Enumerability and ownership of properties

Every property in JavaScript objects can be classified by three factors:

  • Enumerable or non-enumerable;
  • String or symbol;
  • Own property or inherited property from the prototype chain.

Enumerable properties are those properties whose internal enumerable flag is set to true, which is the default for properties created via simple assignment or via a property initializer. Properties defined via Object.defineProperty and such are not enumerable by default. Most iteration means (such as for...in loops and Object.keys) only visit enumerable keys.

Ownership of properties is determined by whether the property belongs to the object directly and not to its prototype chain.

All properties, enumerable or not, string or symbol, own or inherited, can be accessed with dot notation or bracket notation. In this section, we will focus on the means provided by JavaScript to visit a group of object properties one-by-one.

Querying object properties

There are four built-in ways to query a property of an object. They all support both string and symbol keys. The following table summarizes when each method returns true.

Enumerable, own Enumerable, inherited Non-enumerable, own Non-enumerable, inherited
propertyIsEnumerable() true ✅ false ❌ false ❌ false ❌
hasOwnProperty() true ✅ false ❌ true ✅ false ❌
Object.hasOwn() true ✅ false ❌ true ✅ false ❌
in true ✅ true ✅ true ✅ true ✅

Traversing object properties

There are many methods in JavaScript that traverse a group of properties of an object. Sometimes, these properties are returned as an array; sometimes, they are iterated one-by-one in a loop; sometimes, they are used for constructing or mutating another object. The following table summarizes when a property may be visited.

Methods that only visit string properties or only symbol properties will have an extra note. ✅ means a property of this type will be visited; ❌ means it will not.

Enumerable, own Enumerable, inherited Non-enumerable, own Non-enumerable, inherited
Object.keys
Object.values
Object.entries		 ✅
(strings)		 ❌ ❌ ❌
Object.getOwnPropertyNames ✅
(strings)		 ❌ ✅
(strings)		 ❌
Object.getOwnPropertySymbols ✅
(symbols)		 ❌ ✅
(symbols)		 ❌
Object.getOwnPropertyDescriptors ✅ ❌ ✅ ❌
Reflect.ownKeys ✅ ❌ ✅ ❌
for...in ✅
(strings)		 ✅
(strings)		 ❌ ❌
Object.assign
(After the first parameter)		 ✅ ❌ ❌ ❌
Object spread ✅ ❌ ❌ ❌

Obtaining properties by enumerability/ownership

Note that this is not the most efficient algorithm for all cases, but useful for a quick demonstration.

  • Detection can occur by SimplePropertyRetriever.theGetMethodYouWant(obj).includes(prop)
  • Iteration can occur by SimplePropertyRetriever.theGetMethodYouWant(obj).forEach((value, prop) => {}); (or use filter(), map(), etc.)
js
const SimplePropertyRetriever = {
  getOwnEnumProps(obj) {
    return this._getPropertyNames(obj, true, false, this._enumerable);
    // Or could use for...in filtered with Object.hasOwn or just this: return Object.keys(obj);
  },
  getOwnNonEnumProps(obj) {
    return this._getPropertyNames(obj, true, false, this._notEnumerable);
  },
  getOwnProps(obj) {
    return this._getPropertyNames(
      obj,
      true,
      false,
      this._enumerableAndNotEnumerable,
    );
    // Or just use: return Object.getOwnPropertyNames(obj);
  },
  getPrototypeEnumProps(obj) {
    return this._getPropertyNames(obj, false, true, this._enumerable);
  },
  getPrototypeNonEnumProps(obj) {
    return this._getPropertyNames(obj, false, true, this._notEnumerable);
  },
  getPrototypeProps(obj) {
    return this._getPropertyNames(
      obj,
      false,
      true,
      this._enumerableAndNotEnumerable,
    );
  },
  getOwnAndPrototypeEnumProps(obj) {
    return this._getPropertyNames(obj, true, true, this._enumerable);
    // Or could use unfiltered for...in
  },
  getOwnAndPrototypeNonEnumProps(obj) {
    return this._getPropertyNames(obj, true, true, this._notEnumerable);
  },
  getOwnAndPrototypeEnumAndNonEnumProps(obj) {
    return this._getPropertyNames(
      obj,
      true,
      true,
      this._enumerableAndNotEnumerable,
    );
  },
  // Private static property checker callbacks
  _enumerable(obj, prop) {
    return Object.prototype.propertyIsEnumerable.call(obj, prop);
  },
  _notEnumerable(obj, prop) {
    return !Object.prototype.propertyIsEnumerable.call(obj, prop);
  },
  _enumerableAndNotEnumerable(obj, prop) {
    return true;
  },
  // Inspired by http://stackoverflow.com/a/8024294/271577
  _getPropertyNames(obj, iterateSelf, iteratePrototype, shouldInclude) {
    const props = [];
    do {
      if (iterateSelf) {
        Object.getOwnPropertyNames(obj).forEach((prop) => {
          if (props.indexOf(prop) === -1 && shouldInclude(obj, prop)) {
            props.push(prop);
          }
        });
      }
      if (!iteratePrototype) {
        break;
      }
      iterateSelf = true;
      obj = Object.getPrototypeOf(obj);
    } while (obj);
    return props;
  },
};

See also