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;

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;

• javascript

4 Answers 4

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.

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 enpasses 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.

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

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

try doing

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