现在,打字稿3.1引入了mapped tuple types,我希望这个代码示例可以工作:
export interface SettingKey {
General_Language: 'en' | 'sl';
Map_InitialLongitude: number;
Map_InitialLatitude: number;
}
export function fetchSetting<K extends (keyof SettingKey)[]>
(...keys: K): Promise<SettingKey[K]> {
return null as any;
}
fetchSetting('General_Language', 'Map_InitialLongitude').then(x => {
return x['General_Language'] === 'de' // would want compilation error 'de' not in 'en' | 'sl'
})
但事实并非如此。错误是:
ttt.ts:7:83 - error TS2536: Type 'K' cannot be used to index type 'SettingKey'.
7 export function fetchSetting<K extends (keyof SettingKey)[]>(...keys: K): Promise<SettingKey[K]> {
~~~~~~~~~~~~~
ttt.ts:11:12 - error TS2571: Object is of type 'unknown'.
11 return x['General_Language'] === 'de'
~
很明显,第二个错误是第一个错误的结果,因此这并不是真正的问题。第一个是有问题的。
keys是keyof SettingKey
的数组,所以我希望SettingKey[K]
是所列出属性的类型的数组(因此,具体来说,在我放入的代码示例中,它将是['en' | 'sl', number]
。来自pull request introducing the typescript feature:
如果T是数组类型S [],我们将映射到数组类型R [],其中R是X的实例化,其中S替换了T [P]。
但是我认为这仅适用于映射类型,并且这里有一个查找类型,这就是我认为它不起作用的原因吗?
我想表达的很清楚;可以在打字稿中将其设为类型安全吗?
答案 0 :(得分:3)
要拥有映射的元组,您需要一个映射的类型,它将原始的元组(在类型参数K
中)映射到新的元组类型
export interface SettingKey {
General_Language: 'en' | 'sl';
Map_InitialLongitude: number;
Map_InitialLatitude: number;
}
type SettingKeyProp<P extends keyof SettingKey> = SettingKey[P]
type SettingKeyArray<K extends { [n: number]: keyof SettingKey }> = {
[P in keyof K]: K[P] extends keyof SettingKey ? SettingKey[K[P]]: never
}
export function fetchSetting<K extends (keyof SettingKey)[]>
(...keys: K): Promise<SettingKeyArray<K>> {
return null as any;
}
fetchSetting('General_Language', 'Map_InitialLongitude').then(x => {
// x[0] is 'en' | 'sl'
return x[0] === 'de' /// since you want a tuple, you should index by number not name
})
如果您也想按名称索引,但是映射类型应该映射在数组中的值而不是数组中的键上:
type SettingKeyArray<K extends { [n: number]: keyof SettingKey }> = {
[P in K[number]]: SettingKey[P]
}
export function fetchSetting<K extends (keyof SettingKey)[]>
(...keys: K): Promise<SettingKeyArray<K>> {
return null as any;
}
fetchSetting('General_Language', 'Map_InitialLongitude').then(x => {
// you can access by name
return x.General_Language === 'de'
})