Typescript - isEmpty 函数的通用类型保护

Question

我无法正确实现泛型isEmpty(value)，将提供的值的类型限制缩小到空的对应项。

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 ...
}

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 类型守卫实现：

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}`
  )
}

具有定义的类型：

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

这有点奇怪，因为它从类型角度正确缩小，但不在 if/else 分支内：

isEmpty 对于字符串值

isEmpty 对于数组值

isEmpty 用于对象值

代码可以在这里看到：https://github.com/Hotell/rex-tils/pull/13/files#diff-a3cdcb321a05315fcfc3309031eab1d8R177

相关问题：空对象的类型保护

Answer 1

处理此问题的一种方法是将空值检查 ( undefined, null) 与空值检查 ( '', [] {}) 分开。我倾向于对 \xe2\x80\x94isDefined和isEmpty.

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第一个可能看起来像这样。请注意typeof检查 \xe2\x80\x94 这使得它也可以处理未声明的变量。

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function isDefined<T>(value: T | undefined | null): value is T {\n  return (typeof value !== 'undefined') && (value !== null);\n}\n

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对于空值，可以使用以下模型。

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namespace Empty {\n  export type String = '';\n  export type Object = Record<string, never>;\n  export type Array = never[];\n}\n\ntype Empty =\n  | Empty.Array\n  | Empty.Object\n  | Empty.String;\n\nfunction isEmpty<T extends string | any[] | object>(subject: T | Empty): subject is Bottom<T> {\n  switch (typeof subject) {\n    case 'object':\n      return (Object.keys(subject).length === 0);\n    case 'string':\n      return (subject === '');\n    default:\n      return false;\n  }\n}\n\ntype Bottom<T> =\n  T extends string\n    ? Empty.String\n    : T extends any[]\n        ? Empty.Array\n        : T extends object\n            ? Empty.Object\n            : never;\n

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底部值被正确推断。

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declare const foo: 'hello' | Empty.String;\ndeclare const bar: [number, number] | Empty.Array;\ndeclare const baz: Window | Empty.Object;\n\nif (isEmpty(foo) && isEmpty(bar) && isEmpty(baz)) {\n  console.log(foo, bar, baz);\n}\n

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编辑：T按照建议添加限制。

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