Globals.js

var Globals = {
  originals:   [],
  userDefined: [],
  warned:      [],
  root:        this,

  initialize: function() {
    if (this.originals.length > 0) return;
    for (var key in this.root) this.originals.push(key);
  },

  register: function() {
    for (var i = 0, n = arguments.length; i < n; i++)
      this.userDefined.push(arguments[i]);
  },

  check: function() {
    for (var key in this.root) {
      if (this.originals.indexOf(key) == -1
          && this.userDefined.indexOf(key) == -1
          && this.warned.indexOf(key) == -1) {
        console.warn('Global variable: ' + key);
        this.warned.push(key);
      }
    }
  },

  run: function() {
    var self = this;
    setInterval(function() { self.check() }, 1000);
  }
};

// Example -- variables from JS.Class
Globals.register('JS', 'it', 'its', 'require', 'undefined');
Globals.initialize();