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

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 */

Use a temporary variable:

var xxx = function()...

then copy all the properties from the original object:

for (var p in bar) { xxx[p] = bar[p]; }

finally reassign the new function with the old properties to the original variable:

bar = xxx;

var A = function(foo) {                                                                                                      
  var B = function() {                                                                                                       
    return A.prototype.constructor.apply(B, arguments);
  };
  B.prototype = A.prototype;                                                                                                 
  return B;                                                                                                                  
}; 

NB: Post written in the style of how I solved the issue. I'm not 100% sure it is usable in the OP's case.

I found this post while looking for a way to convert objects created on the server and delivered to the client by JSON / ajax.

Which effectively left me in the same situation as the OP, an object that I wanted to be convert into a function so as to be able to create instances of it on the client.

In the end I came up with this, which is working (so far at least):

var parentObj = {}

parentObj.createFunc = function (model)
{
    // allow it to be instantiated
    parentObj[model._type] = function()
    {
        return (function (model)
        {
            // jQuery used to clone the model
            var that = $.extend(true, null, model);
            return that;
        })(model);
    }
}

Which can then be used like:

var data = { _type: "Example", foo: "bar" };
parentObject.createFunc(data);
var instance = new parentObject.Example();

In my case I actually wanted to have functions associated with the resulting object instances, and also be able to pass in parameters at the time of instantiating it.

So my code was:

var parentObj = {};
// base model contains client only stuff
parentObj.baseModel =
{
    parameter1: null,
    parameter2: null,
    parameterN: null, 
    func1: function ()
    {
        return this.parameter2;
    },
    func2: function (inParams) 
    {
        return this._variable2;
    }
}

// create a troop type
parentObj.createModel = function (data)
{
    var model = $.extend({}, parentObj.baseModel, data);
    // allow it to be instantiated
    parentObj[model._type] = function(parameter1, parameter2, parameterN)
    {
        return (function (model)
        {
            var that = $.extend(true, null, model);
            that.parameter1 = parameter1;
            that.parameter2 = parameter2;
            that.parameterN = parameterN;
            return that;
        })(model);
    }
}

And was called thus:

// models received from an AJAX call
var models = [
{ _type="Foo", _variable1: "FooVal", _variable2: "FooVal" },
{ _type="Bar", _variable1: "BarVal", _variable2: "BarVal" },
{ _type="FooBar", _variable1: "FooBarVal", _variable2: "FooBarVal" }
];

for(var i = 0; i < models.length; i++)
{
    parentObj.createFunc(models[i]);
}

And then they can be used:

var test1 = new parentObj.Foo(1,2,3);
var test2 = new parentObj.Bar("a","b","c");
var test3 = new parentObj.FooBar("x","y","z");

// test1.parameter1 == 1
// test1._variable1 == "FooVal"
// test1.func1() == 2

// test2.parameter2 == "a"
// test2._variable2 == "BarVal"
// test2.func2() == "BarVal"

// etc

Here's easiest way to do this that I've found:

let bar = { baz: "qqqq" };
bar = Object.assign(() => console.log("do something"), bar)

This uses Object.assign to concisely make copies of all the the properties of bar onto a function.

Alternatively you could use some proxy magic.

JavaScript allows functions to be treated as objects--you can add a property to a function. How do you do the reverse, and add a function to an object?

You appear to be a bit confused. Functions, in JavaScript, are objects. And variables are variable. You wouldn't expect this to work:

var three = 3;
three = 4;
assert(three === 3);

...so why would you expect that assigning a function to your variable would somehow preserve its previous value? Perhaps some annotations will clarify things for you:

// assigns an anonymous function to the variable "foo"
var foo = function() { return 1; }; 
// assigns a string to the property "baz" on the object 
// referenced by "foo" (which, in this case, happens to be a function)
foo.baz = "qqqq";

var bar = { 
    baz: "qqqq",
    runFunc: function() {
        return 1;
    }
};

alert(bar.baz); // should produce qqqq
alert(bar.runFunc()); // should produce 1

I think you're looking for this.

can also be written like this:

function Bar() {
    this.baz = "qqqq";
    this.runFunc = function() {
        return 1;
    }
}

nBar = new Bar(); 

alert(nBar.baz); // should produce qqqq
alert(nBar.runFunc()); // should produce 1