Enumerability and ownership of properties
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 default enumerable to false.
Enumerable properties show up in for...in loops unless the property's key is a Symbol. Ownership of properties is determined by whether the property belongs to the object directly and not to its prototype chain. Properties of an object can also be retrieved in total.
Detecting, retrieving, and enumerating object properties
There are a number of built-in means of detecting, iterating/enumerating, and retrieving object properties. These are summarized in the tables below.
Detection
Own object | Own object and prototype chain | Prototype chain only | |
---|---|---|---|
Enumerable | Not available without extra code | Not available without extra code | |
Nonenumerable |
|
Not available without extra code | Not available without extra code |
Enumerable and Nonenumerable | in |
Not available without extra code |
Retrieval
Own object | Own object and prototype chain | Prototype chain only | |
---|---|---|---|
Enumerable | Not available without extra code | Not available without extra code | |
Nonenumerable |
getOwnPropertyNames ,
getOwnPropertySymbols
– filtered to exclude enumerables using propertyIsEnumerable
|
Not available without extra code | Not available without extra code |
Enumerable and Nonenumerable | Not available without extra code | Not available without extra code |
Iteration
Own object | Own object and prototype chain | Prototype chain only | |
---|---|---|---|
Enumerable |
(excluding symbols) |
Not available without extra code | |
Nonenumerable |
getOwnPropertyNames ,
getOwnPropertySymbols
– filtered to exclude enumerables using propertyIsEnumerable
|
Not available without extra code | Not available without extra code |
Enumerable and Nonenumerable | Not available without extra code | Not available without extra code |
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).indexOf(prop) > -1
- Iteration can occur by
SimplePropertyRetriever.theGetMethodYouWant(obj).forEach(function (value, prop) {});
(or usefilter()
,map()
, etc.)
var SimplePropertyRetriever = {
getOwnEnumerables: function(obj) {
return this._getPropertyNames(obj, true, false, this._enumerable);
// Or could use for..in filtered with hasOwnProperty or just this: return Object.keys(obj);
},
getOwnNonenumerables: function(obj) {
return this._getPropertyNames(obj, true, false, this._notEnumerable);
},
getOwnEnumerablesAndNonenumerables: function(obj) {
return this._getPropertyNames(obj, true, false, this._enumerableAndNotEnumerable);
// Or just use: return Object.getOwnPropertyNames(obj);
},
getPrototypeEnumerables: function(obj) {
return this._getPropertyNames(obj, false, true, this._enumerable);
},
getPrototypeNonenumerables: function(obj) {
return this._getPropertyNames(obj, false, true, this._notEnumerable);
},
getPrototypeEnumerablesAndNonenumerables: function(obj) {
return this._getPropertyNames(obj, false, true, this._enumerableAndNotEnumerable);
},
getOwnAndPrototypeEnumerables: function(obj) {
return this._getPropertyNames(obj, true, true, this._enumerable);
// Or could use unfiltered for..in
},
getOwnAndPrototypeNonenumerables: function(obj) {
return this._getPropertyNames(obj, true, true, this._notEnumerable);
},
getOwnAndPrototypeEnumerablesAndNonenumerables: function(obj) {
return this._getPropertyNames(obj, true, true, this._enumerableAndNotEnumerable);
},
// Private static property checker callbacks
_enumerable: function(obj, prop) {
return obj.propertyIsEnumerable(prop);
},
_notEnumerable: function(obj, prop) {
return !obj.propertyIsEnumerable(prop);
},
_enumerableAndNotEnumerable: function(obj, prop) {
return true;
},
// Inspired by http://stackoverflow.com/a/8024294/271577
_getPropertyNames: function getAllPropertyNames(obj, iterateSelfBool, iteratePrototypeBool, includePropCb) {
var props = [];
do {
if (iterateSelfBool) {
Object.getOwnPropertyNames(obj).forEach(function(prop) {
if (props.indexOf(prop) === -1 && includePropCb(obj, prop)) {
props.push(prop);
}
});
}
if (!iteratePrototypeBool) {
break;
}
iterateSelfBool = true;
} while (obj = Object.getPrototypeOf(obj));
return props;
}
};
Detection Table
Enumerable | Nonenumerable | Symbols keys | Inherited Enumerable | Inherited Nonenumerable | Inherited Symbols keys | |
---|---|---|---|---|---|---|
in |
true | true | true | true | true | true |
for..in |
true | false | false | true | false | false |
obj.hasOwnProperty |
true | true | true | false | false | false |
obj.propertyIsEnumerable |
true | false | true | false | false | false |
Object.keys |
true | false | false | false | false | false |
Object.getOwnPropertyNames |
true | true | false | false | false | false |
Object.getOwnPropertyDescriptors |
true | true | true | false | false | false |
Reflect.ownKeys() |
true | true | true | false | false | false |