onFCP.ts

/*
 * 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';
import { getActivationStart } from './lib/getActivationStart';
import { getVisibilityWatcher } from './lib/getVisibilityWatcher';
import { initMetric } from './lib/initMetric';
import { observe } from './lib/observe';
import { whenActivated } from './lib/whenActivated';
import type { FCPMetric, MetricRatingThresholds, ReportOpts } from './types';

/** Thresholds for FCP. See https://web.dev/articles/fcp#what_is_a_good_fcp_score */
export const FCPThresholds: MetricRatingThresholds = [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: (metric: FCPMetric) => void, opts: ReportOpts = {}) => {
  whenActivated(() => {
    const visibilityWatcher = getVisibilityWatcher();
    const metric = initMetric('FCP');
    let report: ReturnType<typeof bindReporter>;

    const handleEntries = (entries: FCPMetric['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);
    }
  });
};