List unused custom properties

list-unused-custom-properties.js

// list unused custom properties
// $ node list-unused-custom-properties.js <FILENAME>

const fs = require("fs");
const postcss = require("postcss");

const css = postcss.parse(fs.readFileSync(process.argv[2], "utf8"));
const customProperties = {};
css.walkDecls(decl => {
  if (decl.prop.startsWith("--")) {
    customProperties[decl.prop] = 0;
  }

  const variables = decl.value.match(/\bvar\(.*?\)/g);

  if (!variables) {
    return;
  }

  variables.forEach(variable => {
    const customProperty = variable.replace(/var\((.*?)\)/, "$1");
    delete customProperties[customProperty];
  });
});
console.log(customProperties);

This works against a concatenated CSS file only.