在Flow中细化未初始化的变量

Question

如何正确对?T或T|undefined类型的未初始化变量进行细化？

是什么让uninitialized variable与undefined的值不同？

不会抛出错误，但在下面的情况下：

const test = (nums: Array<number>) => {
  let lastNumber: ?number
  const output = []
  if (lastNumber != null) {
    output.push(lastNumber /*lastNumber is Uncovered code*/)
  }
  return output
}

flow type-at-pos进入lastNumber会(unknown)，因为uninitialized variable（在声明时）

但这完全覆盖了：

const test = (nums: Array<number>) => {
  let lastNumber: ?number = undefined
  const output = []
  if (lastNumber != null) {
    output.push(lastNumber /*Fully covered*/)
  }
  return output
}