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readme.md

rehype-parse

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rehype plugin to add support for parsing HTML input.

Contents

What is this?

This package is a unified (rehype) plugin that defines how to take HTML as input and turn it into a syntax tree. When it’s used, HTML can be parsed and other rehype plugins can be used after it.

See the monorepo readme for info on what the rehype ecosystem is.

When should I use this?

This plugin adds support to unified for parsing HTML. You can alternatively use rehype instead, which combines unified, this plugin, and rehype-stringify.

When you’re in a browser, trust your content, don’t need positional info, and value a smaller bundle size, you can use rehype-dom-parse instead.

This plugin is built on parse5 and hast-util-from-parse5, which deal with HTML-compliant tokenizing, parsing, and creating nodes. rehype focusses on making it easier to transform content by abstracting such internals away.

Install

This package is ESM only. In Node.js (version 12.20+, 14.14+, or 16.0+), install with npm:

npm install rehype-parse

In Deno with esm.sh:

import rehypeParse from 'https://esm.sh/rehype-parse@8'

In browsers with esm.sh:

<script type="module">
  import rehypeParse from 'https://esm.sh/rehype-parse@8?bundle'
</script>

Use

Say we have the following module example.js:

import {unified} from 'unified'
import rehypeParse from 'rehype-parse'
import rehypeRemark from 'rehype-remark'
import remarkStringify from 'remark-stringify'

main()

async function main() {
  const file = await unified()
    .use(rehypeParse)
    .use(rehypeRemark)
    .use(remarkStringify)
    .process('<h1>Hello, world!</h1>')

  console.log(String(file))
}

…running that with node example.js yields:

# Hello, world!

API

This package exports no identifiers. The default export is rehypeParse.

unified().use(rehypeParse[, options])

Add support for parsing HTML input.

options

Configuration (optional).

options.fragment

Specify whether to parse as a fragment (boolean, default: false). The default is to expect a whole document. In document mode, unopened html, head, and body elements are opened.

options.space

Which space the document is in ('svg' or 'html', default: 'html').

When an <svg> element is found in the HTML space, rehype-parse already automatically switches to and from the SVG space when entering and exiting it.

👉 Note: rehype is not an XML parser. It supports SVG as embedded in HTML. It does not support the features available in XML. Passing SVG files might break but fragments of modern SVG should be fine.

👉 Note: make sure to set fragment: true if space: 'svg'.

options.emitParseErrors

Emit HTML parse errors as warning messages (boolean, default: false).

Specific rules can be turned off by setting their IDs in options to false (or 0). The default, when emitParseErrors: true, is true (or 1), and means that rules emit as warnings. Rules can also be configured with 2, to turn them into fatal errors.

The list of parse errors:

options.verbose

Add extra positional info (boolean, default: false).

Examples

Example: fragment versus document

The following example shows the difference between parsing as a document and parsing as a fragment:

import {unified} from 'unified'
import rehypeParse from 'rehype-parse'
import rehypeStringify from 'rehype-stringify'

main()

async function main() {
  const doc = '<title>Hi!</title><h1>Hello!</h1>'

  console.log(
    String(
      await unified()
        .use(rehypeParse, {fragment: true})
        .use(rehypeStringify)
        .process(doc)
    )
  )

  console.log(
    String(
      await unified()
        .use(rehypeParse, {fragment: false})
        .use(rehypeStringify)
        .process(doc)
    )
  )
}

…yields:

<title>Hi!</title><h1>Hello!</h1>
<html><head><title>Hi!</title></head><body><h1>Hello!</h1></body></html>

👉 Note: observe that when a whole document is expected (second example), missing elements are opened and closed.

Example: whitespace around and inside <html>

The following example shows how whitespace is handled when around and directly inside the <html> element:

import {unified} from 'unified'
import rehypeParse from 'rehype-parse'
import rehypeStringify from 'rehype-stringify'

main(`<!doctype html>
<html lang=en>
  <head>
    <title>Hi!</title>
  </head>
  <body>
    <h1>Hello!</h1>
  </body>
</html>`)

async function main(doc) {
  console.log(
    String(await unified().use(rehypeParse).use(rehypeStringify).process(doc))
  )
}

…yields (where ␠ represents a space character):

<!doctype html><html lang="en"><head>
    <title>Hi!</title>
  </head>
  <body>
    <h1>Hello!</h1>
␠␠
</body></html>

👉 Note: observe that the line ending before <html> is ignored, the line ending and two spaces before <head> is moved inside it, and the line ending after </body> is moved before it.

This behavior is described by the HTML standard (see the section 13.2.6.4.1 “The ‘initial’ insertion mode” and adjacent states) which rehype follows.

The changes to this meaningless whitespace should not matter, except when formatting markup, in which case rehype-format can be used to improve the source code.

Example: parse errors

The following example shows how HTML parse errors can be enabled and configured:

import {reporter} from 'vfile-reporter'
import {unified} from 'unified'
import rehypeParse from 'rehype-parse'
import rehypeStringify from 'rehype-stringify'

main()

async function main() {
  const file = await unified()
    .use(rehypeParse, {
      emitParseErrors: true, // Emit all.
      missingWhitespaceBeforeDoctypeName: 2, // Mark one as a fatal error.
      nonVoidHtmlElementStartTagWithTrailingSolidus: false // Ignore one.
    })
    .use(rehypeStringify)
    .process(`<!doctypehtml>
<title class="a" class="b">Hello…</title>
<h1/>World!</h1>`)

  console.log(reporter(file))
}

…yields:

  1:10-1:10  error    Missing whitespace before doctype name  missing-whitespace-before-doctype-name  parse-error
  2:23-2:23  warning  Unexpected duplicate attribute          duplicate-attribute                     parse-error

2 messages (✖ 1 error, ⚠ 1 warning)

🧑‍🏫 Info: messages in unified are warnings instead of errors. Other linters (such as ESLint) almost always use errors. Why? Those tools only check code style. They don’t generate, transform, and format code, which is what rehype and unified focus on, too. Errors in unified mean the same as an exception in your JavaScript code: a crash. That’s why we use warnings instead, because we continue checking more HTML and continue running more plugins.

Syntax

HTML is parsed according to WHATWG HTML (the living standard), which is also followed by browsers such as Chrome and Firefox.

Syntax tree

The syntax tree format used in rehype is hast.

Types

This package is fully typed with TypeScript. The extra types Options, ErrorCode, and ErrorSeverity are exported.

Compatibility

Projects maintained by the unified collective are compatible with all maintained versions of Node.js. As of now, that is Node.js 12.20+, 14.14+, and 16.0+. Our projects sometimes work with older versions, but this is not guaranteed.

Security

As rehype works on HTML, and improper use of HTML can open you up to a cross-site scripting (XSS) attack, use of rehype can also be unsafe. Use rehype-sanitize to make the tree safe.

Use of rehype plugins could also open you up to other attacks. Carefully assess each plugin and the risks involved in using them.

For info on how to submit a report, see our security policy.

Contribute

See contributing.md in rehypejs/.github for ways to get started. See support.md for ways to get help.

This project has a code of conduct. By interacting with this repository, organization, or community you agree to abide by its terms.

Sponsor

Support this effort and give back by sponsoring on OpenCollective!

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License

MIT © Titus Wormer