web-vitals
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Easily measure performance metrics in JavaScript
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JavaScript
/*
* Copyright 2022 Google LLC
*
* 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
*
* https://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.
*/
import { getNavigationEntry } from '../lib/getNavigationEntry.js';
import { getSelector } from '../lib/getSelector.js';
import { onLCP as unattributedOnLCP } from '../onLCP.js';
const attributeLCP = (metric) => {
// Use a default object if no other attribution has been set.
let attribution = {
timeToFirstByte: 0,
resourceLoadDelay: 0,
resourceLoadDuration: 0,
elementRenderDelay: metric.value,
};
if (metric.entries.length) {
const navigationEntry = getNavigationEntry();
if (navigationEntry) {
const activationStart = navigationEntry.activationStart || 0;
const lcpEntry = metric.entries[metric.entries.length - 1];
const lcpResourceEntry = lcpEntry.url &&
performance
.getEntriesByType('resource')
.filter((e) => e.name === lcpEntry.url)[0];
const ttfb = Math.max(0, navigationEntry.responseStart - activationStart);
const lcpRequestStart = Math.max(ttfb,
// Prefer `requestStart` (if TOA is set), otherwise use `startTime`.
lcpResourceEntry
? (lcpResourceEntry.requestStart || lcpResourceEntry.startTime) -
activationStart
: 0);
const lcpResponseEnd = Math.max(lcpRequestStart, lcpResourceEntry ? lcpResourceEntry.responseEnd - activationStart : 0);
const lcpRenderTime = Math.max(lcpResponseEnd, lcpEntry.startTime - activationStart);
attribution = {
element: getSelector(lcpEntry.element),
timeToFirstByte: ttfb,
resourceLoadDelay: lcpRequestStart - ttfb,
resourceLoadDuration: lcpResponseEnd - lcpRequestStart,
elementRenderDelay: lcpRenderTime - lcpResponseEnd,
navigationEntry,
lcpEntry,
};
// Only attribution the URL and resource entry if they exist.
if (lcpEntry.url) {
attribution.url = lcpEntry.url;
}
if (lcpResourceEntry) {
attribution.lcpResourceEntry = lcpResourceEntry;
}
}
}
// Use Object.assign to set property to keep tsc happy.
const metricWithAttribution = Object.assign(metric, { attribution });
return metricWithAttribution;
};
/**
* Calculates the [LCP](https://web.dev/articles/lcp) value for the current page and
* calls the `callback` function once the value is ready (along with the
* relevant `largest-contentful-paint` performance entry used to determine the
* value). The reported value is a `DOMHighResTimeStamp`.
*
* If the `reportAllChanges` configuration option is set to `true`, the
* `callback` function will be called any time a new `largest-contentful-paint`
* performance entry is dispatched, or once the final value of the metric has
* been determined.
*/
export const onLCP = (onReport, opts) => {
unattributedOnLCP((metric) => {
const metricWithAttribution = attributeLCP(metric);
onReport(metricWithAttribution);
}, opts);
};