new_audit: preload-lcp-image

lighthouse-core/audits/preload-lcp-image.js

@@ -0,0 +1,176 @@
+/**
+ * @license Copyright 2020 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 URL = require('../lib/url-shim.js');
+const Audit = require('./audit.js');
+const i18n = require('../lib/i18n/i18n.js');
+const MainResource = require('../computed/main-resource.js');
+const LanternLCP = require('../computed/metrics/lantern-largest-contentful-paint.js');
+const LoadSimulator = require('../computed/load-simulator.js');
+const UnusedBytes = require('./byte-efficiency/byte-efficiency-audit.js');
+
+const UIStrings = {
+  /** Title of a lighthouse audit that tells a user to preload an image in order to improve their LCP time. */
+  title: 'Preload Largest Contentful Paint image',
+  /** Description of a lighthouse audit that tells a user to preload an image in order to improve their LCP time.  */
+  description: 'Preload the image used by ' +
+    'the LCP element in order to improve your LCP time. [Learn more](https://web.dev/optimize-lcp/#preload-important-resources).',
+};
+
+const str_ = i18n.createMessageInstanceIdFn(__filename, UIStrings);
+
+class PreloadLCPImageAudit extends Audit {
+  /**
+   * @return {LH.Audit.Meta}
+   */
+  static get meta() {
+    return {
+      id: 'preload-lcp-image',
+      title: str_(UIStrings.title),
+      description: str_(UIStrings.description),
+      requiredArtifacts: ['traces', 'devtoolsLogs', 'URL', 'TraceElements', 'ImageElements'],
+      scoreDisplayMode: Audit.SCORING_MODES.NUMERIC,
+    };
+  }
+
+  /**
+   *
+   * @param {LH.Artifacts.NetworkRequest} request
+   * @param {LH.Artifacts.NetworkRequest} mainResource
+   * @param {Array<LH.Gatherer.Simulation.GraphNode>} initiatorPath
+   * @return {boolean}
+   */
+  static shouldPreloadRequest(request, mainResource, initiatorPath) {
+    const mainResourceDepth = mainResource.redirects ? mainResource.redirects.length : 0;
+
+    // If it's already preloaded, no need to recommend it.
+    if (request.isLinkPreload) return false;
+    // It's not a request loaded over the network, don't recommend it.
+    if (URL.NON_NETWORK_PROTOCOLS.includes(request.protocol)) return false;
+    // It's already discoverable from the main document, don't recommend it.
+    if (initiatorPath.length <= mainResourceDepth) return false;
+    // Finally, return whether or not it belongs to the main frame
+    return request.frameId === mainResource.frameId;
+  }
+
+  /**
+   * @param {LH.Gatherer.Simulation.GraphNode} graph
+   * @param {string} imageUrl
+   * @return {{lcpNode: LH.Gatherer.Simulation.GraphNetworkNode|undefined, path: Array<LH.Gatherer.Simulation.GraphNetworkNode>|undefined}}
+   */
+  static findLCPNode(graph, imageUrl) {
+    let lcpNode;
+    let path;
+    graph.traverse((node, traversalPath) => {
+      if (node.type !== 'network') return;
+      if (node.record.url === imageUrl) {
+        lcpNode = node;
+        path =
+          traversalPath.slice(1).filter(initiator => initiator.type === 'network');
+      }
+    });
+    return {
+      lcpNode,
+      path,
+    };
+  }
+
+  /**
+   * @param {LH.Artifacts.NetworkRequest} mainResource
+   * @param {LH.Gatherer.Simulation.GraphNode} graph
+   * @param {LH.Artifacts.TraceElement|undefined} lcpElement
+   * @param {Array<LH.Artifacts.ImageElement>} imageElements
+   * @return {LH.Gatherer.Simulation.GraphNetworkNode|undefined}
+   */
