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.
inoperator can check if something exists in the Object. It returnstruewhen 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
