typescript 多种类型检查功能

Question

背景

我正在使用打字稿进行类型检查功能。

const checkType = <T>(value: unknown, isTypeFn: (value: unknown) => value is T): T => {
  if (!isTypeFn(value)) {
    console.error(`${isTypeFn} ${value} does not have correct type`);
    throw new Error(`${value} does not have correct type`);
  }

  return value;
};

const isBoolean = (value: unknown): value is boolean => typeof value === 'boolean';

const isString = (value: unknown): value is string => typeof value === 'string';

const isNumber = (value: unknown): value is number => typeof value === 'number';

const isNull = (value: unknown): value is null => value === null;

我可以像下面这样使用它。

const a = await getNumber() // this should be number
const numA: number = checkType(a, isNumber); // numA is number or throw Error!

问题

我想扩展 checkType 函数，如下所示。

const b = await getNumberOrString();
const bNumOrString: number | string = checkType(b, isNumber, isString);
const bNumOrBoolean: number | boolean = checkType(b, isNumber, isBoolean);
const bStringOrNull: string | null = checkType(b, isString, isNull);

如何改进 checkType 使其像这样工作？

Answer 1

该函数checkType需要一个剩余参数。我们就这样称呼它吧isTypeFns。isTypeFns是一个泛型类型参数T，它将是带有类型谓词的函数数组。

const checkType = <
  T extends ((value: unknown) => value is any)[]
>(value: unknown, ...isTypeFns: T): CheckType<T> => {
  if (isTypeFns.some(fn => fn(value))) {
    console.error(`${value} does not have correct type`);
    throw new Error(`${value} does not have correct type`);
  }

  return value as CheckType<T>;
};

实施过程非常简单。您只需检查其中的函数之一是否isTypeFns返回true给定的value.

返回类型再次变得更加棘手。我们需要采用T并推断类型谓词类型的并集。

type CheckType<T extends ((value: unknown) => value is any)[]> = 
  T[number] extends ((value: unknown) => value is infer U) 
    ? U 
    : never

我们使用它作为函数的返回类型。当涉及到return实现的声明时，TypeScript 无法理解这种复杂的类型。这就是为什么我在那里添加了一个断言。

const b = "b" as number | string;

const bNumOrString = checkType(b, isNumber, isString);
//    ^? const bNumOrString: string | number

const bNumOrBoolean = checkType(b, isNumber, isBoolean);
//    ^? const bNumOrBoolean: number | boolean

const bStringOrNull = checkType(b, isString, isNull);
//    ^? const bStringOrNull: string | null

操场