流类型，`+`符号在属性前面是什么意思？

我遇到了以下用js FlowType编写的代码（我有兴趣知道在FlowType的上下文中+的值不是普通的JS）。

I came across the following code written in js FlowType (I am interested to know the value of + in the context of FlowType not in general JS).

可能您可以解释一下 + 符号在下面的代码中在该属性前面的含义：

Could you please explain me what does the + symbol mean in front of the property in the code below:

  export type User = {
      +name: string,
      +surname: string,
      +personId: PourceId,
    }

我在文档中找不到任何参考，也欢迎任何链接。

I could not find any reference in the documentation, any link is also welcome.