: fenced code is printed as plain text, with its language and a copy button above it. Pass a highlighter to colour it:

import { Markdown } from 'flowdown'
import { highlight } from 'flowdown/highlight'

;<Markdown highlight={highlight}>{text}</Markdown>

flowdown/highlight is built on highlight.js, an optional peer dependency: install highlight.js (version 11) to use it. A page that highlights some other way, or not at all, bundles none of it.

Languages#

The bundled highlighter knows the languages replies and READMEs mostly contain:

In a shell fence the highlighter colours what a reader looks for in an install line: the command (the first word of each command, after a $ prompt, a |, && or ;), its flags, numbers, strings, variables and comments. bun add flowdown --dev shows bun and --dev; the test of bun test is an argument and stays plain.

Only the language the fence names is highlighted. A fence without a language, or with one the highlighter does not know, is printed as plain text: guessing the language mislabels as often as it helps, and costs time on every update while a reply streams.

Colours#

The bundled highlighter emits highlight.js classes (hljs-keyword, hljs-string, …), and flowdown/styles.css colours them from tokens, so code follows your theme and its dark mode:

Set them like any other token; see Styling. A highlight.js theme stylesheet works too, but the tokens are the way that follows the page's light and dark scheme.

Your own highlighter#

A highlighter is a function:

type Highlighter = (code: string, language: string) => string | null

It is called for a fenced block whose fence names a language, with the code and that language. It returns HTML for the inside of the <code> element, or null to leave the code as plain text.

highlight.js with every language, for example:

import hljs from 'highlight.js'
import type { Highlighter } from 'flowdown'

export const highlightAll: Highlighter = (code, language) =>
  hljs.getLanguage(language) ? hljs.highlight(code, { language }).value : null

The highlighter is synchronous, because it runs while React renders. A highlighter that loads its grammars asynchronously (Shiki, for instance) must have them loaded before it is passed, and returns null for a language that is not loaded yet.

Highlighted code gets the class fd-highlighted beside its language-* class.

While a reply streams#

The fence being written is highlighted in pieces of 40 lines, and only its last piece again on each update, so its colours appear as it grows. A construct that spans two pieces (a block comment, a template string) can be coloured wrongly until the fence closes; then the block is highlighted whole. Blocks above it are not highlighted again. A page that draws code itself (components.pre or components.code) receives the growing fence whole, as before. Keep the highlighter's reference stable (a module-level function, as above): a new function on every render makes every code block highlight again.

In a terminal#

flowdown/tui takes the same function: renderToAnsi(reply, { highlight }), or createTerminalView({ highlight }) for a reply that streams. It reads the kind of each token from highlight.js's class names and draws it in one of the terminal theme's colours; the --fd-code-* tokens are a page's and do not apply. A highlighter that names its colours another way (Shiki's inline styles) leaves code in its one colour. The tokens are drawn only when together they are the code that was given, so a highlighter can change a row's colours and never its text.

A view streams code as a page does: the fence being written is highlighted in pieces of 40 lines, only the last of them again on each token, and whole once it closes. A fence that is finished is highlighted once, however long the reply goes on.

While a reply streams, the fence being written is highlighted whole on each update.

Without the renderer#

highlightCode(code, language) from flowdown/highlight is the same highlighter for code outside a reply. It returns escaped plain text, rather than null, for a language it does not know.