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

问题描述

我试图将我的代码从express的请求正文（使用body-parser中间件）转换为接口，但这并没有强制类型安全。

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.

这是我的接口：

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

这是我尝试执行转换的代码：

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，被称为的服务方法：

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，甚至可以'接近匹配接口定义，并且此代码可以正常工作。我希望，如果不可能从响应主体到接口进行强制转换，则将在运行时抛出异常，例如Java或C＃。

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中不存在类型转换，只有类型声明，所以它只会告诉编译器对象的类型为 x ，所以...我错了吗？强制执行并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，因此如果投射失败，则无法抛出。

Typescript < href = https://www.typescriptlang.org/docs/handbook/basic-types.html#type-assertions rel = noreferrer>支持转换，但这仅用于编译时间，您可以这样做：

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以及是否抛出错误，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();
}

---

编辑

正如@huyz所指出的那样，不需要类型断言，因为 isToDoDto 是类型保护，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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