Cast object to interface in TypeScript - object

Question

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.
This is my interface:
export interface IToDoDto {
description: string;
status: boolean;
};
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();
}
And finally, the service method that's being called:
public addToDo(toDo: IToDoDto): void {
toDo.id = this.idCounter;
this.todos.push(toDo);
this.idCounter++;
}
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#.
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?

Answer 1

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;
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();
}
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);

Answer 2

Here's another way to force a type-cast even between incompatible types and interfaces where TS compiler normally complains:
export function forceCast<T>(input: any): T {
// ... do runtime checks here
// #ts-ignore <-- forces TS compiler to compile this as-is
return input;
}
Then you can use it to force cast objects to a certain type:
import { forceCast } from './forceCast';
const randomObject: any = {};
const typedObject = forceCast<IToDoDto>(randomObject);
Note that I left out the part you are supposed to do runtime checks before casting for the sake of reducing complexity. What I do in my project is compiling all my .d.ts interface files into JSON schemas and using ajv to validate in runtime.

Answer 3

If it helps anyone, I was having an issue where I wanted to treat an object as another type with a similar interface. I attempted the following:
Didn't pass linting
const x = new Obj(a as b);
The linter was complaining that a was missing properties that existed on b. In other words, a had some properties and methods of b, but not all. To work around this, I followed VS Code's suggestion:
Passed linting and testing
const x = new Obj(a as unknown as b);
Note that if your code attempts to call one of the properties that exists on type b that is not implemented on type a, you should realize a runtime fault.