hast-util-from-html

v2.0.1

Published

9 months ago

hast utility to parse from HTML

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unist hast hast-util util utility html parse tokenize

hast-util-from-html

hast utility that turns HTML into a syntax tree.

What is this?

This package is a utility that takes HTML input and turns it into a hast syntax tree.

When should I use this?

If you want to handle syntax trees manually, use this.

Use parse5 instead when you just want to parse HTML and don’t care about hast. You can also use hast-util-from-parse5 and parse5 yourself, or use the rehype plugin rehype-parse, which wraps this utility to also parse HTML at a higher-level (easier) abstraction. xast-util-from-xml can be used if you are dealing with XML instead of HTML.

If you might run in a browser and prefer a ligher alternative, while not caring about positional info, parse errors, and consistency across browsers, use hast-util-from-html-isomorphic, which wraps this in Node and uses browser APIs otherwise.

Finally you can use the utility hast-util-to-html for the inverse of this utility. It turns hast into HTML.

Install

This package is ESM only. In Node.js (version 16+), install with npm:

npm install hast-util-from-html

In Deno with esm.sh:

import {fromHtml} from 'https://esm.sh/hast-util-from-html@2'

In browsers with esm.sh:

<script type="module">
  import {fromHtml} from 'https://esm.sh/hast-util-from-html@2?bundle'
</script>

Use

import {fromHtml} from 'hast-util-from-html'

const tree = fromHtml('<h1>Hello, world!</h1>', {fragment: true})

console.log(tree)

Yields:

{
  type: 'root',
  children: [
    {
      type: 'element',
      tagName: 'h1',
      properties: {},
      children: [Array],
      position: [Object]
    }
  ],
  data: { quirksMode: false },
  position: {
    start: { line: 1, column: 1, offset: 0 },
    end: { line: 1, column: 23, offset: 22 }
  }
}

API

This package exports the identifier fromHtml. There is no default export.

`fromHtml(value[, options])`

Turn serialized HTML into a hast tree.

Parameters

value (Compatible) — serialized HTML to parse
options (Options, optional) — configuration

Returns

Tree (Root).

`ErrorCode`

Known names of parse errors (TypeScript type).

Types

type ErrorCode =
  | 'abandonedHeadElementChild'
  | 'abruptClosingOfEmptyComment'
  | 'abruptDoctypePublicIdentifier'
  // … see readme on `options[key in ErrorCode]` above.

`ErrorSeverity`

Error severity (TypeScript type).

Types

export type ErrorSeverity =
  // Turn the parse error off:
  | 0
  | false
  // Turn the parse error into a warning:
  | 1
  | true
  // Turn the parse error into an actual error: processing stops.
  | 2

`OnError`

Function called when encountering HTML parse errors.

Parameters

error (VFileMessage) — message

Returns

Nothing (void).

`Options`

Configuration (TypeScript type).

Fields

`options.space`

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

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

👉 Note: this 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. Use xast-util-from-xml to parse XML.

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

`options.verbose`

Add extra positional info about attributes, start tags, and end tags (boolean, default: false).

`options.fragment`

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.onerror`

Function called when encountering HTML parse errors (OnError, optional).

`options[key in ErrorCode]`

Specific parse errors can be configured by setting their identifiers (see ErrorCode) as keys directly in options to an ErrorSeverity as value.

The list of parse errors:

