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The hasOwnProperty()
method returns a boolean indicating whether the object has the specified property.
Syntax
obj.hasOwnProperty(prop)
Parameters
Return value
A Boolean
indicating whether or not the object has the specified property.
Description
Every object descended from Object
inherits the hasOwnProperty
method. This method can be used to determine whether an object has the specified property as a direct property of that object; unlike the in
operator, this method does not check down the object's prototype chain.
Examples
Using hasOwnProperty
to test for a property's existence
The following example determines whether the o
object contains a property named prop
:
o = new Object(); o.prop = 'exists'; function changeO() { o.newprop = o.prop; delete o.prop; } o.hasOwnProperty('prop'); // returns true changeO(); o.hasOwnProperty('prop'); // returns false
Direct versus inherited properties
The following example differentiates between direct properties and properties inherited through the prototype chain:
o = new Object(); o.prop = 'exists'; o.hasOwnProperty('prop'); // returns true o.hasOwnProperty('toString'); // returns false o.hasOwnProperty('hasOwnProperty'); // returns false
Iterating over the properties of an object
The following example shows how to iterate over the properties of an object without executing on inherit properties. Note that the for...in
loop is already only iterating enumerable items, so one should not assume based on the lack of non-enumerable properties shown in the loop that hasOwnProperty
itself is confined strictly to enumerable items (as with Object.getOwnPropertyNames()
).
var buz = { fog: 'stack' }; for (var name in buz) { if (buz.hasOwnProperty(name)) { console.log('this is fog (' + name + ') for sure. Value: ' + buz[name]); } else { console.log(name); // toString or something else } }
Using hasOwnProperty
as a property name
JavaScript does not protect the property name hasOwnProperty
; thus, if the possibility exists that an object might have a property with this name, it is necessary to use an external hasOwnProperty
to get correct results:
var foo = { hasOwnProperty: function() { return false; }, bar: 'Here be dragons' }; foo.hasOwnProperty('bar'); // always returns false // Use another Object's hasOwnProperty and call it with 'this' set to foo ({}).hasOwnProperty.call(foo, 'bar'); // true // It's also possible to use the hasOwnProperty property from the Object prototype for this purpose Object.prototype.hasOwnProperty.call(foo, 'bar'); // true
Note that in the last case there are no newly created objects.
Specifications
Specification | Status | Comment |
---|---|---|
ECMAScript 3rd Edition (ECMA-262) | Standard | Initial definition. Implemented in JavaScript 1.5. |
ECMAScript 5.1 (ECMA-262) The definition of 'Object.prototype.hasOwnProperty' in that specification. |
Standard | |
ECMAScript 2015 (6th Edition, ECMA-262) The definition of 'Object.prototype.hasOwnProperty' in that specification. |
Standard | |
ECMAScript 2017 Draft (ECMA-262) The definition of 'Object.prototype.hasOwnProperty' in that specification. |
Draft |
Browser compatibility
Feature | Chrome | Edge | Firefox (Gecko) | Internet Explorer | Opera | Safari |
---|---|---|---|---|---|---|
Basic support | (Yes) | (Yes) | (Yes) | (Yes) | (Yes) | (Yes) |
Feature | Android | Chrome for Android | Edge | Firefox Mobile (Gecko) | Firefox OS | IE Mobile | Opera Mobile | iOS WebKit (Safari/Chrome/Firefox/etc) |
---|---|---|---|---|---|---|---|---|
Basic support | (Yes) | (Yes) | (Yes) | (Yes) | (Yes) | (Yes) | (Yes) | (Yes) |