Map data structure with handy deepGet and deepSet methods https://jsfiddle.net/aq93v4g7/7/

Map.js

function Map(keys, values) {return new _Map(keys, values)}
function _Map(keys, values) {this.clear(keys, values);}
_Map.prototype = {

  set: function(key, value) {
    var i = this.indexOf(key);
    this._keys[i] = key;
    this._values[i] = value;
    return i !== this._keys.length;
  },

  get: function(key) {
    var i = this.indexOf(key);
    return this._values[i];
  },

  remove: function(key) {
    var i = this.indexOf(key);
    if (i !== this._keys.length) {
      this._keys.splice(i, 1);
      this._values.splice(i, 1);
    }
    return this;
  },

  clear: function(keys, values) {
    this._keys = keys || [];
    this._values = values || [];
    return this;
  },

  indexOf: function(key) {
    var i = this._keys.indexOf(key);
    return i === -1 ? this._keys.length : i;
  },

  deepGet: function(keyPath) {
    var current = this,
        keyPath = keyPath || [],
        key;
    
    for(var i= 0, len=keyPath.length; i<len; i++) {
      key = keyPath[i];
      current = current.get(key);
      if (current === undefined) {
        break;
      }
    }

    return current;
  },

  deepSet: function(keyPath, value) {
    var current = this,
        keyPath = keyPath || [],
        _cur;

    keyPath.forEach(function(key, index) {
      if (index + 1 === keyPath.length) {
        current.set(key, value);
      } else {
        _cur = current.get(key);
        if (!_cur) {
          _cur = Map();
          current.set(key, _cur);
        }
        current = _cur;
      }
    });

    return this;
  }
};

usage.js

var keys = [{foo: 1}, "foo", [1,2,3], function(foo) {console.log(foo)}];
var _pathListeners = Map(
    [keys[0], keys[1]],
    [
        // keyPath: [{foo: 1}]
        Map( 
            ["__listeners__"],
            
            // keyPath: [{foo: 1}] callbacks
            [[function() {console.log("w00t {foo: 1}")}]]
        ),
        
        // keyPath: ["foo"]
        Map(
            ["__listeners__", "bar", keys[2]], 
            [
                // keyPath: ["foo"] callbacks
                [function() {console.log("w00t foo")}],
                
                // keyPath: ["foo", "bar"]
                Map(
                    ["__listeners__"],
                    
                    // keyPath: ["foo", "bar"] callbacks
                    [[function() {console.log("w00t foo bar")}]]
                ),

                // keyPath: ["foo", [1,2,3]]                    
                Map(
                    ["__listeners__", keys[2]],
                    [
                        // keyPath: ["foo", [1,2,3]] callbacks
                        [
                            function() {console.log("w00t foo [1,2,3]")},
                            function() {console.log("w00t foo [1,2,3] again")}
                        ],
                        
                        // keyPath: ["foo", [1,2,3], function(foo) {console.log(foo)}]
                        Map(
                            ["__listeners__"],
                            
                            // keyPath: ["foo", [1,2,3], function(foo) {console.log(foo)}] callbacks
                            [[function() {console.log("w00t for long keyPath")}]]
                        )
                    ]
                )  
            ]
        )
    ]
);

console.log(_pathListeners);
console.log(_pathListeners.deepGet(["foo"]));
var firstLevelListeners = _pathListeners.deepGet([keys[0], "__listeners__"]);
firstLevelListeners[0]();
var deepListeners = _pathListeners.deepGet([keys[1], keys[2], "__listeners__"]);
deepListeners[0]();

_pathListeners.deepSet([keys[1], keys[2], keys[0], "__listeners__"], [function() {console.log("new callback")}]);
var newListeners = _pathListeners.deepGet([keys[1], keys[2], keys[0], "__listeners__"]);
newListeners[0]();