lib/bind_keys.js
function updateChange (targetKey, path, key) {
  var target = path.target
  var index = path.index
  var replaceKey = key

  if (typeof index === 'number') {
    target = target[key]
    replaceKey = index
  }

  return function handleChange (event) {
    target[replaceKey] = event.target[targetKey]
  }
}
function updateChange (targetKey, path, key) {
  var target = path.target
  var index = path.index
  var replaceKey = key

  if (typeof index === 'number') {
    target = target[key]
    replaceKey = index
  }

  return function handleChange (event) {
    target[replaceKey] = event.target[targetKey]
  }
}
Internal event listener to update data on input change.