Flow工具类型（Utility Types）

工具类型（Utility Types）

Flow提供了一组实用的工具类型，可用于其他类型的操作，并可用于不同的场景。

• $Keys<T>
• $Values<T>
• $ReadOnly<T>
• $Exact<T>
• $Diff<A, B>
• $Rest<A, B>
• $PropertyType<T>
• $ElementType<T>
• $ObjMap<T, F>
• $TupleMap<T, F>
• $Call<F>
• Class<T>
• $Supertype<T>
• $Subtype<T>
• Existential Type (*)

$Keys<T>

参考联合类型。

// @flow
const countries = {
  US: "United States",
  IT: "Italy",
  FR: "France"
};

type Country = $Keys<typeof countries>;

const italy: Country = 'IT';
const nope: Country = 'nope'; // 'nope' is not a Country

$Values<T>

// @flow
type Props = {
  name: string,
  age: number,
};

// The following two types are equivalent:
type PropValues = string | number;
type Prop$Values = $Values<Props>;

const name: Prop$Values = 'Jon';  // OK
const age: Prop$Values = 42;  // OK
const fn: Prop$Values = () => {};  // Error! function is not part of the union type

$ReadOnly<T>

// @flow
type Props = {
  name: string,
  age: number,
  // ...
};

type ReadOnlyProps = $ReadOnly<Props>;

function render(props: ReadOnlyProps) {
  const {name, age} = props;  // OK to read
  props.age = 42;             // Error when writing
  // ...
}

$Exact<T>

参考{| key: type |}

// @flow
type ExactUser = $Exact<{name: string}>;
type ExactUserShorthand = {| name: string |};

const user2 = {name: 'John Wilkes Booth'};
// These will both be satisfied because they are equivalent
(user2: ExactUser);
(user2: ExactUserShorthand);

$Diff<A, B>

// @flow
type Props = { name: string, age: number };
type DefaultProps = { age: number };
type RequiredProps = $Diff<Props, DefaultProps>;

function setProps(props: RequiredProps) {
  // ...
}

setProps({ name: 'foo' });
setProps({ name: 'foo', age: 42, baz: false }); // you can pass extra props too
setProps({ age: 42 }); // error, name is required

请注意，如果要从中删除属性的对象不具有要删除的属性，则$Diff <A，B>将会报错，即如果B具有A中不存在的键：

// @flow
type Props = { name: string, age: number };
type DefaultProps = { age: number, other: string }; // Will error due to this `other` property not being in Props.
type RequiredProps = $Diff<Props, DefaultProps>;

function setProps(props: RequiredProps) {
  props.name;
  // ...
}

解决方案：

type A = $Diff<{}, {nope: number}>; // Error
type B = $Diff<{}, {nope: number | void}>; // OK