web-vitals/src/lib/initMetric.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 {getBFCacheRestoreTime} from './bfcache.js';
import {generateUniqueID} from './generateUniqueID.js';
import {getActivationStart} from './getActivationStart.js';
import {getNavigationEntry} from './getNavigationEntry.js';
import type {MetricType} from '../types.js';

export const initMetric = <MetricName extends MetricType['name']>(
  name: MetricName,
  value: number = -1,
  navigationType?: MetricType['navigationType'],
  navigationId: number = 0,
  navigationInteractionId?: number,
  navigationURL?: string,
  navigationStartTime?: number,
) => {
  const hardNavEntry = getNavigationEntry();
  const hardNavId = hardNavEntry?.navigationId || 0;
  let _navigationType: MetricType['navigationType'] = 'navigate';

  if (navigationType) {
    // If it was passed in, then use that
    _navigationType = navigationType;
  } else if (getBFCacheRestoreTime() >= 0) {
    _navigationType = 'back-forward-cache';
  } else if (hardNavEntry) {
    if (document.prerendering || getActivationStart() > 0) {
      _navigationType = 'prerender';
    } else if (document.wasDiscarded) {
      _navigationType = 'restore';
    } else if (hardNavEntry.type) {
      _navigationType = hardNavEntry.type.replace(
        /_/g,
        '-',
      ) as MetricType['navigationType'];
    }
  }

  // Use `entries` type specific for the metric.
  const entries: Extract<MetricType, {name: MetricName}>['entries'] = [];

  return {
    name,
    value,
    rating: 'good' as const, // If needed, will be updated when reported. `const` to keep the type from widening to `string`.
    delta: 0,
    entries,
    id: generateUniqueID(),
    navigationType: _navigationType,
    navigationId: navigationId || hardNavId,
    navigationInteractionId: navigationInteractionId,
    navigationURL: navigationURL || hardNavEntry?.name,
    navigationStartTime: navigationStartTime || 0,
  };
};