UNPKG

web-vitals

Version:

Easily measure performance metrics in JavaScript

104 lines (103 loc) 4.9 kB
/* * 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 { initUnique } from '../lib/initUnique.js'; import { LCPEntryManager } from '../lib/LCPEntryManager.js'; import { onLCP as unattributedOnLCP } from '../onLCP.js'; /** * 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 = {}) => { // Clone the opts object to ensure it's unique, so we can initialize a // single instance of the `LCPEntryManager` class that's shared only with // this function invocation and the `unattributedOnLCP()` invocation below // (which is passed the same `opts` object). opts = Object.assign({}, opts); const lcpEntryManager = initUnique(opts, LCPEntryManager); const lcpTargetMap = new WeakMap(); lcpEntryManager._onBeforeProcessingEntry = (entry) => { if (entry.element) { const generateTargetFn = opts.generateTarget ?? getSelector; const customTarget = generateTargetFn(entry.element); lcpTargetMap.set(entry, customTarget); } }; 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; // The `metric.entries.length` check ensures there will be an entry. const lcpEntry = metric.entries.at(-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.min( // Cap at LCP time (videos continue downloading after LCP for example) metric.value, Math.max(lcpRequestStart, lcpResourceEntry ? lcpResourceEntry.responseEnd - activationStart : 0)); attribution = { target: lcpTargetMap.get(lcpEntry), timeToFirstByte: ttfb, resourceLoadDelay: lcpRequestStart - ttfb, resourceLoadDuration: lcpResponseEnd - lcpRequestStart, elementRenderDelay: metric.value - lcpResponseEnd, navigationEntry, lcpEntry, }; // Only attribute the URL and resource entry if they exist. if (lcpEntry.url) { attribution.url = lcpEntry.url; } if (lcpResourceEntry) { attribution.lcpResourceEntry = lcpResourceEntry; } } } // Use `Object.assign()` to ensure the original metric object is returned. const metricWithAttribution = Object.assign(metric, { attribution }); return metricWithAttribution; }; unattributedOnLCP((metric) => { const metricWithAttribution = attributeLCP(metric); onReport(metricWithAttribution); }, opts); };