css-selector-parser

A high-performance CSS selector parser with advanced features for modern web development.

Features

🚀 Fast and memory-efficient parsing for all CSS selectors
🌳 AST-based object model for programmatic manipulation
🚶 AST traversal with visitor pattern for analyzing and transforming selectors
📊 Full compliance with all CSS selector specifications
🧪 Comprehensive test coverage
📚 Well-documented API with TypeScript support
🔄 Two-way conversion between CSS selectors and AST
🧩 Modular support for various CSS specifications
🎮 Interactive Playground - Try it in your browser!

Playground

Test CSS selectors in your browser with syntax highlighting, real-time AST visualization, and configurable parser options.

Supported CSS Selector Standards

css1: W3C CSS1 Specification
css2: W3C CSS2 Specification
css3/selectors-3: W3C Selectors Level 3
selectors-4: W3C Selectors Level 4
latest: refers to selectors-4
progressive: latest + accepts unknown pseudo-classes, pseudo-elements and attribute case sensitivity modifiers

Migration Guides

See Changelog for release details.

Installation

npm install css-selector-parser
# or
yarn add css-selector-parser
# or
pnpm add css-selector-parser

Usage

Parsing Selectors

import { createParser } from 'css-selector-parser';

const parse = createParser();
const selector = parse('a[href^="/"], .container:has(nav) > a[href]:nth-child(2)::before');

console.log(selector);

This produces an AST (Abstract Syntax Tree) output:

({
    type: 'Selector',
    rules: [
        {
            type: 'Rule',
            items: [
                { type: 'TagName', name: 'a' },
                {
                    type: 'Attribute',
                    name: 'href',
                    operator: '^=',
                    value: { type: 'String', value: '/' }
                }
            ]
        },
        {
            type: 'Rule',
            items: [
                { type: 'ClassName', name: 'container' },
                {
                    type: 'PseudoClass',
                    name: 'has',
                    argument: {
                        type: 'Selector',
                        rules: [
                            {
                                type: 'Rule',
                                items: [ { type: 'TagName', name: 'nav' } ]
                            }
                        ]
                    }
                }
            ],
            nestedRule: {
                type: 'Rule',
                items: [
                    { type: 'TagName', name: 'a' },
                    { type: 'Attribute', name: 'href' },
                    {
                        type: 'PseudoClass',
                        name: 'nth-child',
                        argument: { type: 'Formula', a: 0, b: 2 }
                    },
                    {
                        type: 'PseudoElement',
                        name: 'before'
                    }
                ],
                combinator: '>'
            }
        }
    ]
})

Building and Rendering Selectors

import { ast, render } from 'css-selector-parser';

const selector = ast.selector({
    rules: [
        ast.rule({
            items: [
                ast.tagName({name: 'a'}),
                ast.attribute({name: 'href', operator: '^=', value: ast.string({value: '/'})})
            ]
        }),
        ast.rule({
            items: [
                ast.className({name: 'container'}),
                ast.pseudoClass({
                    name: 'has',
                    argument: ast.selector({
                        rules: [ast.rule({items: [ast.tagName({name: 'nav'})]})]
                    })
                })
            ],
            nestedRule: ast.rule({
                combinator: '>',
                items: [
                    ast.tagName({name: 'a'}),
                    ast.attribute({name: 'href'}),
                    ast.pseudoClass({
                        name: 'nth-child',
                        argument: ast.formula({a: 0, b: 2})
                    }),
                    ast.pseudoElement({name: 'before'})
                ]
            })
        })
    ]
});

console.log(render(selector)); // a[href^="/"], .container:has(nav) > a[href]:nth-child(2)::before

Traversing the AST

The traverse function allows you to walk through the AST and visit each node, making it easy to analyze or transform selectors.

import { createParser, traverse } from 'css-selector-parser';

const parse = createParser();
const selector = parse('div.foo > span#bar:hover::before');

// Simple visitor function - called for each node
traverse(selector, (node, context) => {
    console.log(node.type, context.parents.length);
});

// Visitor with enter/exit hooks
traverse(selector, {
    enter(node, context) {
        console.log('Entering:', node.type);
        if (node.type === 'ClassName') {
            console.log('Found class:', node.name);
        }
    },
    exit(node, context) {
        console.log('Leaving:', node.type);
    }
});

// Skip visiting children of specific nodes
traverse(selector, (node) => {
    if (node.type === 'PseudoClass') {
        // Don't visit children of pseudo-classes
        return false;
    }
});

// Practical example: collect all class names
const classNames = [];
traverse(selector, (node) => {
    if (node.type === 'ClassName') {
        classNames.push(node.name);
    }
});
console.log(classNames); // ['foo']

// Access parent information
traverse(selector, (node, context) => {
    console.log({
        type: node.type,
        parent: context.parent?.type,
        depth: context.parents.length,
        key: context.key,
        index: context.index
    });
});

The traversal context provides:

node: The current AST node being visited
parent: The parent node (undefined for root)
parents: Array of all ancestor nodes from root to current
key: Property name in parent that references this node
index: Array index if this node is in an array

CSS Modules Support

CSS Modules are specifications that add new selectors or modify existing ones. This parser supports various CSS modules that can be included in your syntax definition:

import { createParser } from 'css-selector-parser';

// Create a parser with specific CSS modules enabled
const parse = createParser({
    syntax: 'selectors-4',
    modules: ['css-position-3', 'css-scoping-1']
});

Supported CSS Modules

Module	Description
`css-position-1/2/3/4`	Position-related pseudo-classes
`css-scoping-1`	Shadow DOM selectors (`:host`, `:host-context()`, `::slotted()`)
`css-pseudo-4`	Modern pseudo-elements (`::selection`, `::backdrop`, etc.)
`css-shadow-parts-1`	`::part()` for styling shadow DOM components
`css-nesting-1`	CSS Nesting selector (`&`)

The latest syntax automatically includes all modules marked as current specifications.

API Documentation

Contributing

Contributions are welcome! Please feel free to submit a Pull Request.

License

MIT

Security Contact

To report a security vulnerability, please use the Tidelift security contact. Tidelift will coordinate the fix and disclosure.

Sponsorship

If you find this project useful, please consider sponsoring the developer or supporting on Patreon.