我无法正确地实现通用isEmpty(value)
在缩小类型约束下提供的值为空的对应物。
使用案例:
function getCountryNameById(countries: LookupItem[] = [], countryId?: number): string | undefined {
if (isEmpty(countries) || !isNumber(countryId)) {
// within this branch I would like to get countries argument to be narrowed to empty array type.
// Same would apply for other function which can have argument type of object or string. Why ? -> to prevent someone to do some mad code hacks like accessing non existent value from empty array ( which would happen on runtime ofc ) on compile time
// $ExpectType []
console.log(countries)
return
}
// continue with code logic ...
// implementation ...
}
关于约束对象的类似情况:
function doSomethingWithObject( data: { foo: string; bar: number } | object ){
if(isEmpty(data)){
// $ExpectType {}
data
// following should throw compile error, as data is empty object
data.foo.toUpercase()
return
}
// here we are sure that data is not empty on both runtime and compile time
}
isEmpty类型后卫实现:
export const isEmpty = <T extends AllowedEmptyCheckTypes>(
value: T | AllowedEmptyCheckTypes
): value is Empty<T> => {
if (isBlank(value)) {
return true
}
if (isString(value) || isArray(value)) {
return value.length === 0
}
if (isObject(value)) {
return Object.keys(value).length === 0
}
throw new Error(
`checked value must be type of string | array | object. You provided ${typeof value}`
)
}
定义类型:
type EmptyArray = Array<never>
type Blank = null | undefined | void
/**
* // object collects {} and Array<any> so adding both {} and Array<any> is not needed
* @private
*/
export type AllowedEmptyCheckTypes = Blank | string | object
/**
* Empty mapped type that will cast any AllowedEmptyCheckTypes to empty equivalent
* @private
*/
export type Empty<T extends AllowedEmptyCheckTypes> = T extends string
? ''
: T extends any[]
? EmptyArray
: T extends object ? {} : T extends Blank ? T : never
它有点奇怪,因为它从类型角度正确缩小,但不在if / else分支内:
代码可以在这里看到:https://github.com/Hotell/rex-tils/pull/13/files#diff-a3cdcb321a05315fcfc3309031eab1d8R177
处理此问题的一种方法是将空检查(undefined
,null
)与空值检查(''
,[]
{}
)分开。我倾向于使用两种类型的防护 - isDefined
和isEmpty
。
第一个可能看起来像这样。请注意typeof
检查 - 这使得它也适用于未声明的变量。
function isDefined<T>(value: T | undefined | null): value is T {
return (typeof value !== 'undefined') && (value !== null);
}
对于空值,可以使用以下模型。
namespace Empty {
export type String = '';
export type Object = Record<string, never>;
export type Array = never[];
}
type Empty =
| Empty.Array
| Empty.Object
| Empty.String;
function isEmpty<T extends string | any[] | object>(subject: T | Empty): subject is Bottom<T> {
switch (typeof subject) {
case 'object':
return (Object.keys(subject).length === 0);
case 'string':
return (subject === '');
default:
return false;
}
}
type Bottom<T> =
T extends string
? Empty.String
: T extends any[]
? Empty.Array
: T extends object
? Empty.Object
: never;
底部值是正确推断的。
declare const foo: 'hello' | Empty.String;
declare const bar: [number, number] | Empty.Array;
declare const baz: Window | Empty.Object;
if (isEmpty(foo) && isEmpty(bar) && isEmpty(baz)) {
console.log(foo, bar, baz);
}
编辑:按照建议在T
上添加约束。
所以在经过几次Twitter讨论和长时间的SO / Github搜索之后,我最终得到了以下解决方案:
{}
|之间基本没有区别object
| any[]
类型后卫缩小将永远不会按预期工作never
所以匹配值将是never
的类型,因为无论如何,在if(isEmpty(value)){ ... }
语句中执行任何进一步的登录没有任何意义,而不是终止程序或抛出错误这是最后的实施:
const isEmpty = <T extends string | object | any[]>(
value: T
): value is never => {
if (isString(value) || isArray(value)) {
return value.length === 0
}
if (isObject(value)) {
return Object.keys(value).length === 0
}
throw new Error(
`checked value must be type of string | array | object. You provided ${
// tslint:disable-next-line:strict-type-predicates
value === null ? 'null' : typeof value
}`
)
}
https://github.com/Hotell/rex-tils/pull/13/files#diff-68ff3b6b6a1354b7277dfc4b23d99901R50
这里
if (isEmpty(countries) || !isNumber(countryId)) {
你有两个条件,其中只有一个是countries
的类型后卫,这就是为什么countries
的类型在if中没有变化的原因。
至于对象,{}
不代表空对象。任何东西,除了null
和undefined
,都可以分配给{}
类型的变量。您可能想要使用{ [prop: string]: never }
或{ [prop: string]: undefined }
。