defaultDict with ES6 proxies. Firefox only!

defaultdict.js

function defaultDict(default_) {
  return new Proxy({}, {
    get: function(target, name, receiver) {
      // iterator and next are two JS1.7 things Firefox looks for when you
      // log an object to the console. pretty sure it's a bug that they trigger 
      // the get trap.
      if (!(name in target) && !(name == '__iterator__'|| name == 'next')) {
        target[name] = default_();
      }
      return target[name];
    }
  });
}

// Here, the default value of a key is simply an empty object.
var singleDict = defaultDict(Object);
singleDict.foo.bar = "Hello world!";
console.log(singleDict);

// This is a bit cooler: the default value of a key is another defaultDict.
var nestedDict = defaultDict(function() { return defaultDict(Object) });
nestedDict.foo.bar.baz = "Hello ES6!";
console.log(nestedDict);

// adapted from http://docs.python.org/2/library/collections.html#defaultdict-examples
// This example defines the default value of a key to be "zero", so it can be incremented
// without manually writing a conditional to set it if it's undefined.
var s = 'mississippi',
    d = defaultDict(function() {return 0});

// bonus thing: "for of" works like "for in" does in literally every other language!
for (k of s) {
  d[k] += 1
}
console.log(d);