lighthouse/core/gather/gatherers/js-usage.js

Automated auditing, performance metrics, and best practices for the web.

/**
 * @license
 * Copyright 2017 Google LLC
 * SPDX-License-Identifier: Apache-2.0
 */

import BaseGatherer from '../base-gatherer.js';

/**
 * @fileoverview Tracks unused JavaScript
 */
class JsUsage extends BaseGatherer {
  /** @type {LH.Gatherer.GathererMeta} */
  meta = {
    supportedModes: ['snapshot', 'timespan', 'navigation'],
  };

  constructor() {
    super();
    /** @type {LH.Crdp.Profiler.ScriptCoverage[]} */
    this._scriptUsages = [];
  }

  /**
   * @param {LH.Gatherer.Context} context
   */
  async startInstrumentation(context) {
    const session = context.driver.defaultSession;
    await session.sendCommand('Profiler.enable');
    await session.sendCommand('Profiler.startPreciseCoverage', {detailed: false});
  }

  /**
   * @param {LH.Gatherer.Context} context
   */
  async stopInstrumentation(context) {
    const session = context.driver.defaultSession;
    const coverageResponse = await session.sendCommand('Profiler.takePreciseCoverage');
    this._scriptUsages = coverageResponse.result;
    await session.sendCommand('Profiler.stopPreciseCoverage');
    await session.sendCommand('Profiler.disable');
  }

  /**
   * @return {Promise<LH.Artifacts['JsUsage']>}
   */
  async getArtifact() {
    /** @type {Record<string, LH.Crdp.Profiler.ScriptCoverage>} */
    const usageByScriptId = {};

    for (const scriptUsage of this._scriptUsages) {
      // If `url` is blank, that means the script was dynamically
      // created (eval, new Function, onload, ...)
      if (scriptUsage.url === '' || scriptUsage.url === '_lighthouse-eval.js') {
        // We currently don't consider coverage of dynamic scripts, and we definitely don't want
        // coverage of code Lighthouse ran to inspect the page, so we ignore this ScriptCoverage.
        // Audits would work the same without this, it is only an optimization (not tracking coverage
        // for scripts we don't care about).
        continue;
      }

      // Scripts run via puppeteer's evaluate interface will have this url.
      if (scriptUsage.url === '__puppeteer_evaluation_script__') {
        continue;
      }

      usageByScriptId[scriptUsage.scriptId] = scriptUsage;
    }

    return usageByScriptId;
  }
}

export default JsUsage;