In this post, you'll read 3 common ways to check for property existence in a JavaScript object.
1. hasOwnProperty[] method
Every JavaScript object has a special method object.hasOwnProperty['myProp']
that returns a boolean indicating whether object
has a property myProp
.
In the
following example, hasOwnProperty[]
determines the presence of properties name
and realName
:
javascript
const hero = {
name: 'Batman'
};
hero.hasOwnProperty['name']; // => true
hero.hasOwnProperty['realName']; // => false
hero.hasOwnProperty['name']
returns true
because the property name
exists in the object hero
.
On the other side, hero
doesn't have realName
property. Thus hero.hasOwnProperty['realName']
returns false
— denoting a missing property.
The method name hasOwnProperty[]
suggests that it looks in the own
properties of the object. The own properties are those defined directly upon the object.
Because of that hasOwnProperty[]
doesn't detect the inherited toString
property:
javascript
const hero = {
name: 'Batman'
};
hero.toString; // => function[] {...}
hero.hasOwnProperty['toString']; // => false
2. in operator
'myProp' in object
also determines whether myProp
property exists in object
.
Let's use in
operator to detect
the existence of name
and realName
in hero
object:
javascript
const hero = {
name: 'Batman'
};
'name' in hero; // => true
'realName' in hero; // => false
'name' in hero
evaluates to true
because hero
has a property name
.
On the other side, 'realName' in hero
evaluates to false
because hero
doesn't have a property named 'realName'
.
in
operator has a short syntax, and I prefer it over hasOwnProperty[]
method.
The main difference between hasOwnProperty[]
method and in
operator is that the latter checks within own and inherited properties of the object.
That's why, in contrast to hasOwnProperty[]
, the in
operator detects that hero
object contains the inherited property toString
:
javascript
const hero = {
name: 'Batman'
};
hero.toString; // => function[] {...}
'toString' in hero; // => true
hero.hasOwnProperty['toString']; // => false
3. Comparing with undefined
Accessing a non-existing property from an object results in undefined
:
javascript
const hero = {
name: 'Batman'
};
hero.name; // => 'Batman'
hero.realName; // => undefined
hero.realName
evaluates
to undefined
because realName
property is missing.
Now you can see an idea: you can compare with undefined
to determine the existence of the property.
javascript
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 that realName
is missing.
Comparing with undefined
to detect the existence of property is a cheap and dirty approach.
But be aware of false-negatives. If the property exists, but has
undefined
value [case, however, rarely happening], comparing against undefined
evaluates incorrectly to false
:
javascript
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.
hasOwnProperty[]
searches only within the own properties of the object.
The second approach makes use of propName in object
operator. The operator evaluates to true
for an existing property, and false
otherwise.
in
operator looks for properties existence in both own and inherited properties.
Finally, you can simply use object.propName !== undefined
and compare against undefined
directly.
What's your preferred way to check for properties existence?