Is Object.prototype.toString a function or a property?

Question

Before asking, I have tried to do my homework and to avoid a duplicate. Thus, I have read about 20 questions and answers (mainly on SO) which all deal with toString(). But unfortunately, none of them did answer my actual question. So here we go ...

Many examples contain code like that:

Object.prototype.toString.call(someVariable);

I just would like to know why toString can be used like a property here. I have read the reference for Object.prototype at MDN and other places. All of them list a function toString() among the members of Object.prototype, but no property toString.

Furthermore, I am using a line like that shown above at several places in my code. For testing purposes, I have added parentheses to make it "clean":

Object.prototype.toString().call(someVariable);

Obviously, that did not make it "clean", but just made it return wrong results or even made the browser stall (I am currently in the process of researching what exactly is going on).

I already have read some questions and answers regarding calling functions without parentheses. But even if I accept that the first of the code lines shown above actually calls a function (although it looks like accessing a property), that still does not explain why it goes wrong horribly when I add the parentheses as shown in the second code line. Being able to call functions without parentheses should not mean being unable to call them with parentheses, should it?

I don't think that question has an answer already (if yes, I apologize), so could anybody please give a short explanation?

Answer 1

Is Object.prototype.toString a function or a property?

Object.prototype.toString is a property. The value of that property is a reference to a function. Exactly like this:

var obj = {f: function() { } };

There, obj.f is a property, the value of which is a reference to a function.

The initial value of Object.prototype.toString is the intrinsic function known in the spec as %ObjProto_toString%. It can be overwritten, but doing so would like break a lot of things.

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The thing to remember is that in JavaScript, functions are just objects that inherit from Function.prototype* and are callable. Just like other objects, you can keep references to them in properties and variables (you can even add properties to functions themselves), pass those references around, etc. This is in marked contrast to many languages which treat "classes" and methods and other kinds of functions as special, non-object things.

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* (host-provided objects aren't required to inherit from Function.prototype, but in modern environments most do; in some obsolete browsers, some don't.)