How to use the spec.undefinedToVoid transformer.
Some JavaScript implementations allow undefined to be overwritten, this
may lead to peculiar bugs that are extremely hard to track down.
This transformer transforms undefined into void 0 which returns undefined
regardless of if it's been reassigned.
require("babel").transform("code", { optional: ["spec.undefinedToVoid"] });
$ babel --optional spec.undefinedToVoid script.js
In
foo === undefined;
Out
foo === void 0;