The "instanceof" operator in JavaScript
The instanceof allows to check if the object has been created by the given constructor or not:
For example:
function Abhi() { }
var abhijeet = new Abhi
alert(abhijeet instanceof Abhi) // true
It follows the __proto__ chain instead.
The logic behind obj instanceof F:
1. Get obj.__proto__
2. Compare obj.__proto__ against F.prototype
3. If no match then set temporarily obj = obj.__proto__ and repeat step 2 until either match is found or the chain ends.
In the above example, the match is found at the first step,
because: abhijeet.__proto__ == Abhi.prototype
Another example:
function Abhi() { }
var abhijeet = new Abhi
alert(abhijeet instanceof Object) // true
Here we can see that the match is found at abhijeet.__proto__.__proto__ == Object.prototype.
Note that the comparison only uses __proto__ and prototype, the function object itself isn’t involved.