Can you alter a Javascript function after declaring it?

Let's say I have var a = function() { return 1; }. Is it possible to alter a so that a() returns 2? Perhaps by editing a property of the a object, since every function is an object?

Update: Wow, thanks for all the responses. However, I'm afraid I wasn't looking to simply reassign a variable but actually edit an existing function. I am thinking along the lines of how you can combine partial functions in Scala to create a new PartialFunction. I am interested in writing something similar in Javascript and was thinking that the existing function could perhaps be updated, rather than creating an entirely new Function object.

You can do all kinds of fun stuff with javascript, including redefining functions:

let a = function() { return 1; }
console.log(a()); // 1
    
// keep a reference
let old = a;
   
// redefine
a = function() {
  // call the original function with any arguments specified, storing the result
  const originalResult = old.apply(old, arguments);
  // add one
  return originalResult + 1;
};

console.log(a()); // 2

Voila.

Edit: Updated to show this in a crazier scenario:

let test = new String("123");
console.log(test.toString()); // logs 123
console.log(test.substring(0)); // logs 123
String.prototype.substring = function(){ return "hahanope"; }
console.log(test.substring(0)); // logs hahanope

You can see here that even though "test" is defined first, and we redefine substring() afterwards, the change still applies.

Side note: you really should reconsider your architecture if you're doing this...you're going to confuse the crap out of some poor developer 5 years down the road when s/he's looking at a function definition that's supposed to return 1, but seems to always return 2....

I used something like this to modify an existing function whose declaration was not accessible to me:

// declare function foo
var foo = function (a) { alert(a); };

// modify function foo
foo = new Function (
  "a",
  foo.toSource()
    .replace("alert(a)", "alert('function modified - ' + a)")
    .replace(/^function[^{]+{/i,"")  // remove everything up to and including the first curly bracket
    .replace(/}[^}]*$/i, "")  // remove last curly bracket and everything after<br>
);

Instead of toSource() you could probably use toString() to get a string containing the function's declaration. Some calls to replace() to prepare the string for use with the Function Constructor and to modify the function's source.

So you want to modify the code of a function directly, in place, and not just reassign a different function to an existing variable.

I hate to say it, but as far as I have been able to figure it out - and I have tried -, it can't be done. True, a function is an object, and as such it has methods and properties which can be tweaked and overwritten on the object itself. Unfortunately, the function body is not one of them. It is not assigned to a public property.

The documentation on MDN lists the properties and methods of the function object. None of them gives us the opportunity to manipulate the function body from the outside.

That's because according to the spec, the function body is stored in the internal [[Code]] property of the function object, which can't be accessed directly.

let a = function() { return 1; }
console.log(a()) // 1

a = function() { return 2; }
console.log(a()) // 2

technically, you're losing one function definition and replacing it with another.

How about this, without having to redefine the function:

var a = function() { return arguments.callee.value || 1; };
alert(a()); // => 1
a.value = 2;
alert(a()); // => 2

I am sticking to jvenema's solution, in which I don't like the global variable "old". It seems better to keep the old function inside of the new one:

function a() { return 1; }

// redefine
a = (function(){
  var _a = a;
  return function() {
  // You may reuse the original function ...
  // Typical case: Conditionally use old/new behaviour
    var originalResult = _a.apply(this, arguments);
  // ... and modify the logic in any way
    return originalResult + 1;
    }
})();
a()  // --> gives 2

All feasible solutions stick to a "function wrapping approach". The most reliable amongst them seems to be the one of rplantiko.

Such function wrapping easily can be abstracted away. The concept / pattern itself might be called "Method Modification". Its implementation definitely belongs to Function.prototype. It would be nice to be backed one day by standard prototypal method modifiers like before, after, around, afterThrowing and afterFinally.

