Typescript - 无法推断参数类型

Question

我正在尝试同时为多个数字类型实现 clamp，如下所示:

import BigNumber from 'bignumber.js'

export const clamp = <T extends number | BigNumber>(min: typeof n, n: T, max: typeof n): T => {
  if (isNumber(n)) {
    return Math.max(min, Math.min(n, max)) as T
  }

  if (isBigNumber(n)) {
    return BigNumber.max(min, BigNumber.min(n, max)) as T
  }
}

const isNumber = (n: number | BigNumber): n is number => {
  return typeof n === 'number'
}

const isBigNumber = (n: number | BigNumber): n is BigNumber => {
  return n instanceof BigNumber
}

但是代码编译失败，出现以下错误:

TypeScript error in clamp.ts(5,21):
Argument of type 'number | BigNumber' is not assignable to parameter of type 'number'.
  Type 'BigNumber' is not assignable to type 'number'.  TS2345

    3 | export const clamp = <T extends number | BigNumber>(min: typeof n, n: T, max: typeof n): T => {
    4 |   if (isNumber(n)) {
  > 5 |     return Math.max(min, Math.min(n, max)) as T
      |                     ^
    6 |   }
    7 | 
    8 |   if (isBigNumber(n)) {

min 和 max 的类型不应该被推断为第 5 行的 number 吗？如果不是，如何保证 Typescript 类型的正确性？

Answer 1

不幸的是，这是 Typescript 类型系统不足的情况之一。有 a proposal允许交集类型守卫，这将使我们能够拥有一个类型守卫，例如:

const isNumber = (n: number | BigNumber, min: typeof n, max: typeof n): n is number & min is number & max is number => {
    return typeof n === 'number';
}

在实现这样的提议之前，您必须显式地对 min 和 max 变量进行类型检查:

import BigNumber from 'bignumber.js'

export const clamp = <T extends number | BigNumber>(min: T, n: T, max: T): number | BigNumber => {
    if (isNumber(n) && isNumber(min) && isNumber(max)) {
        return Math.max(min, Math.min(n, max));
    }

    if (isBigNumber(n) && isBigNumber(min) && isBigNumber(max)) {
        return BigNumber.min(min, BigNumber.max(n, max));
    }

    throw new TypeError("Every parameter of this function must be either of type number or an instance of BigNumber");
}

const isNumber = (n: number | BigNumber): n is number => {
    return typeof n === 'number'
}

const isBigNumber = (n: number | BigNumber): n is BigNumber => {
    return n instanceof BigNumber
}

或者同时转换最小值和最大值:

import BigNumber from 'bignumber.js'

export const clamp = <T extends number | BigNumber>(min: T, n: T, max: T): number | BigNumber => {
    if (isNumber(n)) {
        return Math.max(min as number, Math.min(n, max as number));
    }

    return BigNumber.min(min as BigNumber, BigNumber.max(n, max as BigNumber));
}

const isNumber = (n: number | BigNumber): n is number => {
    return typeof n === 'number'
}