It's like `mkdir -p` but for Javascript objects! (Probably an anti-pattern.)

namespace.js

// I often do stuff like (coffeescript):
//
//   class @Forms.Select extends React.Component
//
// Which, often requires an ugly @Forms ||= {} somewhere, or this will
// explode. Now, I know there's better ways to do this, and CommonJS has
// its nifty resource stuff, but I am old and stuck in my ways and stuff.
//
// With this snippet (if you're in rails, you can include it in application.js
// before your components dir), you can write (again, coffee):
//
//   namespace 'Forms'
//   class @Forms.Select extends React.Component
//
// It will also take a path and call itself recursively. The return value, if
// for whatever reason you want to use it, is a reference to the LAST object
// declared in the namespace.
//
//   namespace 'User.Account'
//   class @User.Account.Menu extends React.Component
//
// Caveat(s):
//
//   1) A Ruby dev wrote this. What even IS Javascript?!
//   2) If anything in the class path isn't an object, this will probably puke
//   3) I'm not responsible for bad habits that result from this pattern.
//   4) You know, this can easily be used on other objects, consider it mkdir -p
//        foo = {}
//        namespace('bar.baz.qux', foo) //=> { foo: { bar: { baz: { qux {} } } }
//
window.namespace = function(ns_path, parent) {
    var spaces, final, current;

    if(!ns_path) return;
    if(!parent) parent = window;

    if(ns_path.unshift) {
        spaces = ns_path;
    } else {
        spaces = ns_path.split('.');
    }

    current = spaces.shift();

    if(typeof window[current] === 'undefined') {
        final = parent[current] = {};
    }

    if(spaces.length > 0) {
        return namespace(spaces, parent[current]);
    } else {
        return final;
    }
};

So for some reason, using let instead of var totally broke all the javascripts in production.

Specifically, I was getting "Undefined variable: $" issues, which on further eval turned out that, eh... nothing was loading? What?

Explain.