This article is going to uncover the usage of Javascript in
operator. in
operator is one of the keywords in Javascript. We use it very often in loops or if conditions.
in
operator can check if something exists in the Object. It returnstrue
when the first operand is in the Object as a property or exists in the prototype chain.
1. Simple basic usage
Object property check
The follow example returns true
because variable obj
has all of them.
let obj = {
javascript: 1,
html: 2,
css: 3
}
'javascript' in obj // true
'html' in obj // true
'css' in obj // true
Prorotype chain
It is false
because python
does not exist in obj
but if we could add it to the prototype chain the result will turn to true
'python' in obj // false
obj.prototype.python = 4
'python' in obj // true
Deleted/Undefined Property
When a property is deleted from obj
, it returns false
because we remove the property completely.
delete obj.javascript
'javascript' in obj // false
But if we sett the property undefined
, the in
operator returns true because the property exists it just doesn't have any value.
obj.javascript = undefined
'javascript' in obj // true
2. Other usages
Other than using in
operator in an hashmap Object, we could also apply it to other situations like String
Array
and some other supported behaviors in Javascript.
If String is in another String
let str = 'string'
'string' in str // true
'str' in str // true
'ing' in str // true
'something else' in str // false
If an index is in an Array
There are only 3 elements in arr
, so the fourth element witn index 3
is out of range.
let arr = ['javascript', 'in', 'operator']
0 in arr // true
1 in arr // true
2 in arr // true
3 in arr // false
Non-inherited property is in Object
obj
is inheriting Object
. But the native function of toString
is an inherited property. To Check if non-inherited property is in Object we need to use Object.property.hasOwnProperty() method.
let obj = {}
'toString' in obj // true
obj.hasOwnProperty('toString') // false
References:
- https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/hasOwnProperty
- https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/in
- https://dmitripavlutin.com/own-and-inherited-properties-in-javascript/
Originally on pitayan.com
https://pitayan.com/posts/javascript-in-operator/
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