As for the aforementioned example by rplantiko ...

function a () { return 1; }

// redefine
a = (function () {
  var _a = a;
  return function () {
    // You may reuse the original function ...
    // Typical case: Conditionally use old/new behaviour
    var originalResult = _a.apply(this, arguments);
    // ... and modify the logic in any way
    return originalResult + 1;
  };
})();

console.log('a() ...', a()); // --> gives 2

.as-console-wrapper { min-height: 100%!important; top: 0; }

... and making use of around, the code would transform to ...

function a () { return 1; }

console.log('original a ...', a);
console.log('a() ...', a()); // 1


a = a.around(function (proceed, handler, args) {
  return (proceed() + 1);
});

console.log('\nmodified a ...', a);
console.log('a() ...', a()); // 2

.as-console-wrapper { min-height: 100%!important; top: 0; }

<script>
(function(d){function f(a){return typeof a==e&&typeof a.call==e&&typeof a.apply==e}function g(a,b){b=null!=b&&b||null;var c=this;return f(a)&&f(c)&&function(){return a.call(b||null!=this&&this||null,c,a,arguments)}||c}var e=typeof d;Object.defineProperty(d.prototype,"around",{configurable:!0,writable:!0,value:g});Object.defineProperty(d,"around",{configurable:!0,writable:!0,value:function(a,b,c){return g.call(a,b,c)}})})(Function);
</script>

This is a Clear Example based on a control timepicker eworld.ui www.eworldui.net

Having a TimePicker eworld.ui where JavaScript is unreachable from outside, you can't find any js related to those controls. So how can you add a onchange event to the timepicker ?

There is a js function called when you Select a time between all the options that the control offer you. This function is: TimePicker_Up_SelectTime

First you have to copy the code inside this function.

Evaluate...quikwatch...TimePicker_Up_SelectTime.toString()

function TimePicker_Up_SelectTime(tbName, lblName, divName, selTime, enableHide, postbackFunc, customFunc) {
    document.getElementById(tbName).value = selTime;
    if(lblName != '')
        document.getElementById(lblName).innerHTML = selTime;
    document.getElementById(divName).style.visibility = 'hidden';
    if(enableHide)
        TimePicker_Up_ShowHideDDL('visible');
    if(customFunc != "")
        eval(customFunc + "('" + selTime + "', '" + tbName + "');");
    eval(postbackFunc + "();");
}

Now

Using the code that you have saved before reassign the same source code but add whatever you want..

TimePicker_Up_SelectTime = function (tbName, lblName, divName, selTime, enableHide, postbackFunc, customFunc) {
    document.getElementById(tbName).value = selTime;
    if (lblName != '')
        document.getElementById(lblName).innerHTML = selTime;
    document.getElementById(divName).style.visibility = 'hidden';
    if (enableHide)
        TimePicker_Up_ShowHideDDL('visible');
    if (customFunc != "")
        eval(customFunc + "('" + selTime + "', '" + tbName + "');");
    eval(postbackFunc + "();");
 
    >>>>>>>  My function  >>>>>   RaiseChange(tbName);
}

I've added My Function to the function so now I can simulate an onchange event when I select a time.

RaiseChange(...) could be whatever you want.

If you're debugging javascript and want to see how changes to the code affects the page, you can use this Firefox extension to view/alter javascripts:

Execute JS firefox extension: https://addons.mozilla.org/en-US/firefox/addon/1729

You can change functions like other objects

var a1 = function(){return 1;}
var b1 = a1;
a1 = function(){
  return b1() + 1;
};
console.log(a1()); // return 2

// OR:
function a2(){return 1;}
var b2 = a2;
a2 = function(){
  return b2() + 1;
};
console.log(a2()); // return 2

const createFunction = function (defaultRealization) {
  let realization = defaultRealization;

  const youFunction = function (...args) {
    return realization(...args);
  };
  youFunction.alterRealization = function (fn) {
    realization = fn;
  };

  return youFunction;
}

const myFunction = createFunction(function () { return 1; });
console.log(myFunction()); // 1

myFunction.alterRealization(function () { return 2; });
console.log(myFunction()); // 2

Absolutely. Just assign to it a new function.

Can you not just define it again later on? When you want the change try just redefining it as:

a = function() { return 2; }