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hasOwnProperty()

2016-06-13 15:21 399 查看

The

hasOwnProperty()

method
returns a boolean indicating whether the object has the specified property.

Syntax

obj.hasOwnProperty(prop)

Parameters

prop

The name of the property to test.

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

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

Desktop

Mobile

Feature	Chrome	Firefox (Gecko)	Internet Explorer	Opera	Safari
Basic support	(Yes)	(Yes)	(Yes)	(Yes)	(Yes)