Shady CSS Parser

The motivation for Shady CSS Parser is to provide a fast, small and flexible CSS parser suitable for facilitating runtime parsing and transformation of CSS. The Polymer library and the Polymer Designer tool are both example cases where fast and flexible CSS parsing and transformation is a critical feature.

Goals

• Feasibility of being used in conjunction with Polymer or Polymer Designer.
• Parse CSS loosely and flexibly. This parser is not spec-compliant, however it will parse all spec-compliant CSS.
• Parse CSS quickly and efficiently. This parser is a suitable tool to aide in the design and implementation of runtime transformations.
• Graceful error recovery. Malformed CSS will be parsed by this parser as closely as possible to the way a browser would parse it.

Installing

With node and npm installed, run the following command:

npm install shady-css-parser

Building

Run the following commands from the project root:

npm run build

This will create a dist directory containing distributable artifacts.

Usage

Basic parsing

import * as shadyCss from 'shady-css-parser';
const css = 'body { color: red; }';
const parser = new shadyCss.Parser();
const ast = parser.parse(css);

Custom parsing

/* Step 1: Inherit from NodeFactory */
class CustomNodeFactory extends shadyCss.NodeFactory {

  /*
   * Step 2: Implement a custom node factory method. Here we override the
   *   default factory for Expression nodes
   */
  expression(text) {
    if (/^darken\(/.test(text)) {
      return {
        type: 'darkenExpression',
        color: text.replace(/^darken\(/, '').replace(/\)$/, ''),
      };
    } else {
      return super.expression.apply(this, arguments);
    }
  }
}

const css = 'body { color: darken(red); }';
/* Step 3: Instantiate a Parser with an instance of the specialized
 * CustomNodeFactory */
const parser = new shadyCss.Parser(new CustomNodeFactory());
const ast = parser.parse(css);

Basic stringification

const stringifier = new shadyCss.Stringifier();
stringifier.stringify(ast);

Note: the built-in Parser and Stringifier discard most insignficiant whitespace from parsed CSS.

Custom stringification

/* Step 1: Inherit from Stringifier. */
class CustomStringifier extends shadyCss.Stringifier {

  /**
  * Step 2: Implement a stringification method named after the type of the node
  * you are interested in stringifying. In this case, we are implementing
  * stringification for the Darken Expression nodes we implemented parsing for
  * above.
  */
  darkenExpression(darkenExpression) {
    // For the sake of brevity, please assume that the darken function returns
    // a darker version of the color parameter:
    return darken(darkenExpression.color);
  }
}

/* Step 3: Use the custom stringifer: */
const stringifier = new CustomStringifier();
const css = stringifier.stringify(ast);

Example ASTs

Custom property declaration

.container {
  --nog: blue;
}

{
  "type": 1, /* stylesheet */
  "rules": [
    {
      "type": 4, /* ruleset */
      "selector": ".container",
      "rulelist": {
        "type": 7, /* rulelist */
        "rules": [
          {
            "type": 6, /* declaration */
            "name": "--nog",
            "value": {
              "type": 5, /* expression */
              "text": "blue"
            }
          }
        ]
      }
    }
  ]
}

Mixin declaration

ruleset {
  --mixin-name: {
    /* rules */
  };
}

{
  "type": 1, /* stylesheet */
  "rules": [
    {
      "type": 4, /* ruleset */
      "selector": "ruleset",
      "rulelist": {
        "type": 7, /* rulelist */
        "rules": [
          {
            "type": 6, /* declaration */
            "name": "--mixin-name",
            "value": {
              "type": 7, /* rulelist */
              "rules": [
                {
                  "type": 2, /* comment */
                  "value": "\/* rules *\/"
                }
              ]
            }
          }
        ]
      }
    }
  ]
}

Mixin application

.title {
  @apply(--my-toolbar-title-theme);
}

{
  "type": 1, /* stylesheet */
  "rules": [
    {
      "type": 4, /* ruleset */
      "selector": ".title",
      "rulelist": {
        "type": 7, /* rulelist */
        "rules": [
          {
            "type": 3, /* at rule */
            "name": "apply",
            "parameters": "(--my-toolbar-title-theme)",
            "rulelist": null
          }
        ]
      }
    }
  ]
}

Pathological comments

/* unclosed
@fiz {
  --huk: {
    /* buz */
    baz: lur;
  };
}

{
  "type": 1, /* stylesheet */
  "rules": [
    {
      "type": 2, /* comment */
      "value": "\/* unclosed\n@fiz {\n  --huk: {\n    \/* buz *\/"
    },
    {
      "type": 6, /* declaration */
      "name": "baz",
      "value": {
        "type": 5, /* expression */
        "text": "lur"
      }
    },
    {
      "type": 8, /* discarded */
      "text": "};\n"
    },
    {
      "type": 8, /* discarded */
      "text": "}"
    }
  ]
}

Example stringification

Basic ruleset

/* before */
body {
  margin: 0;
  padding: 0px
}

/* after */
body{margin:0;padding:0px;}

At rules

/* before */
@import url('foo.css');

@font-face {
  font-family: foo;
}

@charset 'foo';

/* after */
@import url('foo.css');@font-face{font-family:foo;}@charset 'foo';

Custom properties

/* before */
:root {
  --qux: vim;
  --foo: {
    bar: baz;
  };
}

#target {
  gak: var(--qux);
  @apply(--foo);
}

/* after */
:root{--qux:vim;--foo:{bar:baz;};}#target{gak:var(--qux);@apply (--foo);}