Convert CSS to nested CSS

pigeon.js

import { parse } from "https://deno.land/x/[email protected]/mod.ts";

var file = Deno.args[0];

var input = await Deno.readTextFile(file);

var parsed = parse(input, { value: true });

var root = {
  rules: [],
  nest: {}
};
var notNested = [];

function spaces(count = 0) {
  return "".padStart(count, " ");
}

function parseSelector(selector) {
  var buffer = "";
  var chunks = [];
  var inPseudo = false;
  for (var i = 0; i < selector.length; i++) {
    var letter = selector[i];
    switch (letter) {
      case "(":
      case ")":
        inPseudo = letter == "(";
        buffer += letter;
        break;

      case " ":
        if (inPseudo) {
          buffer += letter;
        } else {
          if (buffer) chunks.push(buffer);
          buffer = "";
        }
        break;

      case "+":
      case ">":
      case "~":
        buffer = chunks.pop() + " ";
        buffer += letter + " ";
        while (selector[++i] == " " && i < selector.length);
        i--;
        break;

      default:
        buffer += letter;
    }
  }
  if (buffer) {
    chunks.push(buffer);
  }
  return chunks;
}

for (var rule of parsed.stylesheet.rules) {
  // currently we skip media queries or keyframes and selector lists
  // at some point it would be nice to handle these
  if (!rule.selectors) {
    notNested.push(rule);
    continue;
  }
  var parts;
  if (rule.selectors.length > 1) {
    parts = [rule.selectors.join(", ")];
  } else {
    parts = parseSelector(rule.selectors[0]);
  }
  var branch = root;
  for (var part of parts) {
    // check to see if we can `&` match on anything
    var splitIndex = part.search(/\w[\[.#]/) + 1;
    var prefix = part.slice(0, splitIndex);
    if (prefix) {
      for (var possible in branch.nest) {
        if (possible == prefix) {
          part = "&" + part.slice(splitIndex);
          branch = branch.nest[prefix];
        }
      }
    }
    if (!branch.nest[part]) {
      branch.nest[part] = {
        rules: [],
        nest: {}
      }
    }
    branch = branch.nest[part];
  }
  branch.rules.push(rule);
}

function writeDeclarations(rule, pad = "") {
  for (var { name, value } of rule.declarations) {
    output += `${pad}${name}: ${value};\n`
  }
}

var output = "";
function walk(node, context = "", indentation = 0) {
  var pad = spaces(indentation);
  for (var rule of node.rules) {
    writeDeclarations(rule, pad);
  }
  for (var selector in node.nest) {
    var merged = context + " " + selector;
    var branch = node.nest[selector];
    if (node != root && selector.match(/^[a-z]/i)) {
      selector = "& " + selector;
    }
    output += "\n" + pad + selector + " {\n";
    walk(branch, context = merged, indentation + 2);
    output += pad + "}\n";
  }
}
walk(root, 0);

function writeRule(rule, indentation = 0) {
  var outer = spaces(indentation);
  var inner = spaces(indentation + 2);
  var selector = rule.selectors.join(", ");
  output += `\n${outer}${selector} {\n`;
  writeDeclarations(rule, inner);
  output += `${outer}}\n`;
}

output += "\n/*==== excluded from nesting ===*/\n";

for (var other of notNested) {
  if (other.declarations) {
    writeRule(other);
  } else if (other.rules) {
    output += `\n@${other.type} ${other.name} {`;
    for (var rule of other.rules) {
      writeRule(rule, 2);
    }
    output += "}\n";
  }
}

console.log(output);

Run with deno run pigeon.js inputfile.css.

This would be a great website.

does this work in both ways?

You mean can it un-nest CSS? No, although that would probably not be too hard to write.