FYI, Dart does type checking.

If you want to give your javascript some type-checking, try running it through the closure compiler. This is what we do, and we annotate our methods with type signatures like this

  /**
   * Queries a Baz for items.
   * @param {number} groupNum Subgroup id to query.
   * @param {string|number|null} term An itemName,
   *     or itemId, or null to search everything.
   */
  goog.Baz.prototype.query = function(groupNum, term) {
    ...
  };

And then the Closure Compiler tells us if we've broken some type contracts.