web-vitals/src/lib/bindReporter.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 {finalMetrics} from './finalMetrics.js';
import {Metric, ReportHandler} from '../types.js';


export const bindReporter = (
  callback: ReportHandler,
  metric: Metric,
  reportAllChanges?: boolean,
) => {
  let prevValue: number;
  return () => {
    if (metric.value >= 0) {
      if (reportAllChanges ||
          finalMetrics.has(metric) ||
          document.visibilityState === 'hidden') {
        metric.delta = metric.value - (prevValue || 0);

        // Report the metric if there's a non-zero delta, if the metric is
        // final, or if no previous value exists (which can happen in the case
        // of the document becoming hidden when the metric value is 0).
        // See: https://github.com/GoogleChrome/web-vitals/issues/14
        if (metric.delta || prevValue === undefined) {
          prevValue = metric.value;
          callback(metric);
        }
      }
    }
  }
}