code: function once(func) {
      var result,
          ran = false;
  
      return function() {
        if (ran) {
          return result;
        }
        ran = true;
        result = func.apply(this, arguments);
        return result;
      };
    }
docs: Creates a new function that is restricted to one execution. Repeat calls to
  the function will return the value of the first call.
  
  @static
  @memberOf _
  @category Functions
  @param {Function} func The function to restrict.
  @returns {Function} Returns the new restricted function.
  @example
  
  var initialize = _.once(createApplication);
  initialize();
  initialize();
  // Application is only created once.