turndown

What is turndown?

Turndown is a library that converts HTML into Markdown. It provides a simple API to transform HTML strings into Markdown format, making it useful for applications that need to convert rich text content into a more readable and lightweight format.

What are turndown's main functionalities?

Basic HTML to Markdown Conversion

This feature allows you to convert basic HTML elements into their Markdown equivalents. In this example, an HTML heading is converted into a Markdown heading.

const TurndownService = require('turndown');
const turndownService = new TurndownService();
const markdown = turndownService.turndown('<h1>Hello World</h1>');
console.log(markdown); // # Hello World

Custom Rules

Turndown allows you to add custom rules for converting specific HTML elements. In this example, a custom rule is added to convert <del>, <s>, and <strike> tags into Markdown strikethrough syntax.

const TurndownService = require('turndown');
const turndownService = new TurndownService();
turndownService.addRule('strikethrough', {
  filter: ['del', 's', 'strike'],
  replacement: function (content) {
    return '~~' + content + '~~';
  }
});
const markdown = turndownService.turndown('<del>Deleted text</del>');
console.log(markdown); // ~~Deleted text~~

Handling Complex HTML Structures

Turndown can handle complex HTML structures and convert them into Markdown. This example demonstrates converting a nested HTML structure with headings, paragraphs, and inline formatting.

const TurndownService = require('turndown');
const turndownService = new TurndownService();
const html = '<div><h1>Title</h1><p>This is a <strong>paragraph</strong> with <em>emphasis</em>.</p></div>';
const markdown = turndownService.turndown(html);
console.log(markdown); // # Title\n\nThis is a **paragraph** with *emphasis*.

Other packages similar to turndown

showdown

Showdown is a bidirectional converter that can convert Markdown to HTML and vice versa. It offers a wide range of options and extensions for customization. Compared to Turndown, Showdown is more versatile as it supports both directions of conversion.

markdown-it

Markdown-it is a fast and flexible Markdown parser that can be extended with plugins. While it primarily focuses on converting Markdown to HTML, it can be extended to support HTML to Markdown conversion with additional plugins. It is known for its speed and extensibility.

remark

Remark is a Markdown processor powered by plugins. It can parse, transform, and compile Markdown. With the right plugins, it can also handle HTML to Markdown conversion. Remark is highly modular and customizable, making it suitable for complex transformations.

README

Turndown

Convert HTML into Markdown with JavaScript.

Project Updates

• to-markdown has been renamed to Turndown. See the migration guide for details.
• Turndown repository has changed its URL to https://github.com/mixmark-io/turndown.

Installation

npm:

npm install turndown

Browser:

<script src="https://unpkg.com/turndown/dist/turndown.js"></script>

For usage with RequireJS, UMD versions are located in lib/turndown.umd.js (for Node.js) and lib/turndown.browser.umd.js for browser usage. These files are generated when the npm package is published. To generate them manually, clone this repo and run npm run build.

Usage

// For Node.js
var TurndownService = require('turndown')

var turndownService = new TurndownService()
var markdown = turndownService.turndown('<h1>Hello world!</h1>')

Turndown also accepts DOM nodes as input (either element nodes, document nodes, or document fragment nodes):

var markdown = turndownService.turndown(document.getElementById('content'))

Options

Options can be passed in to the constructor on instantiation. For example:

var turndownService = new TurndownService({ option: 'value' })

Option	Valid values	Default
headingStyle	setext or atx	setext
hr	Any Thematic break	* * *
bulletListMarker	-, +, or *	*
codeBlockStyle	indented or fenced	indented
fence	``` or ~~~	```
emDelimiter	_ or *	_
strongDelimiter	** or __	**
linkStyle	inlined or referenced	inlined
linkReferenceStyle	full, collapsed, or shortcut	full
preformattedCode	false or true	false

Advanced Options

Option	Valid values	Default
blankReplacement	rule replacement function	See Special Rules below
keepReplacement	rule replacement function	See Special Rules below
defaultReplacement	rule replacement function	See Special Rules below

Methods

addRule(key, rule)

The key parameter is a unique name for the rule for easy reference. Example:

turndownService.addRule('strikethrough', {
  filter: ['del', 's', 'strike'],
  replacement: function (content) {
    return '~' + content + '~'
  }
})

addRule returns the TurndownService instance for chaining.

