web-vitals/src/lib/bfcache.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.
 */

interface onBFCacheRestoreCallback {
  (event: PageTransitionEvent): void;
}

let bfcacheRestoreTime = -1;

export const getBFCacheRestoreTime = () => bfcacheRestoreTime;

export const onBFCacheRestore = (cb: onBFCacheRestoreCallback) => {
  addEventListener(
    'pageshow',
    (event) => {
      if (event.persisted) {
        bfcacheRestoreTime = event.timeStamp;
        cb(event);
      }
    },
    true,
  );
};