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.

• Peter Ajtai: there's Arrays too... they have different native prototypes than Objects (note that an Array will return a typeof as an object, but if you check its constructor you will see that it is in fact Array).
• user123444555621: @Peter: Still, Arrays are objects. The ECMA standard defines "Array objects", "Function objects", ... and "Object objects"

Answer 3

In Javascript, a function is an object, so you can set properties on it:

function a() {};
a.foo = 30;
a.foo; // evaluates to 30

A number literal however creates a primitive value (which has no properties) and not an object.

a = 20; // Create primitive value using number literal

When you set a property on a primitive value, in fact you create a wrapper object around the primitive value and set the property on the wrapper object, not the primitive value.

a.foo = 30; // wrapper object created and directly thrown away
(new Number(a)).foo = 30; // --> equivalent

When you read the property, again you create a wrapper object, whihch hasn't the property defined.

a.foo; // wrapper object created, has no foo property
(new Number(a)).foo; // --> equivalent, evaluates to undefined

• danday74: In Javascript, a function is an object, so you can set properties on it ... nice!

Answer 4

In javascript functions are objects. Or they inherit from objects, or something.

try doing

a = function(){};
alert(typeof a.prototype);