web-vitals/dist/modules/getLCP.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 { bindReporter } from './lib/bindReporter.js';
import { finalMetrics } from './lib/finalMetrics.js';
import { getFirstHidden } from './lib/getFirstHidden.js';
import { initMetric } from './lib/initMetric.js';
import { observe } from './lib/observe.js';
import { onBFCacheRestore } from './lib/onBFCacheRestore.js';
import { onHidden } from './lib/onHidden.js';
export const getLCP = (onReport, reportAllChanges) => {
    const firstHidden = getFirstHidden();
    let metric = initMetric('LCP');
    let report;
    const entryHandler = (entry) => {
        // The startTime attribute returns the value of the renderTime if it is not 0,
        // and the value of the loadTime otherwise.
        const value = entry.startTime;
        // If the page was hidden prior to paint time of the entry,
        // ignore it and mark the metric as final, otherwise add the entry.
        if (value < firstHidden.timeStamp) {
            metric.value = value;
            metric.entries.push(entry);
        }
        report();
    };
    const po = observe('largest-contentful-paint', entryHandler);
    if (po) {
        report = bindReporter(onReport, metric, reportAllChanges);
        const stopListening = () => {
            if (!finalMetrics.has(metric)) {
                po.takeRecords().map(entryHandler);
                po.disconnect();
                finalMetrics.add(metric);
                report();
            }
        };
        // Stop listening after input. Note: while scrolling is an input that
        // stop LCP observation, it's unreliable since it can be programmatically
        // generated. See: https://github.com/GoogleChrome/web-vitals/issues/75
        ['keydown', 'click'].forEach((type) => {
            addEventListener(type, stopListening, { once: true, capture: true });
        });
        onHidden(stopListening, true);
        onBFCacheRestore((event) => {
            metric = initMetric('LCP');
            report = bindReporter(onReport, metric, reportAllChanges);
            requestAnimationFrame(() => {
                requestAnimationFrame(() => {
                    metric.value = performance.now() - event.timeStamp;
                    finalMetrics.add(metric);
                    report();
                });
            });
        });
    }
};