Can you alter a Javascript function after declaring it?

Question

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.

Answer 1

var a = function() { return 1; }
alert(a()) // 1
a = function() { return 2; }
alert(a()) // 2

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

Answer 2

Why would you not just assign a new function to a that returned 2?

Answer 3

Found a Google link that might help: http://www.coryhudson.com/blog/2007/02/07/modify-existing-javascript-functions/

Answer 4

Absolutely. Just assign to it a new function.

Answer 5

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

a = function() { return 2; }

Answer 6

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

Answer 7

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

var a = function(){ return 1; }

alert(a()); //1

var old = a;
var a = function(){
  // call the original function, storing the result
  var originalResult = old.apply(a, arguments);
  // add one
  return originalResult + 1;
};
alert(a()); //2

Voila.

Answer 8

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

Answer 9

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.