function Store(data) {
  const events = {};

  let value = data.value;

  function getValue() { return value; }
  function setValue(val) { value = val; emit('value'); }

  return {
    getValue,
    setValue,
    on, off
  };

  // ...
}

function Input(data) {
  let elem;
  const store = data.store;

  function construct() {
    elem = document.createElement('input');
    bind();
    update();
  }

  function update() {
    elem.value = store.getValue();
  }

  function enter() {}
  function exit() {}

  function destroy() {
    unbind();
  }

  function onInput() {
    store.setValue(elem.value);
  }

  function bind() {
    elem.addEventListener('input', onInput);
    store.on('value', update);
  }
  function unbind() {
    elem.removeEventListener('input', onInput);
    store.off('value', update);
  }

  construct();
  return { elem, data, update, enter, exit, destroy };
}

function App(data) {
  let elem;
  const store = data.store;

  let textElem;
  let input;

  function construct() {
    elem = document.createElement('div');
    {
      textElem = document.createElement('div');
      elem.appendChild(textElem);
      input = new Input({ store });
      elem.appendChild(input.elem);
    }
    bind();
    update();
  }

  function update() {
    textElem.innerHTML = store.getValue() || 'Enter a value...';
  }

  function enter() {}
  function exit() {}

  function destroy() {
    input.destroy();
    unbind();
  }

  function bind() {
    store.on('value', update);
  }
  function unbind() {
    store.off('value', update);
  }

  construct();
  return { elem, data, update, enter, exit, destroy };
}