The posthtml-parser npm package is a tool used to parse HTML into an Abstract Syntax Tree (AST). This allows developers to manipulate HTML content programmatically, making it easier to perform tasks such as transforming HTML structures, extracting specific elements, and integrating with other tools in the PostHTML ecosystem.

What are posthtml-parser's main functionalities?

Parsing HTML to AST

This feature allows you to parse a string of HTML into an Abstract Syntax Tree (AST). The AST can then be manipulated programmatically.

const parse = require('posthtml-parser');
const html = '<div class="example">Hello World</div>';
const ast = parse(html);
console.log(ast);

Handling HTML fragments

This feature allows you to parse HTML fragments and control parsing options such as whether to convert tag names to lowercase.

const parse = require('posthtml-parser');
const fragment = '<span>Fragment</span>';
const ast = parse(fragment, { lowerCaseTags: false });
console.log(ast);

Integration with PostHTML plugins

This feature demonstrates how to integrate the parser with PostHTML plugins to transform HTML content. In this example, a plugin is used to change all <div> tags to <section> tags.

const posthtml = require('posthtml');
const parse = require('posthtml-parser');
const html = '<div class="example">Hello World</div>';
posthtml()
  .use(tree => {
    tree.match({ tag: 'div' }, node => {
      node.tag = 'section';
      return node;
    });
  })
  .process(html)
  .then(result => console.log(result.html));

PostHTML Parser

Parse HTML/XML to PostHTML AST

Installation

npm install posthtml-parser

Usage

Input HTML:

<a class="animals" href="#">
  <span class="animals__cat" style="background: url(cat.png)">Cat</span>
</a>

Parse with posthtml-parser:

import fs from 'fs'
import { parser } from 'posthtml-parser'

const html = fs.readFileSync('path/to/input.html', 'utf-8')

console.log(parser(html))

Resulting PostHTML AST:

[
  {
    tag: 'a',
    attrs: {
      class: 'animals',
      href: '#'
    },
    content: [
      '\n    ',
      {
        tag: 'span',
        attrs: {
          class: 'animals__cat',
          style: 'background: url(cat.png)'
        },
        content: ['Cat']
      },
      '\n'
    ]
  }
]

PostHTML AST Format

Any parser used with PostHTML should return a standard PostHTML Abstract Syntax Tree (AST).

Fortunately, this is a very easy format to produce and understand. The AST is an array that can contain strings and objects. Strings represent plain text content, while objects represent HTML tags.

Tag objects generally look like this:

{
  tag: 'div',
  attrs: {
    class: 'foo'
  },
  content: ['hello world!']
}

Tag objects can contain three keys:

The tag key takes the name of the tag as the value. This can include custom tags.
The optional attrs key takes an object with key/value pairs representing the attributes of the html tag. A boolean attribute has an empty string as its value.
The optional content key takes an array as its value, which is a PostHTML AST. In this manner, the AST is a tree that should be walked recursively.

Options

`directives`

Type: Array
Default: [{name: '!doctype', start: '<', end: '>'}]

Adds processing of custom directives.

The property name in custom directives can be of String or RegExp type.