Javascript function properties

Question

Why is it that I can do the following in javascript:

function a() {};
a.foo = 30;

Specifically, why can I set a property on the function a? After all, I cannot do this:

var a = 20;
a.foo = 30;

Answer 1

You really can't do this because it's a syntax error

function a = function() {};

I suppose you simply want to say:

function a() {}

Anyway. The reason that you cannot get a property out of a number, is that it is not a real Object.

a = 20;
a.foo = 30;   // this works
alert(a.foo); // this alerts nothing

Believe it or not, the same goes for strings:

a = "ohai";
a.foo = 30;   // this works
alert(a.foo); // this alerts nothing

However if it's String object, then it works as expected:

a = new String("ohai");
a.foo = 30;   // this works
alert(a.foo); // this alerts 30

Or if it's an Number object. You get the point.

String and number literals are not objects in Javascript. That's the reason.

Answer 2

In JavaScript the dot (.) operator expects it's left value to be an object. And in JavaScript, functions are objects.

Basically, in JavaScript, there are four main datatypes:

1. Number
2. String
3. Boolean
4. Object

Objects encompasses functions, arrays, date objects, and (for lack of a better word) regular objects. The function object is unique in that it contains executable code, and can be invoked.

Numbers are primitives, and thus you cannot access/assign properties to/from them.