legacy-circular-equal-support.md

Legacy environment support for circular equal comparators

Starting in 4.x.x, WeakMap is expected to be available in the environment. All modern browsers support this global object, however there may be situations where a legacy environmental support is required (example: IE11). If you need to support such an environment and polyfilling is not an option, creating a custom comparator that uses a custom cache implementation with the same contract is a simple solution.

import { createCustomEqual, sameValueZeroEqual } from 'fast-equals';
import type { Cache } from 'fast-equals';

function getCache(): Cache<any, any> {
  const entries: Array<[object, any]> = [];

  return {
    delete(key) {
      for (let index = 0; index < entries.length; ++index) {
        if (entries[index][0] === key) {
          entries.splice(index, 1);
          return true;
        }
      }

      return false;
    },

    get(key) {
      for (let index = 0; index < entries.length; ++index) {
        if (entries[index][0] === key) {
          return entries[index][1];
        }
      }
    },

    set(key, value) {
      for (let index = 0; index < entries.length; ++index) {
        if (entries[index][0] === key) {
          entries[index][1] = value;
          return this;
        }
      }

      entries.push([key, value]);

      return this;
    },
  };
}

interface Meta {
  customMethod(): void;
  customValue: string;
}

const meta = {
  customMethod() {
    console.log('hello!');
  },
  customValue: 'goodbye',
};

const circularDeepEqual = createCustomEqual<Cache>({
  circular: true,
  createState: () => ({
    cache: getCache(),
    meta,
  }),
});

const circularShallowEqual = createCustomEqual<Cache>({
  circular: true,
  comparator: sameValueZeroEqual,
  createState: () => ({
    cache: getCache(),
    meta,
  }),
});