penthouse/lib/browser-sandbox/pruneNonCriticalSelectors.js

Generate critical path CSS for web pages

"use strict";

Object.defineProperty(exports, "__esModule", {
  value: true
});
exports.default = pruneNonCriticalSelectors;

// executed inside sandboxed browser environment,
// no access to scrope outside of function
function pruneNonCriticalSelectors({
  selectors,
  maxElementsToCheckPerSelector
}) {
  console.log('debug: pruneNonCriticalSelectors init');
  var h = window.innerHeight; // cache whether elements are above fold,
  // primarily because getBoundingClientRect() can be slow to query,
  // and some stylesheets have lots of generic selectors (like '.button', '.fa' etc)

  var isElementAboveFoldCache = new Map();

  function isElementAboveFold(element) {
    if (isElementAboveFoldCache.has(element)) {
      return isElementAboveFoldCache.get(element);
    } // temporarily force clear none in order to catch elements that clear previous
    // content themselves and who w/o their styles could show up unstyled in above
    // the fold content (if they rely on f.e. 'clear:both;' to clear some main content)
    // but do that only if elements have their style attribute defined
    // (ref: https://github.com/pocketjoso/penthouse/issues/342)


    const isElementStyleDefined = typeof element.style !== 'undefined';

    if (isElementStyleDefined) {
      var originalClearStyle = element.style.clear || '';
      element.style.clear = 'none';
    }

    var aboveFold = element.getBoundingClientRect().top < h; // cache so we dont have to re-query DOM for this value

    isElementAboveFoldCache.set(element, aboveFold);

    if (isElementStyleDefined) {
      // set clear style back to what it was
      element.style.clear = originalClearStyle;
    } // Should not be needed anymore with Chrome Headless:
    // do some monitoring before complete removing the code (below)
    // if (!aboveFold) {
    //   // phantomJS/QT browser has some bugs regarding fixed position;
    //   // sometimes positioning elements outside of screen incorrectly.
    //   // just keep all fixed position elements - normally very few in a stylesheet anyway
    //   var styles = window.getComputedStyle(element, null)
    //   if (styles.position === 'fixed') {
    //     console.log('debug: force keeping fixed position styles')
    //     return true
    //   }
    // }


    return aboveFold;
  }

  function isSelectorCritical(selector) {
    // we have a selector to test, first grab any matching elements
    let elements;

    try {
      elements = document.querySelectorAll(selector);
    } catch (e) {
      // not a valid selector, remove it.
      return false;
    }

    let nrElementsToCheck = elements.length;

    if (maxElementsToCheckPerSelector && nrElementsToCheck > maxElementsToCheckPerSelector) {
      console.log(`debug: isSelectorCritical, selector: ${selector} appearing ${nrElementsToCheck} time on page, ONLY checking first ${maxElementsToCheckPerSelector}...`);
      nrElementsToCheck = maxElementsToCheckPerSelector;
    } // only keep selectors that match at least one elements on the page above the fold


    for (let idx = 0; idx < nrElementsToCheck; idx++) {
      if (isElementAboveFold(elements[idx])) {
        return true;
      }
    }

    return false;
  }

  function filterSelectors(selectors) {
    console.log('debug: filterSelectors START');
    selectors = selectors.filter(isSelectorCritical);
    console.log('debug: filterSelectors DONE');
    return selectors;
  }

  return filterSelectors(selectors);
}