Why typescript doesn’t know that i checked for object type?

I want to build such a function:

const recursionProxy = <T extends object>(subject: T) =>
  new Proxy(subject, {
    get(target, key: keyof T) {
      const nestedSubject = target[key];

      if (typeof nestedSubject === "object") {
        return recursionProxy(nestedSubject);
      }

      return nestedSubject ?? target._ ?? "Message not set";
    },
  });

but under the line recursionProxy(nestedSubject); there is an error that says

[i] Argument of type 'T[keyof T]' is not assignable to parameter of type 'object'.

why typescript doesn’t take that if staement in consideration, in side the if statement nestedSubject is of type object

Answer

It does seem to work if you use a type predicate :

const isObject = (f: any): f is object => {
   if(typeof f === "object") return true;
return false;
}
const recursionProxy = <T extends object>(subject: T) =>
  new Proxy(subject, {
    get(target, key: keyof T) {
      const nestedSubject = target[key];

      if (isObject(nestedSubject)) {
        return recursionProxy(nestedSubject);
      }

      return nestedSubject ?? target._ ?? "Message not set";
    },
  });

Link

A null check can also be added in the predicate, if that is required:

const isObject = (f: any): f is object => {
  if(f === null) return false;
   if(typeof f === "object") return true;
return false;
}