TypeScript：经断言验证后变量类型为何重置为unknown？

在assertValidString函数中，通过assert<string>断言参数str为string类型后，str的类型确实变为string。但执行if(trimBeforeLenghtValidation) str = str.trim();这行代码后，str的类型变回unknown，导致后续访问str.length时出现错误：Object is of type 'unknown'.(2571)。希望了解该问题的根本原因，不使用any作为临时解决方案。

相关代码片段：

// this class is imported by the validator.ts module
class EWC extends Error {
    constructor(public message: string, public code: string) {
        super(message);
    }
}

// The assert function is coming from an external library
// and it has the following signature    
interface AssertOtherArg {
    name: string;
    code: string;
}

function assert<Type>(schema: string, value: unknown, other: AssertOtherArg): asserts value is Type {
    throw "unimplemented";
}
// end of assert


// ------- [string-validator.ts] ----------------------------------------------

interface OtherInfo {
    name: string;
    typeErrorCode?: string; 

    trimBeforeLenghtValidation?: boolean;

    minLength?: number;
    minLengthErrorCode?: string;

    maxLength?: number;
    maxLengthErrorCode?: string;
}

function assertValidString(str: unknown, otherInfo: OtherInfo): asserts str is string {
    const {name, trimBeforeLenghtValidation = false} = otherInfo;

    {
        const code = "typeErrorCode" in otherInfo ? otherInfo.typeErrorCode : `invalid_${name.toLocaleUpperCase()}`;
        assert<string>("string", str, {name, code});
    }

    str; // here str: string

    if(trimBeforeLenghtValidation) str = str.trim();

    str; // here str: unknown! why?

    // min length vlaidation
    if("minLength" in otherInfo) {
        const code = "minLengthErrorCode" in otherInfo ? otherInfo.minLengthErrorCode : `${name.toUpperCase()}_TOO_SHORT`;

        const {minLength} = otherInfo;

        // the problem is happening here
        // Error: Object is of type 'unknown'.(2571)
        if(str.length < minLength)
            throw new EWC(`"${name}" cannot be shorter than ${minLength} character(s) long!`, code);
    }

    // maxLength validation
    // ...
}

TypeScript中，函数参数的类型断言（比如asserts value is Type）只会临时窄化参数的类型范围，不会修改参数的原始类型声明。str的原始类型是unknown，当你对它执行赋值操作（str = str.trim()）时，TypeScript会重新校验赋值内容是否符合参数的原始类型声明——尽管str.trim()返回string，但由于参数本身被声明为unknown，TypeScript不会保留之前通过断言得到的窄化类型，而是将其重置为原始的unknown类型。

简言之：参数的类型窄化是临时状态，一旦对参数重新赋值，之前的窄化类型信息就会丢失，回到参数最初的声明类型。

可以通过以下两种方式避免类型丢失，且无需使用any：

• 使用临时变量存储窄化后的类型
  将断言后的str赋值给一个临时变量，后续所有操作都基于这个临时变量：

  function assertValidString(str: unknown, otherInfo: OtherInfo): asserts str is string {
      const {name, trimBeforeLenghtValidation = false} = otherInfo;
  
      {
          const code = "typeErrorCode" in otherInfo ? otherInfo.typeErrorCode : `invalid_${name.toLocaleUpperCase()}`;
          assert<string>("string", str, {name, code});
      }
  
      // 用临时变量保存窄化后的string类型
      let validatedStr = str;
      if(trimBeforeLenghtValidation) validatedStr = validatedStr.trim();
  
      if("minLength" in otherInfo) {
          const code = "minLengthErrorCode" in otherInfo ? otherInfo.minLengthErrorCode : `${name.toUpperCase()}_TOO_SHORT`;
          const {minLength} = otherInfo;
          if(validatedStr.length < minLength)
              throw new EWC(`"${name}" cannot be shorter than ${minLength} character(s) long!`, code);
      }
  
      // maxLength validation
      // ...
  }
  

• 避免重新赋值原参数
  直接使用trim()后的结果进行判断，不修改原参数：

  function assertValidString(str: unknown, otherInfo: OtherInfo): asserts str is string {
      const {name, trimBeforeLenghtValidation = false} = otherInfo;
  
      {
          const code = "typeErrorCode" in otherInfo ? otherInfo.typeErrorCode : `invalid_${name.toLocaleUpperCase()}`;
          assert<string>("string", str, {name, code});
      }
  
      // 直接生成要检查的字符串，不修改原参数
      const strToCheck = trimBeforeLenghtValidation ? str.trim() : str;
  
      if("minLength" in otherInfo) {
          const code = "minLengthErrorCode" in otherInfo ? otherInfo.minLengthErrorCode : `${name.toUpperCase()}_TOO_SHORT`;
          const {minLength} = otherInfo;
          if(strToCheck.length < minLength)
              throw new EWC(`"${name}" cannot be shorter than ${minLength} character(s) long!`, code);
      }
  
      // maxLength validation
      // ...
  }
  

内容的提问来源于stack exchange，提问作者h-sifat