How can dynmically change a line of a JavaScript function?

Question

Suppose I had the following function:

function alertMesg()
{
   alert("This ok function alerts message!");
}

Now at run time I would like to change the alertMesg function to do something else. My thought was to do somehting like this.

var temp = window.alertMesg.toString.replace("ok","great")
temp = temp.replace('function alertMesg()',"");
window.alertMesg = new Function(temp);

Basically, the problem is I have no control over the source in the alertMesg function. I would like to change the function, but I can't actually change the source of it because it is produced server side. That being said, I need it to act differently.

PS: I forgot to mention an important part: I have to keep most of the function. I can't just replace the function out right. I have to keep 95% of the function the way it is, and change the other five percent.

@Barlow Tucker, quixoto, pekka Thanks, for the interest.

Basically, I don't think the proxy idea will work because I am not just adding functionality, I am changing the functionality of the code. I want for example, the third line of the function to be different. In my real life example I have to add a line right in the middle of a function.

Answer 1

If you must replace the content of a function, it is possible:

function alertMesg()
{
   alert ("This ok function alerts my message!");
}

alertMesg(); // alerts "This ok function alerts my message!"

// do whatever you want to the function here
var temp = alertMesg.toString();
temp = temp.replace('my', 'your');

// now replace the original function
alertMesg=new Function(temp.substring(temp.indexOf('{')+1,temp.lastIndexOf('}')));

alertMesg(); // alerts "This ok function alerts your message!"

This probably isn't the best way to do what you're trying to achieve, but I can't really suggest anything else unless you provide more details.

Answer 2

Dynamic code replacement like you're suggesting might work in some cases, but it's a scary road to go down-- fragile, one wrong step and you're busted, and it quickly becomes a maintenance nightmare.

As a commenter said, your cleaner bet is to just wrap the native window.alert and then do the right thing when the string you care about comes through, per the answer here: JavaScript: Overriding alert()

(Insert standard comment here about how you should get your server side people on the same page/team as you on this so you don't need to hack around your own page.)

UPDATE: You're not asking about alert, you're asking about this problem generally. Yes, you could what others are suggesting. But if you have the original code for the function, why not just replace it entirely? If the function you want is a global function called foo(), you can run JS that does:

window.foo = function() {
    // The stuff I know is there (A)
    ...
    // Some new stuff I want to change (B)
    ...
    // More stuff I know is there (C)
}

Which will throw away the original and replace it with your version. This would work reasonably well, although "monkey patching" the stuff in the page definitely comes with some maintenance headaches.

I will definitely note here that if you can't do this for some reason and thus insist on doing textual code replacement in the middle of existing functions, you're abusing the language/environment for the purposes of maintainable anything, and you are truly screwed in multiple ways.