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?
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