function joint(a){var b;return b=a[a.length-1],a.pop(),a=a.length>1?joint(a):a[0],function(){b.apply(new a)}}

​

​

// My original function that I want to extend

function my_func() {

  console.log( "Original functionality." );

}

​

// Function with the new functionality

function new_func() {

  console.log( "New functionality." );

}

​

// Add the functionality of new_func to my_func 

my_func = joint([ my_func, new_func ]);

​

// Now run the original function with the new functionality

my_func();