How to modify a function in a page using Bookmarklet?

Lets say I have an HTML page which has its own function myFunction()

<!DOCTYPE html>
<html>
<body>

<p>Click the button to trigger a function that will output "Hello World" in a p element with id="demo".</p>

<button onclick="myFunction()">Click me</button>

<p id="demo"></p>

<script>
function myFunction() {
    alert("stack overflow");
    document.getElementById("demo").innerHTML = "Hello World";
}
</script>

</body>
</html>

I want to change the myFunction()definition like this

function myFunction() {
    //alert("stack overflow");
    document.getElementById("demo").innerHTML = "Hello World";
}

using bookmarklet.So is there any way to change the def temporarily ? I tried using the Chrome Console to change its def and also remain successful,but I want to change it using bookmarklets.So please help me.

Solution

As it's a global, you can do this in your bookmarklet:

var oldFunction = myFunction;
myFunction = function() {
    alert("Hello world!");
};

Or if you enclose your bookmarklet in a scping function (usually a good idea), probably store the old value on a window property instead of a local var (so you can access it from another bookmarklet to restore it later):

window.oldFunction = myFunction;
myFunction = function() {
    alert("Hello world!");
};

Another bookmarklet could set it back again:

myFunction = window.oldFunction; // or just = oldFunction if using a local var

Live Example:

function myFunction() {
  alert("Original function");
}
document.getElementById("btn-override").addEventListener("click", function() {
  window.oldFunction = myFunction;
  myFunction = function() {
    alert("New function");
  };
}, false);
document.getElementById("btn-restore").addEventListener("click", function() {
  myFunction = window.oldFunction;
}, false);

<input type="button" id="btn-call" value="Click to call myFunction" onclick="myFunction()">
<input type="button" id="btn-override" value="Click to override it">
<input type="button" id="btn-restore" value="Click to restore it">