Typescript 接口在用作运算符时获取错误的属性类型

Question

Since I am new to typescript I am not sure how the following code works, and why it is not typesafe:

interface MyNumber {
    x: number;
}

let data = JSON.parse(
    `{
        "x": "Hello"
    }`
);

let number: MyNumber = data as MyNumber ;
console.log(typeof number.x);

Webstorm tells me that x is of type number, but console prints type string. How can this be since the interface clearly defined the type of x as number not as string?

How would I do this correctly, to get a typesafe object (with the correct type?)

This question was derived from a more practical code like:

app.post('/', (req: Request<{},{}, Person>, res: Response) => {
   req.body.age; // <-- also can be either string or number depending what I provide in the body, so no typesafety
}

Answer 1

I think you misunderstood the use of typescript. Obviously your editor says the variable is of type number as defined by the typescript code. The content then is parsed at run time, so you will have no errors in your ide but during execution the resulting javascript has the content unchanged, ie as a string.