abandonedHeadElementChild — unexpected metadata element after head (example)
abruptClosingOfEmptyComment — unexpected abruptly closed empty comment (example)
abruptDoctypePublicIdentifier — unexpected abruptly closed public identifier (example)
abruptDoctypeSystemIdentifier — unexpected abruptly closed system identifier (example)
absenceOfDigitsInNumericCharacterReference — unexpected non-digit at start of numeric character reference (example)
cdataInHtmlContent — unexpected CDATA section in HTML (example)
characterReferenceOutsideUnicodeRange — unexpected too big numeric character reference (example)
closingOfElementWithOpenChildElements — unexpected closing tag with open child elements (example)
controlCharacterInInputStream — unexpected control character (example)
controlCharacterReference — unexpected control character reference (example)
disallowedContentInNoscriptInHead — disallowed content inside <noscript> in <head> (example)
duplicateAttribute — unexpected duplicate attribute (example)
endTagWithAttributes — unexpected attribute on closing tag (example)
endTagWithTrailingSolidus — unexpected slash at end of closing tag (example)
endTagWithoutMatchingOpenElement — unexpected unopened end tag (example)
eofBeforeTagName — unexpected end of file (example)
eofInCdata — unexpected end of file in CDATA (example)
eofInComment — unexpected end of file in comment (example)
eofInDoctype — unexpected end of file in doctype (example)
eofInElementThatCanContainOnlyText — unexpected end of file in element that can only contain text (example)
eofInScriptHtmlCommentLikeText — unexpected end of file in comment inside script (example)
eofInTag — unexpected end of file in tag (example)
incorrectlyClosedComment — incorrectly closed comment (example)
incorrectlyOpenedComment — incorrectly opened comment (example)
invalidCharacterSequenceAfterDoctypeName — invalid sequence after doctype name (example)
invalidFirstCharacterOfTagName — invalid first character in tag name (example)
misplacedDoctype — misplaced doctype (example)
misplacedStartTagForHeadElement — misplaced <head> start tag (example)
missingAttributeValue — missing attribute value (example)
missingDoctype — missing doctype before other content (example)
missingDoctypeName — missing doctype name (example)
missingDoctypePublicIdentifier — missing public identifier in doctype (example)
missingDoctypeSystemIdentifier — missing system identifier in doctype (example)
missingEndTagName — missing name in end tag (example)
missingQuoteBeforeDoctypePublicIdentifier — missing quote before public identifier in doctype (example)
missingQuoteBeforeDoctypeSystemIdentifier — missing quote before system identifier in doctype (example)
missingSemicolonAfterCharacterReference — missing semicolon after character reference (example)
missingWhitespaceAfterDoctypePublicKeyword — missing whitespace after public identifier in doctype (example)
missingWhitespaceAfterDoctypeSystemKeyword — missing whitespace after system identifier in doctype (example)
missingWhitespaceBeforeDoctypeName — missing whitespace before doctype name (example)
missingWhitespaceBetweenAttributes — missing whitespace between attributes (example)
missingWhitespaceBetweenDoctypePublicAndSystemIdentifiers — missing whitespace between public and system identifiers in doctype (example)
nestedComment — unexpected nested comment (example)
nestedNoscriptInHead — unexpected nested <noscript> in <head> (example)
nonConformingDoctype — unexpected non-conforming doctype declaration (example)
nonVoidHtmlElementStartTagWithTrailingSolidus — unexpected trailing slash on start tag of non-void element (example)
noncharacterCharacterReference — unexpected noncharacter code point referenced by character reference (example)
noncharacterInInputStream — unexpected noncharacter character (example)
nullCharacterReference — unexpected NULL character referenced by character reference (example)
openElementsLeftAfterEof — unexpected end of file (example)
surrogateCharacterReference — unexpected surrogate character referenced by character reference (example)
surrogateInInputStream — unexpected surrogate character
unexpectedCharacterAfterDoctypeSystemIdentifier — invalid character after system identifier in doctype (example)
unexpectedCharacterInAttributeName — unexpected character in attribute name (example)
unexpectedCharacterInUnquotedAttributeValue — unexpected character in unquoted attribute value (example)
unexpectedEqualsSignBeforeAttributeName — unexpected equals sign before attribute name (example)
unexpectedNullCharacter — unexpected NULL character (example)
unexpectedQuestionMarkInsteadOfTagName — unexpected question mark instead of tag name (example)
unexpectedSolidusInTag — unexpected slash in tag (example)
unknownNamedCharacterReference — unexpected unknown named character reference (example)

Examples

Example: fragment versus document

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

import {fromHtml} from 'hast-util-from-html'

const doc = '<title>Hi!</title><h1>Hello!</h1>'

console.log(fromHtml(doc))

console.log(fromHtml(doc, {fragment: true}))

…yields (positional info and data omitted for brevity):

{
  type: 'root',
  children: [
    {type: 'element', tagName: 'html', properties: {}, children: [Array]}
  ]
}

{
  type: 'root',
  children: [
    {type: 'element', tagName: 'title', properties: {}, children: [Array]},
    {type: 'element', tagName: 'h1', properties: {}, children: [Array]}
  ]
}

👉 Note: observe that when a whole document is expected (first 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 {fromHtml} from 'hast-util-from-html'
import {inspect} from 'unist-util-inspect'

const doc = `<!doctype html>
<html lang=en>
  <head>
    <title>Hi!</title>
  </head>
  <body>
    <h1>Hello!</h1>
  </body>
</html>`

console.log(inspect(fromHtml(doc)))

