is.js

import { isValueObject } from './predicates/isValueObject';

/**
 * An extension of the "same-value" algorithm as [described for use by ES6 Map
 * and Set](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Map#Key_equality)
 *
 * NaN is considered the same as NaN, however -0 and 0 are considered the same
 * value, which is different from the algorithm described by
 * [`Object.is`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/is).
 *
 * This is extended further to allow Objects to describe the values they
 * represent, by way of `valueOf` or `equals` (and `hashCode`).
 *
 * Note: because of this extension, the key equality of Immutable.Map and the
 * value equality of Immutable.Set will differ from ES6 Map and Set.
 *
 * ### Defining custom values
 *
 * The easiest way to describe the value an object represents is by implementing
 * `valueOf`. For example, `Date` represents a value by returning a unix
 * timestamp for `valueOf`:
 *
 *     var date1 = new Date(1234567890000); // Fri Feb 13 2009 ...
 *     var date2 = new Date(1234567890000);
 *     date1.valueOf(); // 1234567890000
 *     assert( date1 !== date2 );
 *     assert( Immutable.is( date1, date2 ) );
 *
 * Note: overriding `valueOf` may have other implications if you use this object
 * where JavaScript expects a primitive, such as implicit string coercion.
 *
 * For more complex types, especially collections, implementing `valueOf` may
 * not be performant. An alternative is to implement `equals` and `hashCode`.
 *
 * `equals` takes another object, presumably of similar type, and returns true
 * if it is equal. Equality is symmetrical, so the same result should be
 * returned if this and the argument are flipped.
 *
 *     assert( a.equals(b) === b.equals(a) );
 *
 * `hashCode` returns a 32bit integer number representing the object which will
 * be used to determine how to store the value object in a Map or Set. You must
 * provide both or neither methods, one must not exist without the other.
 *
 * Also, an important relationship between these methods must be upheld: if two
 * values are equal, they *must* return the same hashCode. If the values are not
 * equal, they might have the same hashCode; this is called a hash collision,
 * and while undesirable for performance reasons, it is acceptable.
 *
 *     if (a.equals(b)) {
 *       assert( a.hashCode() === b.hashCode() );
 *     }
 *
 * All Immutable collections are Value Objects: they implement `equals()`
 * and `hashCode()`.
 */
export function is(valueA, valueB) {
  if (valueA === valueB || (valueA !== valueA && valueB !== valueB)) {
    return true;
  }
  if (!valueA || !valueB) {
    return false;
  }
  if (
    typeof valueA.valueOf === 'function' &&
    typeof valueB.valueOf === 'function'
  ) {
    valueA = valueA.valueOf();
    valueB = valueB.valueOf();
    if (valueA === valueB || (valueA !== valueA && valueB !== valueB)) {
      return true;
    }
    if (!valueA || !valueB) {
      return false;
    }
  }
  return !!(
    isValueObject(valueA) &&
    isValueObject(valueB) &&
    valueA.equals(valueB)
  );
}