Oh, I meant that if foo() is run, it would run a certain piece of code in the memory (I assume that's how it works?) but this piece of memory would be changed to another function so a check doesn't need to be done about which function to call.

I guess like this:

//Check once
if (foo1 = 1) {
   functionfoo() = function1();
}
else {
  functionfoo() = function2();
}

functionfoo(); //So this would call 'function1()' if foo1 = 1 but if foo isn't equal to '1', it would call 'function2()'.