en.javascript.info/1-js/5-functions-closures/4-closures-usage/2-stringbuffer/_js.view/solution.js

function makeBuffer() {
  var text = ''; 
    
  return function(piece) {
    if (arguments.length == 0) { // вызов без аргументов
      return text;
    }
    text += piece; 
  };
};