Handling special (custom) objects

The standard equality comparators use Object.prototype.toString.call() to decide which well-known comparator to call. However, some values might not work for this check. Examples:

• Provide an explicit comparison for unhandled items, such as WeakMap
• Support class instances with a custom Symbol.toStringTag value
• Support polyfills / new built-in objects for proposals to the language

In that case, you can leverage createCustomEqual with the getUnsupportedCustomComparator handler to create your own equality comparators for those unsupported values.

import { createCustomEqual } from 'fast-equals';

const areZonedDateTimesEqual = (a: unknown, b: unknown) =>
  a instanceof Temporal.ZonedDateTime && b instanceof Temporal.ZonedDateTime && a.equals(b);

const isSpecialObjectEqual = createCustomEqual({
  createCustomConfig: () => ({
    getUnsupportedCustomComparator(a) {
      if (a?.[Symbol.toStringTag] === 'Temporal.ZonedDateTime') {
        return areZonedDateTimesEqual;
      }
    },
  }),
});