How do I modify/extend a javascript function in place by an inject script?

Lets say this function...

function foo(param){
// original works
}
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is already in-place in an html document.

I have a bookmarklet that injects an external script to the document. From that script, I want to modify the behavior of foo() function into this...

function foo(param){
// original works
bar(param);
}

bar() is a new function in the injected script. I have no problem duplicating foo in the injected script.

How do I do this?

Everything in javascript can be an object, including functions. With this in mind, you can create a duplicate of the old function, then override the new one while referencing the duplicate:

function foo(param){
// original works
}

var old_foo = foo;

function foo(param) {
 old_foo(param);
 bar(param);
}