Clarify my understanding of a bit of JavaScript

Question

I am working through a book that is all about functional javascript. In that book, there is the following bit of code:

const forEachObj = (obj,fn) =>{
    for(let prop in obj){
        if(obj.hasOwnProperty(prop)){
            fn(prop,obj[prop]);
        }
    }
};

I understand this as going through each property of an object then asking IF that object has that property then running a provided function.

What I don't understand is how the if does anything. Won't it always be true? Isn't it just asking if the prop it got from obj is a property of the obj?

Answer 1

From the MDN

The hasOwnProperty() method returns a boolean indicating whether the object has the specified property as its own property (as opposed to inheriting it).

Lots of properties (toString, for example) are inherited but likely not something you would want to iterate over when checking keys defined on an object.