/* * Copyright 2020 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 { initMetric } from './lib/initMetric.js'; const afterLoad = (callback) => { if (document.readyState === 'complete') { // Queue a task so the callback runs after `loadEventEnd`. setTimeout(callback, 0); } else { // Queue a task so the callback runs after `loadEventEnd`. addEventListener('load', () => setTimeout(callback, 0)); } }; const getNavigationEntryFromPerformanceTiming = () => { // Really annoying that TypeScript errors when using `PerformanceTiming`. const timing = performance.timing; const navigationEntry = { entryType: 'navigation', startTime: 0, }; for (const key in timing) { if (key !== 'navigationStart' && key !== 'toJSON') { navigationEntry[key] = Math.max(timing[key] - timing.navigationStart, 0); } } return navigationEntry; }; export const getTTFB = (onReport) => { const metric = initMetric('TTFB'); afterLoad(() => { try { // Use the NavigationTiming L2 entry if available. const navigationEntry = performance.getEntriesByType('navigation')[0] || getNavigationEntryFromPerformanceTiming(); metric.value = metric.delta = navigationEntry.responseStart; // In some cases the value reported is negative or is larger // than the current page time. Ignore these cases: // https://github.com/GoogleChrome/web-vitals/issues/137 // https://github.com/GoogleChrome/web-vitals/issues/162 if (metric.value < 0 || metric.value > performance.now()) return; metric.entries = [navigationEntry]; onReport(metric); } catch (error) { // Do nothing. } }); };