css-search.js

const fs = require('fs');
const postcss = require('postcss');
const selectorParser = require('postcss-selector-parser');
const combineDuplicates = require('postcss-combine-duplicated-selectors');
const discardDuplicates = require('postcss-discard-duplicates');
const perfectionist = require('perfectionist');

const search = postcss.plugin('postcss-css-search', (options) => {
  if (!options.what) {
    throw new Error("Please set options.what to the class you're looking for");
  }

  let check; 

  if (options.mode === 'contains') {
    if (options.what instanceof Array) {
      check = (classes) => {
        return !!classes.filter(c => options.what.indexOf(c.value) !== -1).length;
      };
    } else {
      check = (classes) => {
        return !!classes.filter(c => c.value === options.what).length;
      };
    }
  } else if (options.mode === 'isLast') {
    if (options.what instanceof Array) {
      check = (classes) => {
        return options.what.indexOf(classes[classes.length - 1].value) !== -1;
      };
    } else {
      check = (classes) => {
        return classes[classes.length - 1].value === options.what;
      };
    }
  } else {
    throw new Error(`Unrecognized mode ${options.mode}`);
  }

  return root => {
    root.walkRules(rule => {
      const rawSelectors = rule.selector;

      let shouldDeleteRule = true;
      selectorParser((selectors) => {
        selectors.each((selector) => {
          const classes = selector.filter(n => n.type === 'class');

          if (classes.length) {
            shouldDeleteRule = !check(classes);

            if (shouldDeleteRule) {
              return false;
            }
          }

        });
      }).process(rawSelectors);

      if (shouldDeleteRule) {
        rule.remove();
      }
    });

    root.walkAtRules(atRule => {
      if (!atRule.nodes.length || atRule.name === 'font-face') {
        atRule.remove();
      }
    });
  };
});

const inputFilename = 'test.css';
const outputFilename = 'test-output.css';

fs.readFile(inputFilename, (error, css) => {
  if (error) {
    throw error;
  }

  postcss([
    search({ mode: 'contains', what: 'embed' }),
    combineDuplicates,
    discardDuplicates,
    perfectionist({ format: 'expanded', cascade: false, indentSize: 2 })
  ]).process(css, { from: inputFilename, to: outputFilename })
    .then(result => {
      fs.writeFile(outputFilename, result.css, (error) => {
        if (error) {
          throw error;
        }
      });
    });
});