immutability-helper 源码的疑惑

Question

在看immutability-helper的源码,对其中一段函数很疑惑

  update.isEquals = function(a, b) { return a === b; };

  function update(object, spec) {
    if (typeof spec === 'function') {
      return spec(object);
    }

    if (!(Array.isArray(object) && Array.isArray(spec))) {
      invariant(
        !Array.isArray(spec),
        'update(): You provided an invalid spec to update(). The spec may ' +
        'not contain an array except as the value of $set, $push, $unshift, ' +
        '$splice or any custom command allowing an array value.'
      );
    }

    invariant(
      typeof spec === 'object' && spec !== null,
      'update(): You provided an invalid spec to update(). The spec and ' +
      'every included key path must be plain objects containing one of the ' +
      'following commands: %s.',
      Object.keys(commands).join(', ')
    );

    var nextObject = object;
    var index, key;
    getAllKeys(spec).forEach(function(key) {
      if (hasOwnProperty.call(commands, key)) {
        var objectWasNextObject = object === nextObject;
        nextObject = commands[key](spec[key], nextObject, spec, object);
        // update.isEquals(nextObject, object) 为true那不就是 nextObject 一定等于 object, 再写 nextObject = object 有什么意义
        if (objectWasNextObject && update.isEquals(nextObject, object)) {
          nextObject = object;
        }
      } else {
        var nextValueForKey =
          type(object) === 'Map'
            ? update(object.get(key), spec[key])
            : update(object[key], spec[key]);
        if (!update.isEquals(nextValueForKey, nextObject[key]) || typeof nextValueForKey === 'undefined' && !hasOwnProperty.call(object, key)) {
          if (nextObject === object) {
            nextObject = copy(object);
          }
          if (type(nextObject) === 'Map') {
            nextObject.set(key, nextValueForKey);
          } else {
            nextObject[key] = nextValueForKey;
          }
        }
      }
    })
    return nextObject;
  }

这段函数有什么意义? update.isEquals(nextObject, object)不就表示两者相等了

if (objectWasNextObject && update.isEquals(nextObject, object)) {
          nextObject = object;
}
        

Answer 1

isEquals为暴露的接口,用户可以自身定义 myUpdate.isEquals = ...
比如将isEquals定义为深度检查,那仍将nextObject = object