How to convert an "object" into a function in JavaScript?

JavaScript allows functions to be treated as objects--if you first define a variable as a function, you can subsequently add properties to that function. How do you do the reverse, and add a function to an "object"?

This works:

var foo = function() { return 1; };
foo.baz = "qqqq";

At this point, foo() calls the function, and foo.baz has the value "qqqq".

However, if you do the property assignment part first, how do you subsequently assign a function to the variable?

var bar = { baz: "qqqq" };

What can I do now to arrange for bar.baz to have the value "qqqq" and bar() to call the function?

It's easy to be confused here, but you can't (easily or clearly or as far as I know) do what you want. Hopefully this will help clear things up.

First, every object in Javascript inherits from the Object object.

//these do the same thing
var foo = new Object();
var bar = {};

Second, functions ARE objects in Javascript. Specifically, they're a Function object. The Function object inherits from the Object object. Checkout the Function constructor

var foo = new Function();
var bar = function(){};
function baz(){};

Once you declare a variable to be an "Object" you can't (easily or clearly or as far as I know) convert it to a Function object. You'd need to declare a new Object of type Function (with the function constructor, assigning a variable an anonymous function etc.), and copy over any properties of methods from your old object.

Finally, anticipating a possible question, even once something is declared as a function, you can't (as far as I know) change the functionBody/source.

There doesn't appear to be a standard way to do it, but this works.

WHY however, is the question.

function functionize( obj , func )
{ 
   out = func; 
   for( i in obj ){ out[i] = obj[i]; } ; 
   return out; 
}

x = { a: 1, b: 2 }; 
x = functionize( x , function(){ return "hello world"; } );
x()   ==> "hello world"

There is simply no other way to acheive this, doing

x={}
x()

WILL return a "type error". because "x" is an "object" and you can't change it. its about as sensible as trying to do

 x = 1
 x[50] = 5
 print x[50]

it won't work. 1 is an integer. integers don't have array methods. you can't make it.

Object types are functions and an object itself is a function instantiation.

alert([Array, Boolean, Date, Function, Number, Object, RegExp, String].join('\n\n'))

displays (in FireFox):

function Array() {
    [native code]
}

function Boolean() {
    [native code]
}

function Date() {
    [native code]
}

function Function() {
    [native code]
}

function Number() {
    [native code]
}

function Object() {
    [native code]
}

function RegExp() {
    [native code]
}

function String() {
    [native code]
}

In particular, note a Function object, function Function() { [native code] }, is defined as a recurrence relation (a recursive definition using itself).

Also, note that the answer 124402#124402 is incomplete regarding 1[50]=5. This DOES assign a property to a Number object and IS valid Javascript. Observe,

alert([
  [].prop="a",
  true.sna="fu",
  (new Date()).tar="fu",
  function(){}.fu="bar",
  123[40]=4,
  {}.forty=2,
  /(?:)/.forty2="life",
  "abc".def="ghi"
].join("\t"))

displays

a   fu  fu  bar 4   2   life    ghi

interpreting and executing correctly according to Javascript's "Rules of Engagement".

Of course there is always a wrinkle and manifest by =. An object is often "short-circuited" to its value instead of a full fledged entity when assigned to a variable. This is an issue with Boolean objects and boolean values.

Explicit object identification resolves this issue.

x=new Number(1);  x[50]=5;  alert(x[50]);

"Overloading" is quite a legitimate Javascript exercise and explicitly endorsed with mechanisms like prototyping though code obfuscation can be a hazard.

Final note:

alert(  123 . x = "not"  );

alert( (123). x = "Yes!" );  /* ()'s elevate to full object status */

How to convert an "object" into a function in JavaScript?

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