Narrowing from `typeof x !== 'object'` incorrectly excludes functions.

Bug Report

🔎 Search Terms

typeof object function narrow, type narrowing

🕗 Version & Regression Information

Present in all recent versions from at least 3.3.3333 through the current nightly 4.4.0-dev.20210701

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about "typeof" and "narrow"

⏯ Playground Link

Playground link

💻 Code

let x!: string|object; // |Function;
if (1 < 2) x = () => 1;

if (typeof x !== 'object') {
  useString(x); // should fail, but doesn't
}

declare function useString(s: string): void;

🙁 Actual behavior

The type narrowing for typeof x !== 'object' pruned the entire |object branch from x's type union, but typeof can return either "object" or "function" for values of type object, so this is overzealous. This causes x inside the conditional to be incorrectly narrowed to just string when we can clearly see from the earlier line that it's been assigned a function, which is incorrectly passed to useString with no complaint.

🙂 Expected behavior

Excluding "object" from typeof x should not remove |object from its type, in the same way that it doesn't narrow unknown any further. This would cause the appropriate type error from passing a function to useString.

Note that if an explicit |Function branch is added to to x's type union then the narrowing proceeds as expected.

Bắt đầu với TypeScript Playground được liên kết và ví dụ narrowing được cung cấp, sau đó lần theo logic narrowing kiểu cho typeof x !== "object". Bản sửa hoàn tất khi một hàm được gán cho x không bị loại bỏ bởi narrowing và việc truyền x vào useString tạo ra lỗi kiểu như mong đợi; hãy kiểm tra hành vi dựa trên các phiên bản đã nêu.