oop.js

function createStringBuilder() {
  let str = '';
  return function self(key) {
    switch (key) {
      case 'append':
        return (s) => {
          str += s;
          return self;
        };
      case 'toString':
        return () => str;      
      default:
        throw new TypeError('Unknown key: ' + key);
    }
  };
}

const sb = createStringBuilder();
assert.equal(
  sb('append')('Hello')('append')(' world!')('toString')(),
  'Hello world!');