index.js

const postcss = require('postcss');

const regViewportUnit = /\d(vw|vh|vmax|vmin)\b/;
const CONTENT_PROP = 'content';
const PREFIX = 'viewport-units-buggyfill';

module.exports = postcss.plugin('postcss-viewport-units', options => (root, result) => {
  const opts = options || {};
  const test = opts.test || (value => regViewportUnit.test(value));
  const { silence = false } = opts;

  if (opts.filterFile && !opts.filterFile(root.source.input.file || '')) return;

  root.walkRules((rule) => {
    let hasContent;
    const viewportUnitDecls = [];
    if (opts.filterRule && !opts.filterRule(rule)) return;

    rule.nodes.slice(0).forEach((decl) => {
      if (decl.prop === CONTENT_PROP && !hasContent) {
        hasContent = true;
      }

      if (test(decl.value)) {
        if (opts.onlyCalc && decl.value.indexOf('calc') !== 0) return;
        viewportUnitDecls.push(`${decl.prop}: ${decl.value}`);
      }
    });

    if (viewportUnitDecls.length === 0) return;

    if (hasContent) {
      if (!silence) {
        rule.warn(result, `'${rule.selector}' already has a 'content' property, give up to overwrite it.`);
      }
    } else {
      rule.append({
        prop: CONTENT_PROP,
        value: `'${[PREFIX].concat(viewportUnitDecls).join('; ')}'`,
      });
    }
  });
});