web-vitals/dist/modules/onFCP.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 { onBFCacheRestore } from './lib/bfcache.js';
import { bindReporter } from './lib/bindReporter.js';
import { doubleRAF } from './lib/doubleRAF.js';
import { getActivationStart } from './lib/getActivationStart.js';
import { getVisibilityWatcher } from './lib/getVisibilityWatcher.js';
import { initMetric } from './lib/initMetric.js';
import { observe } from './lib/observe.js';
import { whenActivated } from './lib/whenActivated.js';
/** Thresholds for FCP. See https://web.dev/articles/fcp#what_is_a_good_fcp_score */
export const FCPThresholds = [1800, 3000];
/**
 * Calculates the [FCP](https://web.dev/articles/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) => {
    // Set defaults
    opts = opts || {};
    whenActivated(() => {
        const visibilityWatcher = getVisibilityWatcher();
        let metric = initMetric('FCP');
        let report;
        const handleEntries = (entries) => {
            entries.forEach((entry) => {
                if (entry.name === 'first-contentful-paint') {
                    po.disconnect();
                    // Only report if the page wasn't hidden prior to the first paint.
                    if (entry.startTime < visibilityWatcher.firstHiddenTime) {
                        // The activationStart reference is used because FCP should be
                        // relative to page activation rather than navigation start if the
                        // page was prerendered. But in cases where `activationStart` occurs
                        // after the FCP, this time should be clamped at 0.
                        metric.value = Math.max(entry.startTime - getActivationStart(), 0);
                        metric.entries.push(entry);
                        report(true);
                    }
                }
            });
        };
        const po = observe('paint', handleEntries);
        if (po) {
            report = bindReporter(onReport, metric, FCPThresholds, opts.reportAllChanges);
            // Only report after a bfcache restore if the `PerformanceObserver`
            // successfully registered or the `paint` entry exists.
            onBFCacheRestore((event) => {
                metric = initMetric('FCP');
                report = bindReporter(onReport, metric, FCPThresholds, opts.reportAllChanges);
                doubleRAF(() => {
                    metric.value = performance.now() - event.timeStamp;
                    report(true);
                });
            });
        }
    });
};