将对象转换为 TypeScript 中的接口 [英] Cast object to interface in TypeScript

问题描述

我正在尝试将我的代码从 express 中的请求正文(使用正文解析器中间件)转换为接口，但它没有强制执行类型安全.

I'm trying to make a cast in my code from the body of a request in express (using body-parser middleware) to an interface, but it's not enforcing type safety.

这是我的界面:

export interface IToDoDto {
  description: string;
  status: boolean;
};

这是我尝试进行转换的代码:

This is the code where I'm trying to do the cast:

@Post()
addToDo(@Response() res, @Request() req) {
  const toDo: IToDoDto = <IToDoDto> req.body; // <<< cast here
  this.toDoService.addToDo(toDo);
  return res.status(HttpStatus.CREATED).end();
}

最后，被调用的服务方法:

And finally, the service method that's being called:

public addToDo(toDo: IToDoDto): void {
  toDo.id = this.idCounter;
  this.todos.push(toDo);
  this.idCounter++;
}

我可以传递任何参数，即使是那些与接口定义不匹配的参数，并且这段代码可以正常工作.我希望，如果从响应主体到接口的转换是不可能的，在运行时会像 Java 或 C# 一样抛出异常.

I can pass whatever arguments, even ones that don't come close to matching the interface definition, and this code will work fine. I would expect, if the cast from response body to interface is not possible, that an exception would be thrown at runtime like Java or C#.

我已经读到在 TypeScript 中不存在强制转换，只有类型断言，所以它只会告诉编译器一个对象是 x 类型，所以......我错了吗?强制执行和确保类型安全的正确方法是什么?

I have read that in TypeScript casting doesn't exist, only Type Assertion, so it will only tell the compiler that an object is of type x, so... Am I wrong? What's the right way to enforce and ensure type safety?

推荐答案

javascript 中没有强制转换，因此如果强制转换失败"，您将无法抛出.
Typescript 支持转换 但这仅适用于编译时间，并且你可以这样做:

There's no casting in javascript, so you cannot throw if "casting fails".
Typescript supports casting but that's only for compilation time, and you can do it like this:

const toDo = <IToDoDto> req.body;
// or
const toDo = req.body as IToDoDto;

您可以在运行时检查该值是否有效，如果没有则抛出错误，即:

You can check at runtime if the value is valid and if not throw an error, i.e.:

function isToDoDto(obj: any): obj is IToDoDto {
    return typeof obj.description === "string" && typeof obj.status === "boolean";
}

@Post()
addToDo(@Response() res, @Request() req) {
    if (!isToDoDto(req.body)) {
        throw new Error("invalid request");
    }

    const toDo = req.body as IToDoDto;
    this.toDoService.addToDo(toDo);
    return res.status(HttpStatus.CREATED).end();
}

<小时>

编辑

正如@huyz 指出的，不需要类型断言，因为 isToDoDto 是一个类型保护，所以这应该足够了:

---

Edit

As @huyz pointed out, there's no need for the type assertion because isToDoDto is a type guard, so this should be enough:

if (!isToDoDto(req.body)) {
    throw new Error("invalid request");
}

this.toDoService.addToDo(req.body);

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