See Extending with Rules below.

keep(filter)

Determines which elements are to be kept and rendered as HTML. By default, Turndown does not keep any elements. The filter parameter works like a rule filter (see section on filters belows). Example:

turndownService.keep(['del', 'ins'])
turndownService.turndown('<p>Hello <del>world</del><ins>World</ins></p>') // 'Hello <del>world</del><ins>World</ins>'

This will render <del> and <ins> elements as HTML when converted.

keep can be called multiple times, with the newly added keep filters taking precedence over older ones. Keep filters will be overridden by the standard CommonMark rules and any added rules. To keep elements that are normally handled by those rules, add a rule with the desired behaviour.

keep returns the TurndownService instance for chaining.

remove(filter)

Determines which elements are to be removed altogether i.e. converted to an empty string. By default, Turndown does not remove any elements. The filter parameter works like a rule filter (see section on filters belows). Example:

turndownService.remove('del')
turndownService.turndown('<p>Hello <del>world</del><ins>World</ins></p>') // 'Hello World'

This will remove <del> elements (and contents).

remove can be called multiple times, with the newly added remove filters taking precedence over older ones. Remove filters will be overridden by the keep filters, standard CommonMark rules, and any added rules. To remove elements that are normally handled by those rules, add a rule with the desired behaviour.

remove returns the TurndownService instance for chaining.

use(plugin|array)

Use a plugin, or an array of plugins. Example:

// Import plugins from turndown-plugin-gfm
var turndownPluginGfm = require('turndown-plugin-gfm')
var gfm = turndownPluginGfm.gfm
var tables = turndownPluginGfm.tables
var strikethrough = turndownPluginGfm.strikethrough

// Use the gfm plugin
turndownService.use(gfm)

// Use the table and strikethrough plugins only
turndownService.use([tables, strikethrough])

use returns the TurndownService instance for chaining.

See Plugins below.

Extending with Rules

Turndown can be extended by adding rules. A rule is a plain JavaScript object with filter and replacement properties. For example, the rule for converting <p> elements is as follows:

{
  filter: 'p',
  replacement: function (content) {
    return '\n\n' + content + '\n\n'
  }
}

The filter selects <p> elements, and the replacement function returns the <p> contents separated by two new lines.

filter String|Array|Function

The filter property determines whether or not an element should be replaced with the rule's replacement. DOM nodes can be selected simply using a tag name or an array of tag names:

• filter: 'p' will select <p> elements
• filter: ['em', 'i'] will select <em> or <i> elements

The tag names in the filter property are expected in lowercase, regardless of their form in the document.

Alternatively, the filter can be a function that returns a boolean depending on whether a given node should be replaced. The function is passed a DOM node as well as the TurndownService options. For example, the following rule selects <a> elements (with an href) when the linkStyle option is inlined:

filter: function (node, options) {
  return (
    options.linkStyle === 'inlined' &&
    node.nodeName === 'A' &&
    node.getAttribute('href')
  )
}

replacement Function

The replacement function determines how an element should be converted. It should return the Markdown string for a given node. The function is passed the node's content, the node itself, and the TurndownService options.

The following rule shows how <em> elements are converted:

rules.emphasis = {
  filter: ['em', 'i'],

  replacement: function (content, node, options) {
    return options.emDelimiter + content + options.emDelimiter
  }
}

Special Rules

Blank rule determines how to handle blank elements. It overrides every rule (even those added via addRule). A node is blank if it only contains whitespace, and it's not an <a>, <td>,<th> or a void element. Its behaviour can be customised using the blankReplacement option.

Keep rules determine how to handle the elements that should not be converted, i.e. rendered as HTML in the Markdown output. By default, no elements are kept. Block-level elements will be separated from surrounding content by blank lines. Its behaviour can be customised using the keepReplacement option.

Remove rules determine which elements to remove altogether. By default, no elements are removed.

Default rule handles nodes which are not recognised by any other rule. By default, it outputs the node's text content (separated by blank lines if it is a block-level element). Its behaviour can be customised with the defaultReplacement option.

Rule Precedence

Turndown iterates over the set of rules, and picks the first one that matches the filter. The following list describes the order of precedence:

1. Blank rule
2. Added rules (optional)
3. Commonmark rules
4. Keep rules
5. Remove rules
6. Default rule

Plugins

The plugin API provides a convenient way for developers to apply multiple extensions. A plugin is just a function that is called with the TurndownService instance.

Escaping Markdown Characters

Turndown uses backslashes (\) to escape Markdown characters in the HTML input. This ensures that these characters are not interpreted as Markdown when the output is compiled back to HTML. For example, the contents of <h1>1. Hello world</h1> needs to be escaped to 1\. Hello world, otherwise it will be interpreted as a list item rather than a heading.

To avoid the complexity and the performance implications of parsing the content of every HTML element as Markdown, Turndown uses a group of regular expressions to escape potential Markdown syntax. As a result, the escaping rules can be quite aggressive.

Overriding TurndownService.prototype.escape

If you are confident in doing so, you may want to customise the escaping behaviour to suit your needs. This can be done by overriding TurndownService.prototype.escape. escape takes the text of each HTML element and should return a version with the Markdown characters escaped.

Note: text in code elements is never passed toescape.

License

turndown is copyright © 2017+ Dom Christie and released under the MIT license.