+  static getLCPNodeToPreload(mainResource, graph, lcpElement, imageElements) {
+    if (!lcpElement) return undefined;
+
+    const lcpImageElement = imageElements.find(elem => {
+      return elem.devtoolsNodePath === lcpElement.devtoolsNodePath;
+    });
+
+    if (!lcpImageElement) return undefined;
+    const lcpUrl = lcpImageElement.src;
+    const {lcpNode, path} = PreloadLCPImageAudit.findLCPNode(graph, lcpUrl);
+    if (!lcpNode || !path) return undefined;
+    // eslint-disable-next-line max-len
+    const shouldPreload = PreloadLCPImageAudit.shouldPreloadRequest(lcpNode.record, mainResource, path);
+    return shouldPreload ? lcpNode : undefined;
+  }
+
+  /**
+   * Computes the estimated effect of preloading the LCP image.
+   * @param {LH.Gatherer.Simulation.GraphNetworkNode} lcpNode
+   * @param {LH.Gatherer.Simulation.GraphNode} graph
+   * @param {LH.Gatherer.Simulation.Simulator} simulator
+   * @return {{wastedMs: number, results: Array<{url: string, wastedMs: number}>}}
+   */
+  static computeWasteWithGraph(lcpNode, graph, simulator) {
+    const simulation = simulator.simulate(graph, {flexibleOrdering: true});
+    // For now, we are simply using the duration of the LCP image request as the wastedMS value
+    const lcpTiming = simulation.nodeTimings.get(lcpNode);
+    const wastedMs = lcpTiming && lcpTiming.duration || 0;
+    return {
+      wastedMs,
+      results: [{
+        url: lcpNode.record.url,
+        wastedMs,
+      }],
+    };
+  }
+
+  /**
+   * @param {LH.Artifacts} artifacts
+   * @param {LH.Audit.Context} context
+   * @return {Promise<LH.Audit.Product>}
+   */
+  static async audit(artifacts, context) {
+    const trace = artifacts.traces[PreloadLCPImageAudit.DEFAULT_PASS];
+    const devtoolsLog = artifacts.devtoolsLogs[PreloadLCPImageAudit.DEFAULT_PASS];
+    const URL = artifacts.URL;
+    const simulatorOptions = {trace, devtoolsLog, settings: context.settings};
+    const lcpElement = artifacts.TraceElements
+      .find(element => element.traceEventType === 'largest-contentful-paint');
+
+    const [mainResource, lanternLCP, simulator] = await Promise.all([
+      MainResource.request({devtoolsLog, URL}, context),
+      LanternLCP.request(simulatorOptions, context),
+      LoadSimulator.request(simulatorOptions, context),
+    ]);
+
+    const graph = lanternLCP.pessimisticGraph;
+    // eslint-disable-next-line max-len
+    const lcpNode = PreloadLCPImageAudit.getLCPNodeToPreload(mainResource, graph, lcpElement, artifacts.ImageElements);
+    if (!lcpNode) {
+      return {
+        score: 1,
+        notApplicable: true,
+      };
+    }
+
+    const {results, wastedMs} =
+      PreloadLCPImageAudit.computeWasteWithGraph(lcpNode, graph, simulator);
+
+    /** @type {LH.Audit.Details.Opportunity['headings']} */
+    const headings = [
+      {key: 'url', valueType: 'url', label: str_(i18n.UIStrings.columnURL)},
+      {key: 'wastedMs', valueType: 'timespanMs', label: str_(i18n.UIStrings.columnWastedMs)},
+    ];
+    const details = Audit.makeOpportunityDetails(headings, results, wastedMs);
+
+    return {
+      score: UnusedBytes.scoreForWastedMs(wastedMs),
+      numericValue: wastedMs,
+      numericUnit: 'millisecond',
+      displayValue: wastedMs ? str_(i18n.UIStrings.displayValueMsSavings, {wastedMs}) : '',
+      details,
+    };
+  }
+}
+
+module.exports = PreloadLCPImageAudit;
+module.exports.UIStrings = UIStrings;

