JavaScript: for...in Loop

In javascript, a for-in loop can perform an advanced loop and fetches all the keys in a property and all the indexes in an array.
Typically we can fetch the value from a property through an everyday notation object[variable].

Syntax

for(variable in object)
{
    //condition
}

Example:

let details = 
{
    firstName : "Stephen",
    lastName : "Strange",
    occupation : "Sorcerer Supreme",
    gender : "male"
}

for(let finalDetail in details)
{
    console.log(finalDetail+ " : " +details[finalDetail])
}

Output:

firstName : Stephen
lastName : Strange
occupation : Sorcerer Supreme
gender : Male

Here we decided to create an object with properties of key-value pair.
We are using the for in to fetch the values of the properties.
So first of all, we are declaring a variable of our own called finalDetail it can be any name we want.
We are pointing that variable to the object details.
So now, obviously, when we try to print finalDetail, it'll fetch all the keys from the property because for in does that, and it is its sole purpose to do that.
When we try to use the notation of object[variable]/details[finalDetail] and print it, it'll fetch all the fundamental values, as you can witness from the code.

let infinityStones = ['Soul', 'Time', 'Reality', 'Power', 'Mind', 'Space']

for(let stones in infinityStones)
{
    console.log(stones)
}

Output:

As you can see, when we try to use for in in an array, it fetches and returns the indexes of each element present in the array.