A chained function with closures.

closures.js

function fn (str) {
    let appended = str;
  const stringBuilder = {
    fn (newStr) {
      if (typeof newStr === 'string') {
          appended += ' ';
          appended += newStr;
        return stringBuilder;
      }
      return appended
    }
  };
  
  return stringBuilder;
}

console.log(fn('hello').fn('world').fn('!!!').fn()) // 'hello world !!!'

console.log(fn('This').fn('is').fn('just').fn('a').fn('test').fn()) // 'This is just a test'