I am experimenting with TypeScript in the playground and found that it allows me to assign the object returned by JSON.parse() to a class, even though the object won’t have any methods, and this causes a runtime error. This is surprising to me as my understanding was that TypeScript is typesafe as long as you don’t use as or any.

Is it possible to have the type system catch this case, using only type annotations, not changing the behaviour of the code?

class Foo {
    inner: Number;
    constructor(inner: Number) {
        this.inner = inner;
    }
    getInner() {
        return this.inner;
    }
}
let result: Foo = JSON.parse('{ "inner": 5 }');
console.log(result.getInner());