Odd javascript behavior for checking “constructor” key in object

Question

I am actually not sure if I just stumbled upon an unwanted behavior in javascript or if this is somehow intended behavior.

The following code results in a true statement:

var test= {"test":1}
document.write("constructor" in test);    

http://jsfiddle.net/xyatxm2g/2/

If I change it to the following code, it returns false as it should:

var test= {"test":1}
document.write(test.hasOwnProperty("constructor"));

http://jsfiddle.net/fg06ovvc/2/

Answer 1

The hasOwnProperty method , as the name says, look into the object to see if it has the property itself.

But when you use 'propertyName' in test , you're not only looking into the object's own properties, but also the properties that come from inheritance.

In that case, constructor is a property that resides inside the Object 's prototype , so all objects have that property, because they all inherit from Object .

Quote from MDN

Every object descended from Object inherits the hasOwnProperty method. This method can be used to determine whether an object has the specified property as a direct property of that object; unlike the in operator , this method does not check down the object's prototype chain.

Answer 2

From the MDN documentation :

Inherited properties
The in operator returns true for properties in the prototype chain.
"toString" in {}; // returns true

Whereas the hasOwnProperty() method only checks for properties directly on the object, not inherited (ie not on the prototype chain).

Answer 3

Following MDN documentation, it is not an enumerable field.

https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/Object/propertyIsEnumerable

You can perform following test:

var obj = {"t": 23};
obj.propertyIsEnumerable("t")

results: true

obj.propertyIsEnumerable("constructor")

results: false

There is a complete example on this document, under section:

Direct versus inherited properties

Answer 4

I think it may be normal behaviour here that the key in object operator is searching through the prototype chain and returning true for Object.prototype.constructor . See this discussion - it goes over a related topic.

How do I check if an object has a property in JavaScript?