lighthouse-core/config/experimental-config.js

@@ -18,6 +18,7 @@ const config = {
     'full-page-screenshot',
     'large-javascript-libraries',
     'script-treemap-data',
+    'preload-lcp-image',
   ],
   passes: [{
     passName: 'defaultPass',
@@ -31,6 +32,7 @@ const config = {
     'performance': {
       auditRefs: [
         {id: 'large-javascript-libraries', weight: 0, group: 'diagnostics'},
+        {id: 'preload-lcp-image', weight: 0, group: 'load-opportunities'},
       ],
     },
     // @ts-ignore: `title` is required in CategoryJson. setting to the same value as the default

lighthouse-core/lib/page-functions.js

@@ -398,7 +398,7 @@ function getNodeLabel(node) {
 
 /**
  * @param {HTMLElement} element
- * @param {LH.Artifacts.Rect}
+ * @return {LH.Artifacts.Rect}
  */
 /* istanbul ignore next */
 function getBoundingClientRect(element) {

lighthouse-core/test/audits/preload-lcp-image-test.js

@@ -0,0 +1,135 @@
+/**
+ * @license Copyright 2020 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';
+
+/* eslint-env jest */
+
+const PreloadLCPImage = require('../../audits/preload-lcp-image.js');
+const networkRecordsToDevtoolsLog = require('../network-records-to-devtools-log.js');
+const createTestTrace = require('../create-test-trace.js');
+
+const rootNodeUrl = 'http://example.com:3000';
+const mainDocumentNodeUrl = 'http://www.example.com:3000';
+const scriptNodeUrl = 'http://www.example.com/script.js';
+const imageUrl = 'http://www.example.com/image.png';
+
+describe('Performance: preload-lcp audit', () => {
+  const mockArtifacts = (networkRecords, finalUrl, imageUrl) => {
+    return {
+      traces: {
+        [PreloadLCPImage.DEFAULT_PASS]: createTestTrace({
+          traceEnd: 6e3,
+          largestContentfulPaint: 45e2,
+        }),
+      },
+      devtoolsLogs: {[PreloadLCPImage.DEFAULT_PASS]: networkRecordsToDevtoolsLog(networkRecords)},
+      URL: {finalUrl},
+      TraceElements: [
+        {
+          traceEventType: 'largest-contentful-paint',
+          devtoolsNodePath: '1,HTML,1,BODY,3,DIV,2,IMG',
+        },
+      ],
+      ImageElements: [
+        {
+          src: imageUrl,
+          devtoolsNodePath: '1,HTML,1,BODY,3,DIV,2,IMG',
+        },
+      ],
+    };
+  };
+
+  const mockNetworkRecords = () => {
+    return [
+      {
+        requestId: '2',
+        priority: 'High',
+        isLinkPreload: false,
+        startTime: 0,
+        endTime: 0.5,
+        timing: {receiveHeadersEnd: 500},
+        url: rootNodeUrl,
+      },
+      {
+        requestId: '2:redirect',
+        resourceType: 'Document',
+        priority: 'High',
+        isLinkPreload: false,
+        startTime: 0.5,
+        endTime: 1,
+        timing: {receiveHeadersEnd: 500},
+        url: mainDocumentNodeUrl,
+      },
+      {
+        requestId: '3',
+        resourceType: 'Script',
+        priority: 'High',
+        isLinkPreload: false,
+        startTime: 1,
+        endTime: 5,
+        timing: {receiveHeadersEnd: 4000},
+        url: scriptNodeUrl,
+        initiator: {type: 'parser', url: mainDocumentNodeUrl},
+      },
+      {
+        requestId: '4',
+        resourceType: 'Image',
+        priority: 'High',
+        isLinkPreload: false,
+        startTime: 2,
+        endTime: 4.5,
+        timing: {receiveHeadersEnd: 2500},
+        url: imageUrl,
+        initiator: {type: 'script', url: scriptNodeUrl},
+      },
+    ];
+  };
+
+  it('should suggest preloading a lcp image', async () => {
+    const networkRecords = mockNetworkRecords();
+    const artifacts = mockArtifacts(networkRecords, mainDocumentNodeUrl, imageUrl);
+    const context = {settings: {}, computedCache: new Map()};
+    const results = await PreloadLCPImage.audit(artifacts, context);
+    expect(results.numericValue).toEqual(180);
+    expect(results.details.overallSavingsMs).toEqual(180);
+    expect(results.details.items[0].url).toEqual(imageUrl);
+    expect(results.details.items[0].wastedMs).toEqual(180);
+  });
+
+  it('shouldn\'t be applicable if lcp image is not found', async () => {
+    const networkRecords = mockNetworkRecords();
+    const artifacts = mockArtifacts(networkRecords, mainDocumentNodeUrl, imageUrl);
+    artifacts.ImageElements = [];
+    const context = {settings: {}, computedCache: new Map()};
+    const results = await PreloadLCPImage.audit(artifacts, context);
+    expect(results.score).toEqual(1);
+    expect(results.notApplicable).toBeTruthy();
+    expect(results.details).toBeUndefined();
+  });
+
+  it('shouldn\'t be applicable if the lcp is already preloaded', async () => {
+    const networkRecords = mockNetworkRecords();
+    networkRecords[3].isLinkPreload = true;
+    const artifacts = mockArtifacts(networkRecords, mainDocumentNodeUrl, imageUrl);
+    const context = {settings: {}, computedCache: new Map()};
+    const results = await PreloadLCPImage.audit(artifacts, context);
+    expect(results.score).toEqual(1);
+    expect(results.notApplicable).toBeTruthy();
+    expect(results.details).toBeUndefined();
+  });
+
+  it('shouldn\'t be applicable if the lcp request is not from over the network', async () => {
+    const networkRecords = mockNetworkRecords();
+    networkRecords[3].protocol = 'data';
+    const artifacts = mockArtifacts(networkRecords, mainDocumentNodeUrl, imageUrl);
+    const context = {settings: {}, computedCache: new Map()};
+    const results = await PreloadLCPImage.audit(artifacts, context);
+    expect(results.score).toEqual(1);
+    expect(results.notApplicable).toBeTruthy();
+    expect(results.details).toBeUndefined();
+  });
+});

lighthouse-core/test/audits/uses-rel-preconnect-test.js

@@ -22,7 +22,7 @@ const mainResource = {
 function buildArtifacts(networkRecords) {
   const trace = createTestTrace({
     timeOrigin: 0,
-    largestContentfulPaint: 5000e3,
+    largestContentfulPaint: 5000,
     topLevelTasks: [{ts: 1000, duration: 50}],
   });
   const devtoolsLog = networkRecordsToDevtoolsLog(networkRecords);

lighthouse-core/test/create-test-trace.js

@@ -126,7 +126,7 @@ function createTestTrace(options) {
   if (options.largestContentfulPaint) {
     traceEvents.push({
       name: 'largestContentfulPaint::Candidate',
-      ts: options.largestContentfulPaint,
+      ts: options.largestContentfulPaint * 1000,
       pid,
       tid,
       ph: 'R',