FLOW IS A STATIC TYPECHECKER FOR JAVASCRIPT.
Flow 是一个静态类型检测工具；在现有项目中加上类型标注后，可以在代码阶段就检测出对变量的不恰当使用。Flow 弥补了 JavaScript 天生的类型系统缺陷。利用 Flow 进行类型检查，可以使你的项目代码更加健壮，确保项目的其他参与者也可以写出规范的代码；而 Flow 的使用更是方便渐进式的给项目加上严格的类型检测。那么这么好的 Flow，要怎么开始使用呢？
如果你想对一个文件进行类型检查，必须在文件头部加上一行注释；
// @flow
// 或者下面这种
/* @flow */
意思简单明了，AT 一下 Flow 检查器，意思是：“嘿， Flow，来验一下我”。
你一定注意到了，这样改写 JavaScript 代码之后，这段代码是不符合 ECMA 语法规范的；所以在上线时，需要用插件把 Flow 的标注内容去掉；Flow 提供 flow-remove-types 和 babel 插件 两种方式来去除标注内容，感兴趣的话可以点击相应链接，本文不详述。
WebStorm里面，需要把languages 设置为 Javascripts => flow
Maybe types are for places where a value is optional and you can create them by adding a question mark in front of the type such as ?string or ?number.
In addition to the type in ?type, maybe types can also be null or void.
function acceptsMaybeString(value: ?string) {
}
acceptsMaybeString("bar");
acceptsMaybeString(undefined);
acceptsMaybeString(null);
acceptsMaybeString();
Optional object properties
Object types can have optional properties where a question mark ? comes after the property name.
{ propertyName?: string }
In addition to their set value type, these optional properties can either be void or omitted altogether. However, they cannot be null.
function acceptsObject(value: { foo?: string }) {
}
acceptsObject({ foo: "bar" });
acceptsObject({ foo: undefined });
acceptsObject({ foo: null });
acceptsObject({});
Optional function parameters 可选的函数参数
Functions can have optional parameters where a question mark ? comes after the parameter name.
function method(param?: string) { /* ... */ }
In addition to their set type, these optional parameters can either be void or omitted altogether. However, they cannot be null.
function acceptsOptionalString(value?: string) {
}
acceptsOptionalString("bar");
acceptsOptionalString(undefined);
acceptsOptionalString(null);
acceptsOptionalString();
https://zhuanlan.zhihu.com/p/26204569