web-vitals
Version:
Easily measure performance metrics in JavaScript
57 lines (56 loc) • 2.19 kB
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 { getBFCacheRestoreTime } from '../lib/bfcache.js';
import { getLoadState } from '../lib/getLoadState.js';
import { getNavigationEntry } from '../lib/getNavigationEntry.js';
import { onFCP as unattributedOnFCP } from '../onFCP.js';
const attributeFCP = (metric) => {
if (metric.entries.length) {
const navigationEntry = getNavigationEntry();
const fcpEntry = metric.entries[metric.entries.length - 1];
if (navigationEntry) {
const activationStart = navigationEntry.activationStart || 0;
const ttfb = Math.max(0, navigationEntry.responseStart - activationStart);
metric.attribution = {
timeToFirstByte: ttfb,
firstByteToFCP: metric.value - ttfb,
loadState: getLoadState(metric.entries[0].startTime),
navigationEntry,
fcpEntry,
};
}
}
else {
// There are no entries when restored from bfcache.
metric.attribution = {
timeToFirstByte: 0,
firstByteToFCP: metric.value,
loadState: getLoadState(getBFCacheRestoreTime()),
};
}
};
/**
* Calculates the [FCP](https://web.dev/fcp/) value for the current page and
* calls the `callback` function once the value is ready, along with the
* relevant `paint` performance entry used to determine the value. The reported
* value is a `DOMHighResTimeStamp`.
*/
export const onFCP = (onReport, opts) => {
unattributedOnFCP(((metric) => {
attributeFCP(metric);
onReport(metric);
}), opts);
};