web-vitals/src/lib/initUnique.ts

Easily measure performance metrics in JavaScript

/*
 * Copyright 2024 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.
 */

const instanceMap: WeakMap<
  new () => unknown,
  WeakMap<object, unknown>
> = new WeakMap();

/**
 * A function that accepts and identity object and a class object and returns
 * either a new instance of that class or an existing instance, if the
 * identity object was previously used.
 */
export function initUnique<T>(identityObj: object, ClassObj: new () => T): T {
  let classInstances = instanceMap.get(ClassObj);
  if (!classInstances) {
    classInstances = new WeakMap();
    instanceMap.set(ClassObj, classInstances);
  }
  if (!classInstances.get(identityObj)) {
    classInstances.set(identityObj, new ClassObj());
  }
  return classInstances.get(identityObj)! as T;
}