1 | /*
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2 | * Copyright 2020 Google LLC
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3 | *
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4 | * Licensed under the Apache License, Version 2.0 (the "License");
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5 | * you may not use this file except in compliance with the License.
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6 | * You may obtain a copy of the License at
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7 | *
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8 | * https://www.apache.org/licenses/LICENSE-2.0
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9 | *
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10 | * Unless required by applicable law or agreed to in writing, software
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11 | * distributed under the License is distributed on an "AS IS" BASIS,
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12 | * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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13 | * See the License for the specific language governing permissions and
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14 | * limitations under the License.
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15 | */
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16 |
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17 | import {onBFCacheRestore} from './lib/bfcache.js';
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18 | import {bindReporter} from './lib/bindReporter.js';
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19 | import {doubleRAF} from './lib/doubleRAF.js';
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20 | import {getActivationStart} from './lib/getActivationStart.js';
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21 | import {getVisibilityWatcher} from './lib/getVisibilityWatcher.js';
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22 | import {initMetric} from './lib/initMetric.js';
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23 | import {observe} from './lib/observe.js';
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24 | import {onHidden} from './lib/onHidden.js';
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25 | import {runOnce} from './lib/runOnce.js';
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26 | import {whenActivated} from './lib/whenActivated.js';
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27 | import {LCPMetric, ReportCallback, ReportOpts} from './types.js';
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28 |
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29 | const reportedMetricIDs: Record<string, boolean> = {};
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30 |
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31 | /**
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32 | * Calculates the [LCP](https://web.dev/lcp/) value for the current page and
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33 | * calls the `callback` function once the value is ready (along with the
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34 | * relevant `largest-contentful-paint` performance entry used to determine the
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35 | * value). The reported value is a `DOMHighResTimeStamp`.
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36 | *
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37 | * If the `reportAllChanges` configuration option is set to `true`, the
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38 | * `callback` function will be called any time a new `largest-contentful-paint`
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39 | * performance entry is dispatched, or once the final value of the metric has
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40 | * been determined.
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41 | */
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42 | export const onLCP = (onReport: ReportCallback, opts?: ReportOpts) => {
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43 | // Set defaults
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44 | opts = opts || {};
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45 |
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46 | whenActivated(() => {
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47 | // https://web.dev/lcp/#what-is-a-good-lcp-score
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48 | const thresholds = [2500, 4000];
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49 |
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50 | const visibilityWatcher = getVisibilityWatcher();
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51 | let metric = initMetric('LCP');
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52 | let report: ReturnType<typeof bindReporter>;
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53 |
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54 | const handleEntries = (entries: LCPMetric['entries']) => {
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55 | const lastEntry = entries[entries.length - 1] as LargestContentfulPaint;
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56 | if (lastEntry) {
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57 | // The startTime attribute returns the value of the renderTime if it is
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58 | // not 0, and the value of the loadTime otherwise. The activationStart
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59 | // reference is used because LCP should be relative to page activation
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60 | // rather than navigation start if the page was prerendered. But in cases
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61 | // where `activationStart` occurs after the LCP, this time should be
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62 | // clamped at 0.
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63 | const value = Math.max(lastEntry.startTime - getActivationStart(), 0);
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64 |
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65 | // Only report if the page wasn't hidden prior to LCP.
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66 | if (value < visibilityWatcher.firstHiddenTime) {
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67 | metric.value = value;
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68 | metric.entries = [lastEntry];
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69 | report();
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70 | }
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71 | }
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72 | };
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73 |
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74 | const po = observe('largest-contentful-paint', handleEntries);
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75 |
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76 | if (po) {
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77 | report = bindReporter(
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78 | onReport,
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79 | metric,
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80 | thresholds,
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81 | opts!.reportAllChanges
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82 | );
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83 |
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84 | const stopListening = runOnce(() => {
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85 | if (!reportedMetricIDs[metric.id]) {
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86 | handleEntries(po!.takeRecords() as LCPMetric['entries']);
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87 | po!.disconnect();
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88 | reportedMetricIDs[metric.id] = true;
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89 | report(true);
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90 | }
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91 | });
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92 |
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93 | // Stop listening after input. Note: while scrolling is an input that
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94 | // stops LCP observation, it's unreliable since it can be programmatically
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95 | // generated. See: https://github.com/GoogleChrome/web-vitals/issues/75
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96 | ['keydown', 'click'].forEach((type) => {
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97 | addEventListener(type, stopListening, true);
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98 | });
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99 |
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100 | onHidden(stopListening);
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101 |
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102 | // Only report after a bfcache restore if the `PerformanceObserver`
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103 | // successfully registered.
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104 | onBFCacheRestore((event) => {
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105 | metric = initMetric('LCP');
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106 | report = bindReporter(
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107 | onReport,
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108 | metric,
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109 | thresholds,
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110 | opts!.reportAllChanges
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111 | );
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112 |
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113 | doubleRAF(() => {
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114 | metric.value = performance.now() - event.timeStamp;
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115 | reportedMetricIDs[metric.id] = true;
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116 | report(true);
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117 | });
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118 | });
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119 | }
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120 | });
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121 | };
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