1EnumerateObjectProperties ( O )

When the abstract operation EnumerateObjectProperties is called with argument O, the following steps are taken:

The iterator's throw and return methods are null and are never invoked. The iterator's next method processes object properties to determine whether the property key should be returned as an iterator value. Returned property keys do not include keys that are Symbols. Properties of the target object may be deleted during enumeration. A property that is deleted before it is processed by the iterator's next method is ignored. If new properties are added to the target object during enumeration, the newly added properties are not guaranteed to be processed in the active enumeration. A property name will be returned by the iterator's next method at most once in any enumeration.

Enumerating the properties of the target object includes enumerating properties of its prototype, and the prototype of the prototype, and so on, recursively; but a property of a prototype is not processed if it has the same name as a property that has already been processed by the iterator's next method. The values of [[Enumerable]] attributes are not considered when determining if a property of a prototype object has already been processed. The enumerable property names of prototype objects must be obtained by invoking EnumerateObjectProperties passing the prototype object as the argument. EnumerateObjectProperties must obtain the own property keys of the target object by calling its [[OwnPropertyKeys]] internal method. Property attributes of the target object must be obtained by calling its [[GetOwnProperty]] internal method.

In addition, if neither O nor any object in its prototype chain is a Proxy exotic object, Integer-Indexed exotic object, Module Namespace exotic object, or implementation provided exotic object, then the iterator must behave as if it were the iterator given by CreateForInIterator ( O ) until one of the following occurs:

• the value of the [[Prototype]] internal slot of O or an object in its prototype chain changes,
• a property is removed from O or an object in its prototype chain,
• a property is added to an object in O's prototype chain, or
• the value of the [[Enumerable]] attribute of a property of O or an object in its prototype chain changes.

function* EnumerateObjectProperties(obj) {
  const visited = new Set();
  for (const key of Reflect.ownKeys(obj)) {
    if (typeof key === "symbol") continue;
    const desc = Reflect.getOwnPropertyDescriptor(obj, key);
    if (desc) {
      visited.add(key);
      if (desc.enumerable) yield key;
    }
  }
  const proto = Reflect.getPrototypeOf(obj);
  if (proto === null) return;
  for (const protoKey of EnumerateObjectProperties(proto)) {
    if (!visited.has(protoKey)) yield protoKey;
  }
}

2For In Iterator Objects

A For In Iterator is an object that represents a specific iteration over some specific object. For In Iterator objects are never directly accessible to ECMAScript code; they exist soley to illustrate the behavior of EnumerateObjectProperties.