web-vitals/dist/modules/getCLS.js

/*
 * 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';
import { observe } from './lib/observe.js';
import { onHidden } from './lib/onHidden.js';
import { onBFCacheRestore } from './lib/onBFCacheRestore.js';
import { bindReporter } from './lib/bindReporter.js';
import { getFCP } from './getFCP.js';
let isMonitoringFCP = false;
let fcpValue = -1;
export const getCLS = (onReport, reportAllChanges) => {
    // Start monitoring FCP so we can only report CLS if FCP is also reported.
    // Note: this is done to match the current behavior of CrUX.
    if (!isMonitoringFCP) {
        getFCP((metric) => {
            fcpValue = metric.value;
        });
        isMonitoringFCP = true;
    }
    const onReportWrapped = (arg) => {
        if (fcpValue > -1) {
            onReport(arg);
        }
    };
    let metric = initMetric('CLS', 0);
    let report;
    let sessionValue = 0;
    let sessionEntries = [];
    const entryHandler = (entry) => {
        // Only count layout shifts without recent user input.
        if (!entry.hadRecentInput) {
            const firstSessionEntry = sessionEntries[0];
            const lastSessionEntry = sessionEntries[sessionEntries.length - 1];
            // If the entry occurred less than 1 second after the previous entry and
            // less than 5 seconds after the first entry in the session, include the
            // entry in the current session. Otherwise, start a new session.
            if (sessionValue &&
                entry.startTime - lastSessionEntry.startTime < 1000 &&
                entry.startTime - firstSessionEntry.startTime < 5000) {
                sessionValue += entry.value;
                sessionEntries.push(entry);
            }
            else {
                sessionValue = entry.value;
                sessionEntries = [entry];
            }
            // If the current session value is larger than the current CLS value,
            // update CLS and the entries contributing to it.
            if (sessionValue > metric.value) {
                metric.value = sessionValue;
                metric.entries = sessionEntries;
                report();
            }
        }
    };
    const po = observe('layout-shift', entryHandler);
    if (po) {
        report = bindReporter(onReportWrapped, metric, reportAllChanges);
        onHidden(() => {
            po.takeRecords().map(entryHandler);
            report();
        });
        onBFCacheRestore(() => {
            sessionValue = 0;
            fcpValue = -1;
            metric = initMetric('CLS', 0);
            report = bindReporter(onReportWrapped, metric, reportAllChanges);
        });
    }
};