What is property in hasOwnProperty in JavaScript?

Consider:

if (someVar.hasOwnProperty('someProperty') ) {
 // Do something();
} else {
 // Do somethingElse();
}

What is the right use/explanation of hasOwnProperty('someProperty')?

Why can't we simply use someVar.someProperty to check if an object someVar contains property with name someProperty?

What is a property in this case?

What property does this JavaScript check?

hasOwnProperty returns a boolean value indicating whether the object on which you are calling it has a property with the name of the argument. For example:

var x = {
    y: 10
};
console.log(x.hasOwnProperty("y")); //true
console.log(x.hasOwnProperty("z")); //false

However, it does not look at the prototype chain of the object.

It's useful to use it when you enumerate the properties of an object with the for...in construct.

If you want to see the full details, the ES5 specification is, as always, a good place to look.

Here is a short and precise answer:

In JavaScript, every object has a bunch of built-in key-value pairs that have meta information about the object. When you loop through all the key-value pairs using for...in construct/loop for an object you're looping through this meta-information key-value pairs too (which you definitely don't want).

Using hasOwnPropery(property) filters-out these unnecessary looping through meta information and directly checks that is the parameter property is a user-given property in the object or not. By filters-out, I mean, that hasOwnProperty(property) does not look if, property exists in Object's prototype chain aka meta information.

It returns boolean true/false based on that.

Here is an example:

var fruitObject = {"name": "Apple", "shape": "round", "taste": "sweet"};
console.log(fruitObject.hasOwnProperty("name"));  //true
console.log(Object.prototype.hasOwnProperty("toString");) //true because in above snapshot you can see, that there is a function toString in meta-information

I hope it's clear!

It checks:

Returns a Boolean value indicating whether an object has a property with the specified name

The hasOwnProperty method returns true if the object has a property of the specified name, false if it does not. This method does not check if the property exists in the object's prototype chain; the property must be a member of the object itself.

Example:

var s = new String("Sample");
document.write(s.hasOwnProperty("split"));                        //false
document.write(String.prototype.hasOwnProperty("split"));         //true