工具類型（Utility Types）
Flow提供了一組實用的工具類型板祝，可用于其他類型的操作擂涛，并可用于不同的場景未荒。
- $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>
參考聯(lián)合類型狞甚。
// @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>
參考{| 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 = $Diff<Props, DefaultProps>;
function setProps(props: RequiredProps) {
// ...
}
setProps({ name: 'foo' });
setProps({ name: 'foo', age: 42, baz: false }); // you can pass extra props too
setProps({ age: 42 }); // error, name is required
請注意等曼，如果要從中刪除屬性的對象不具有要刪除的屬性，則$Diff <A损离，B>將會報錯哥艇，即如果B具有A中不存在的鍵：
// @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;
// ...
}
解決方案：
type A = $Diff<{}, {nope: number}>; // Error
type B = $Diff<{}, {nope: number | void}>; // OK