…yields:

root[2] (1:1-9:8, 0-119)
│ data: {"quirksMode":false}
├─0 doctype (1:1-1:16, 0-15)
└─1 element<html>[3] (2:1-9:8, 16-119)
    │ properties: {"lang":"en"}
    ├─0 element<head>[3] (3:3-5:10, 33-72)
    │   │ properties: {}
    │   ├─0 text "\n    " (3:9-4:5, 39-44)
    │   ├─1 element<title>[1] (4:5-4:23, 44-62)
    │   │   │ properties: {}
    │   │   └─0 text "Hi!" (4:12-4:15, 51-54)
    │   └─2 text "\n  " (4:23-5:3, 62-65)
    ├─1 text "\n  " (5:10-6:3, 72-75)
    └─2 element<body>[3] (6:3-8:10, 75-111)
        │ properties: {}
        ├─0 text "\n    " (6:9-7:5, 81-86)
        ├─1 element<h1>[1] (7:5-7:20, 86-101)
        │   │ properties: {}
        │   └─0 text "Hello!" (7:9-7:15, 90-96)
        └─2 text "\n  \n" (7:20-9:1, 101-112)

👉 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 we follow.

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 {fromHtml} from 'hast-util-from-html'

const doc = `<!doctypehtml>
<title class="a" class="b">Hello…</title>
<h1/>World!</h1>`

fromHtml(doc, {
  onerror: console.log,
  missingWhitespaceBeforeDoctypeName: 2, // Mark one as a fatal error.
  nonVoidHtmlElementStartTagWithTrailingSolidus: false // Ignore one.
})

…yields:

[1:10-1:10: Missing whitespace before doctype name] {
  ancestors: undefined,
  cause: undefined,
  column: 10,
  fatal: true,
  line: 1,
  place: {
    start: { line: 1, column: 10, offset: 9 },
    end: { line: 1, column: 10, offset: 9 }
  },
  reason: 'Missing whitespace before doctype name',
  ruleId: 'missing-whitespace-before-doctype-name',
  source: 'hast-util-from-html',
  note: 'Unexpected `h`. Expected ASCII whitespace instead',
  url: 'https://html.spec.whatwg.org/multipage/parsing.html#parse-error-missing-whitespace-before-doctype-name'
}
[2:23-2:23: Unexpected duplicate attribute] {
  ancestors: undefined,
  cause: undefined,
  column: 23,
  fatal: false,
  line: 2,
  place: {
    start: { line: 2, column: 23, offset: 37 },
    end: { line: 2, column: 23, offset: 37 }
  },
  reason: 'Unexpected duplicate attribute',
  ruleId: 'duplicate-attribute',
  source: 'hast-util-from-html',
  note: 'Unexpectedly double attribute. Expected attributes to occur only once',
  url: 'https://html.spec.whatwg.org/multipage/parsing.html#parse-error-duplicate-attribute'
}

🧑‍🏫 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 we 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 can continue doing work.

Syntax

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

Types

This package is fully typed with TypeScript. It exports the additional types ErrorCode, ErrorSeverity, OnError, and Options.

Compatibility

Projects maintained by the unified collective are compatible with maintained versions of Node.js.

When we cut a new major release, we drop support for unmaintained versions of Node. This means we try to keep the current release line, hast-util-from-html@^2, compatible with Node.js 16.

Security

Parsing HTML is safe but using user-provided content can open you up to a cross-site scripting (XSS) attack. Use hast-util-santize to make the hast tree safe.

hast-util-to-html — serialize hast
hast-util-sanitize — sanitize hast
xast-util-from-xml — parse XML

Contribute

See contributing.md in syntax-tree/.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.

Published

Vulnerabilities

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Maintainers

Keywords

Readme

hast-util-from-html

Contents

What is this?

When should I use this?

Install

Use

API

fromHtml(value[, options])

Parameters

Returns

ErrorCode

Types

ErrorSeverity

Types

OnError

Parameters

Returns

Options

Fields

options.space

options.verbose

options.fragment

options.onerror

options[key in ErrorCode]

Examples

Example: fragment versus document

Example: whitespace around and inside <html>

Example: parse errors

Syntax

Types

Compatibility

Security

Related

Contribute

License

`fromHtml(value[, options])`

`ErrorCode`

`ErrorSeverity`

`OnError`

`Options`

`options.space`

`options.verbose`

`options.fragment`

`options.onerror`

`options[key in ErrorCode]`

Example: whitespace around and inside `<html>`