Enumerability and ownership of properties - JavaScript 编辑

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. There are a number of built-in means of detecting, iterating/enumerating, and retrieving object properties, with the chart showing below which are available. Some sample code follows which demonstrates how to obtain the missing categories.

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

See also

• in
• for..in
• Object.hasOwnProperty()
• Object.propertyIsEnumerable()
• Object.getOwnPropertyNames()
• Object.keys()
• Object.getOwnPropertyDescriptors()