For this certain usage, you can use EcmaScript 6 .find
function. Similar method exists in Underscore.js, if you want to be compatible with browsers not supporting EcmaScript 6.
result = lst.find cond
if result?
do_stuff_with result
else
do_other_stuff()
However, there is no direct replacement for for else
loop from Python. In general case, you will need to declare a boolean to store the state.