内容简介:Due to the dynamic nature of JavaScript, you might need to verify if a specific property exists in an object.In this post, you’ll read about the 3 common ways to check whether a property exists in an object.The JavaScript object has a special method
Due to the dynamic nature of JavaScript, you might need to verify if a specific property exists in an object.
In this post, you’ll read about the 3 common ways to check whether a property exists in an object.
1. hasOwnProperty() method
The JavaScript object has a special method object.hasOwnProperty(propName)
that returns a boolean that indicates whether object
has a property propName
.
In the following example hasOwnProperty()
determines the presence of properties:
const hero = { name: 'Batman' }; hero.hasOwnProperty('name'); // => true hero.hasOwnProperty('realName'); // => false
The property name
exists in the object hero
: thus hero.hasOwnProperty('name')
returns true
.
On the other side, realName
property doesn’t exist in the object hero
. As expected, hero.hasOwnProperty('realName')
returns false
— denoting a missing property.
The method name hasOwnProperty()
suggests that it looks for properties in theown properties of the object. In simple words, the own properties are those defined directly upon the object.
Even if every JavaScript object has an inherited property toString
(which is a method inherited from the object’s prototype), hasOwnProperty()
doesn’t detect it as a property:
const hero = { name: 'Batman' }; hero.toString; // => function() {...} hero.hasOwnProperty('toString'); // => false
2. in operator
The in
operator propName in object
also determines whether propName
property exists in object
.
Let’s use in
operator to detect the existence of a property:
const hero = { name: 'Batman' }; 'name' in hero; // => true 'realName' in hero; // => false
'name' in hero
evaluates as expected to true
because hero
has a property name
.
However, hero
doesn’t have a property named 'realName'
. As a result, 'realName' in hero
evaluates to false
.
in
operator has a short syntax, and I prefer it over hasOwnProperty()
method in most of the cases.
Here’s the main difference between hasOwnProperty()
method and in
operator: the latter checks within the own properties, but also in the list of inherited properties.
That’s why, in contrast to hasOwnProperty()
, the in
operator detects that the inherited property toString
exists inside hero
object:
const hero = { name: 'Batman' }; hero.toString; // => function() {...} 'toString' in hero; // => true
3. Comparing with undefined
If you access a non-existing property from an object, the result is undefined
. Let’s try an example:
const hero = { name: 'Batman' }; hero.name; // => 'Batman' hero.realName; // => undefined
hero.realName
evaluates to undefined
because realName
property is missing.
Now you can see the idea: let’s compare against undefined
to determine the existence of the property.
const hero = { name: 'Batman' }; hero.name !== undefined; // => true hero.realName !== undefined; // => false
hero.name !== undefined
evaluates to true
, which shows the existence of property.
On the other side, hero.realName !== undefined
is false
, which indicates the realName
is missing.
I’ve been comparing with undefined
to detect the existence of property for quite some time. It’s a cheap and dirty approach.
Note that this approach can generate a false-negative. If the property exists, but has undefined
value (case, however, rarely happening), comparing against undefined
evaluates incorrectly to false
:
const hero = { name: undefined }; hero.name !== undefined; // => false
Even if the property name
exists (but has undefined
value), hero.name !== undefined
evaluates to false
: which incorrectly indicates a missing property.
4. Summary
There are mainly 3 ways to check if the property exists.
The first way is to invoke object.hasOwnProperty(propName)
. The method returns true
if the propName
exists inside object
, and false
otherwise.
Note that hasOwnProperty()
searches only within theown properties of the object.
The second approach makes use of propName in object
operator. As well, the operator evaluates to true
for an existing property, and false
otherwise.
in
operator looks for properties existence in bothown andinherited object properties.
Finally, you can simply use object.propName !== undefined
and compare against undefined
directly.
What’s your preferred way to check for properties existence?
以上所述就是小编给大家介绍的《3 Ways to Check If an Object Has a Property》,希望对大家有所帮助,如果大家有任何疑问请给我留言,小编会及时回复大家的。在此也非常感谢大家对 码农网 的支持!
猜你喜欢:本站部分资源来源于网络,本站转载出于传递更多信息之目的,版权归原作者或者来源机构所有,如转载稿涉及版权问题,请联系我们。
开发高质量PHP框架与应用的实际案例解析
Sebastian Bergmann / 刘文瀚、刘海燕 / 清华大学出版社 / 2012-6 / 49.00元
PHP已经成为最受欢迎的编程语言之一,这使得用PHP创建高质量、易维护的应用程序和框架比以往受到更多的青睐。通过使用来自于知名公司的真实案例研究,《开发高质量PHP框架与应用的实际案例解析》为Web软件体系结构的不同层次介绍了规划、执行以及测试自动化方面的内容,并解释了这些公司如何测量和测试软件质量。《开发高质量PHP框架与应用的实际案例解析》作者Sebastian Bergmann、Stefan......一起来看看 《开发高质量PHP框架与应用的实际案例解析》 这本书的介绍吧!