Dynamic Replacement of JavaScript Function Content

Is it possible to completely replace a function in JavaScript?

I have come across this code snippet that supposedly replaces a function, but for some reason, the code is not functioning as expected. The DOM seems to update, though. Can anyone shed some light on what might be happening here?

<html>
<head>
    <script id="myScript" type="text/javascript">
        function someFunction() {
            alert("Same old.");
        }
    </script>
</head>
<body>

<input type="button" onclick="someFunction();" value="A button." />
<script>
    function replace() {
        var oldFunctionString = someFunction.toString();
        var oldContents = oldFunctionString.substring(oldFunctionString.indexOf("{") + 1, oldFunctionString.lastIndexOf("}") );
        var newCode = "alert(New code!);";        
        var newFunctionString = "function someFunction(){"+newCode+"}";
        var scriptTag = document.getElementById('myScript');

        scriptTag.innerHTML = scriptTag.innerHTML.replace(oldFunctionString,newFunctionString);
    }

    replace();
</script>

</body>
</html>

JSfiddle here

Answer №1

When using .innerHTML, it does not trigger a script to run again. To achieve that, you would need to create a new script element and insert it into the DOM. This will overwrite any actions performed by the previous script (although this may not work in every scenario).

If you wish to substitute that function with something else, you can do so by:

somefunction = function() {
    alert("New code!"); // Please note, there is a syntax error here
};

If you only want to modify certain parts of the code (without knowing all of it), regex and similar techniques could be used. However, ultimately, you just need to assign the updated function to the variable like this:

somefunction = eval("("
  + somefunction.toString().replace(/(alert\().*?(\);)/, "$1New code!$2")
  + ")");

Answer №2

If you're dealing with strings, it appears that you may be misunderstanding the function itself. Here's a simple adjustment:

newFunction = function () { /* insert your function code here */ }

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