Why, look at the senior developer's brilliant, perfectly intuitive code, and know everything about the system instantly, of course!
I live this life right now. No documentation, and the senior devs have left, so you just have to figure out what the hell they're thinking as best as you can with the code left behind, sometimes stepping through everything multiple times to see how the data flows through it. Thankfully one of the architects that used to work here knew how to make readable, modular code, so our newer projects aren't too bad to modify, but our legacy systems are awful nests of spaghetti.
I live this life right now. No documentation, and the senior devs have left, so you just have to figure out what the hell they're thinking as best as you can with the code left behind, sometimes stepping through everything multiple times to see how the data flows through it. Thankfully one of the architects that used to work here knew how to make readable, modular code, so our newer projects aren't too bad to modify, but our legacy systems are awful nests of spaghetti.