javascript: call a function in a closure scope

Question

Is there any way I can override a closure so it does part of what the original closure does? I know there's no straightforward way, but is there some hack? I'm willing to be messy...

<html>
    <head>

        // I DON'T CONTROL THIS CODE!!!
        <script>
        ;(function() {

            function _dothing() {
                alert("_dothing");
            }

            function _doit() {
                _dothing();
                alert("_doit");
            }

            window.K = { doit : _doit };

        })();
        </script>
    </head>
<body>

    // I DO CONTROL THIS CODE
    <script>

        function mydoit() {
            alert("mydoit");
            _dothing();         <-- THIS FAILS, IS THERE ANY WAY TO SUCCEED? :(
        }

        window.K.doit = mydoit;
        window.K.doit();
    </script>

</body>
</html>

I see it out of your scope. I would copy&paste, is that an option? — Trylks, Oct 18 '12 at 18:09
Unfortunately no: for other reasons I can't duplicate _dothing() (pretend it does something magical that's not reproducible), and I can't touch the top script body. — Colin, Oct 18 '12 at 18:14
No. Closure scope comes from the location of the function *declaration*, which can't be changed. — Bergi, Oct 18 '12 at 18:17
Would it matter if the old (overwritten) `_doit` was called (alerting `"_doit"`)? — Bergi, Oct 18 '12 at 18:19
_doit() just calls _dothing() or it does something else that you don't want? — Pato, Oct 18 '12 at 18:28

score 1 · Answer 1 · edited May 23 '17 at 12:18

1

I think you can use jQuery to get the content of the script tag and after that you can use eval see this question.

edited May 23 '17 at 12:18

Community

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1

answered Oct 18 '12 at 18:14

dignifiedquire

1,360
9
14

Yes it exists outside your scope so I think the only way to get access to it is parse the script and use eval. – Donagh Hatton Oct 18 '12 at 18:18

score 1 · Answer 2 · answered Oct 18 '12 at 18:18

When you put code inside this:

(function() {

})();

it's called a self invoking function, and creates a scope that you can't access (just like not being able to access a function's scope anywhere else...only inside of it) - it is run as soon as it is declared. The fact that you can call window.K.doit is because the code extends the global window object. Since you have access to window everywhere, it can be called, but only with window.K.doit or K.doit. This is how jQuery plugins are normally defined - they extend the global jQuery object without exposing any of their code directly. Sooooo no, you are not able to access it unless you do something like what the other answerer proposes - but be careful with using eval, as TECHNICALLY, any script could be inserted and you could "assume" it's right/trusted and eval it.

Doh. Unfortunately I don't think I can even do the eval trick. The first block of code (the one I don't control) actually is 'injected' into a UIWebView on iOS using stringByEvaluatingJavascriptFromString. So I don't even know where that context would be. I guess I'm hosed :( — Colin, Oct 18 '12 at 18:27
But can't you wait until it's injected and read in the javascript? Or is it already evaluated when it's injected? — dignifiedquire, Oct 18 '12 at 19:03

Pato · Answer 3 · 2012-10-18T20:15:15.727

1

You can try something like this (I know this is very nasty, but, as others pointed out, it seems to be the only way):

function mydoit() {
    alert("mydoit");
    _dothing();
}

var f = new Function(document.scripts[0].text.replace(/(}\)\(\);\s*)$/, "window._dothing = _dothing;\n$1"));
f();
window.K.doit = mydoit;
window.K.doit();

Tested on Firefox, Chrome and IE8.

Beware: this is creating a hole new context, it's not the same as the already created. It is calling just a copy of _dothing, not the original.

edited Oct 18 '12 at 20:15

answered Oct 18 '12 at 20:07

Pato

671
6
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Massively hacky, and it just might work! Testing now with our (a bit more complicated) actual use case... – Colin Oct 18 '12 at 21:58

score -1 · Answer 4 · answered Oct 18 '12 at 18:22

-1

You have to create a global variable from within the anonymous / self executing function: this tutorial will show you how: http://professionalaspnet.com/archive/2012/07/29/Make-Your-JavaScript-Better-With-Self-Executing-Anonymous-Functions.aspx

answered Oct 18 '12 at 18:22

Jeff Wooden

5,339
2
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The OP said they have no control over the code they are trying to reach. – Ian Oct 18 '12 at 18:22

javascript: call a function in a closure scope

4 Answers4