gistfile1.coffee

~/bin/src/coffee-script $ coffee -pe "thing[stuff()] ?= 5"
(function() {
  var _a, _b;
  thing[(_a = stuff())] = (typeof (_b = thing[(_a = stuff())]) !== "undefined" && _b !== null) ? thing[_a] : 5;
})();

should be

~/bin/src/coffee-script $ coffee -pe "thing[stuff()] ?= 5"
(function() {
  var _a, _b;
  thing[(_a = stuff())] = (typeof (_b = thing[_a]) !== "undefined" && _b !== null) ? thing[_a] : 5;
})();