web-vitals/src/onFID.ts

Easily measure performance metrics in JavaScript

/*
 * 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 {getVisibilityWatcher} from './lib/getVisibilityWatcher.js';
import {initMetric} from './lib/initMetric.js';
import {observe} from './lib/observe.js';
import {onHidden} from './lib/onHidden.js';
import {
  firstInputPolyfill,
  resetFirstInputPolyfill,
} from './lib/polyfills/firstInputPolyfill.js';
import {runOnce} from './lib/runOnce.js';
import {whenActivated} from './lib/whenActivated.js';
import {
  FIDMetric,
  FirstInputPolyfillCallback,
  MetricRatingThresholds,
  ReportOpts,
} from './types.js';

/** Thresholds for FID. See https://web.dev/articles/fid#what_is_a_good_fid_score */
export const FIDThresholds: MetricRatingThresholds = [100, 300];

/**
 * Calculates the [FID](https://web.dev/articles/fid) value for the current page and
 * calls the `callback` function once the value is ready, along with the
 * relevant `first-input` performance entry used to determine the value. The
 * reported value is a `DOMHighResTimeStamp`.
 *
 * _**Important:** since FID is only reported after the user interacts with the
 * page, it's possible that it will not be reported for some page loads._
 */
export const onFID = (
  onReport: (metric: FIDMetric) => void,
  opts?: ReportOpts,
) => {
  // Set defaults
  opts = opts || {};

  whenActivated(() => {
    const visibilityWatcher = getVisibilityWatcher();
    let metric = initMetric('FID');
    let report: ReturnType<typeof bindReporter>;

    const handleEntry = (entry: PerformanceEventTiming) => {
      // Only report if the page wasn't hidden prior to the first input.
      if (entry.startTime < visibilityWatcher.firstHiddenTime) {
        metric.value = entry.processingStart - entry.startTime;
        metric.entries.push(entry);
        report(true);
      }
    };

    const handleEntries = (entries: FIDMetric['entries']) => {
      entries.forEach(handleEntry);
    };

    const po = observe('first-input', handleEntries);

    report = bindReporter(
      onReport,
      metric,
      FIDThresholds,
      opts!.reportAllChanges,
    );

    if (po) {
      onHidden(
        runOnce(() => {
          handleEntries(po.takeRecords() as FIDMetric['entries']);
          po.disconnect();
        }),
      );

      onBFCacheRestore(() => {
        metric = initMetric('FID');
        report = bindReporter(
          onReport,
          metric,
          FIDThresholds,
          opts!.reportAllChanges,
        );

        // Browsers don't re-emit FID on bfcache restore so fake it until you make it
        resetFirstInputPolyfill();
        firstInputPolyfill(handleEntry as FirstInputPolyfillCallback);
      });
    }
  });
};