unused-css.js

/**
 * @license Copyright 2019 The Lighthouse Authors. All Rights Reserved.
 * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You may obtain a copy of the License at http://www.apache.org/licenses/LICENSE-2.0
 * Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.
 */
'use strict';

const makeComputedArtifact = require('./computed-artifact.js');
const ByteEfficiencyAudit = require('../audits/byte-efficiency/byte-efficiency-audit.js');
const NetworkRecords = require('./network-records.js');

const PREVIEW_LENGTH = 100;

/** @typedef {LH.Artifacts.CSSStyleSheetInfo & {networkRecord: LH.Artifacts.NetworkRequest, usedRules: Array<LH.Crdp.CSS.RuleUsage>}} StyleSheetInfo */

class UnusedCSS {
  /**
   * @param {Array<LH.Artifacts.CSSStyleSheetInfo>} styles The output of the Styles gatherer.
   * @param {Array<LH.Artifacts.NetworkRequest>} networkRecords
   * @return {Object<string, StyleSheetInfo>} A map of styleSheetId to stylesheet information.
   */
  static indexStylesheetsById(styles, networkRecords) {
    const indexedNetworkRecords = networkRecords
        // Some phantom network records appear with a 0 resourceSize that aren't real.
        // A network record that has no size data is just as good as no network record at all for our
        // purposes, so we'll just filter them out. https://github.com/GoogleChrome/lighthouse/issues/9684#issuecomment-532381611
        .filter(record => record.resourceSize > 0)
        .reduce((indexed, record) => {
          indexed[record.url] = record;
          return indexed;
        }, /** @type {Object<string, LH.Artifacts.NetworkRequest>} */ ({}));

    return styles.reduce((indexed, stylesheet) => {
      indexed[stylesheet.header.styleSheetId] = Object.assign({
        usedRules: [],
        networkRecord: indexedNetworkRecords[stylesheet.header.sourceURL],
      }, stylesheet);
      return indexed;
    }, /** @type {Object<string, StyleSheetInfo>} */ ({}));
  }

  /**
   * Adds used rules to their corresponding stylesheet.
   * @param {Array<LH.Crdp.CSS.RuleUsage>} rules The output of the CSSUsage gatherer.
   * @param {Object<string, StyleSheetInfo>} indexedStylesheets Stylesheet information indexed by id.
   */
  static indexUsedRules(rules, indexedStylesheets) {
    rules.forEach(rule => {
      const stylesheetInfo = indexedStylesheets[rule.styleSheetId];

      if (!stylesheetInfo) {
        return;
      }

      if (rule.used) {
        stylesheetInfo.usedRules.push(rule);
      }
    });
  }

  /**
   * @param {StyleSheetInfo} stylesheetInfo
   * @return {{wastedBytes: number, totalBytes: number, wastedPercent: number}}
   */
  static computeUsage(stylesheetInfo) {
    let usedUncompressedBytes = 0;
    const totalUncompressedBytes = stylesheetInfo.content.length;

    for (const usedRule of stylesheetInfo.usedRules) {
      usedUncompressedBytes += usedRule.endOffset - usedRule.startOffset;
    }

    const totalTransferredBytes = ByteEfficiencyAudit.estimateTransferSize(
        stylesheetInfo.networkRecord, totalUncompressedBytes, 'Stylesheet');
    const percentUnused = (totalUncompressedBytes - usedUncompressedBytes) / totalUncompressedBytes;
    const wastedBytes = Math.round(percentUnused * totalTransferredBytes);

    return {
      wastedBytes,
      wastedPercent: percentUnused * 100,
      totalBytes: totalTransferredBytes,
    };
  }

  /**
   * Trims stylesheet content down to the first rule-set definition.
   * @param {string=} content
   * @return {string}
   */
  static determineContentPreview(content) {
    let preview = (content || '')
        .slice(0, PREVIEW_LENGTH * 5)
        .replace(/( {2,}|\t)+/g, '  ') // remove leading indentation if present
        .replace(/\n\s+}/g, '\n}') // completely remove indentation of closing braces
        .trim(); // trim the leading whitespace

    if (preview.length > PREVIEW_LENGTH) {
      const firstRuleStart = preview.indexOf('{');
      const firstRuleEnd = preview.indexOf('}');

      if (firstRuleStart === -1 || firstRuleEnd === -1 ||
          firstRuleStart > firstRuleEnd ||
          firstRuleStart > PREVIEW_LENGTH) {
        // We couldn't determine the first rule-set or it's not within the preview
        preview = preview.slice(0, PREVIEW_LENGTH) + '...';
      } else if (firstRuleEnd < PREVIEW_LENGTH) {
        // The entire first rule-set fits within the preview
        preview = preview.slice(0, firstRuleEnd + 1) + ' ...';
      } else {
        // The first rule-set doesn't fit within the preview, just show as many as we can
        const lastSemicolonIndex = preview.slice(0, PREVIEW_LENGTH).lastIndexOf(';');
        preview = lastSemicolonIndex < firstRuleStart ?
            preview.slice(0, PREVIEW_LENGTH) + '... } ...' :
            preview.slice(0, lastSemicolonIndex + 1) + ' ... } ...';
      }
    }

    return preview;
  }

  /**
   * @param {StyleSheetInfo} stylesheetInfo The stylesheetInfo object.
   * @return {LH.Audit.ByteEfficiencyItem}
   */
  static mapSheetToResult(stylesheetInfo) {
    let url = stylesheetInfo.header.sourceURL;
    if (!url || stylesheetInfo.header.isInline) {
      const contentPreview = UnusedCSS.determineContentPreview(stylesheetInfo.content);
      url = contentPreview;
    }

    const usage = UnusedCSS.computeUsage(stylesheetInfo);
    return {url, ...usage};
  }

  /**
   * @param {{CSSUsage: LH.Artifacts['CSSUsage'], devtoolsLog: LH.DevtoolsLog}} data
   * @param {LH.Artifacts.ComputedContext} context
   * @return {Promise<LH.Audit.ByteEfficiencyItem[]>}
  */
  static async compute_(data, context) {
    const {CSSUsage, devtoolsLog} = data;
    const networkRecords = await NetworkRecords.request(devtoolsLog, context);
    const indexedSheets = UnusedCSS.indexStylesheetsById(CSSUsage.stylesheets, networkRecords);
    UnusedCSS.indexUsedRules(CSSUsage.rules, indexedSheets);

    const items = Object.keys(indexedSheets)
      .map(sheetId => UnusedCSS.mapSheetToResult(indexedSheets[sheetId]));
    return items;
  }
}

module.exports = makeComputedArtifact(UnusedCSS, ['CSSUsage', 'devtoolsLog']);