Rethinking relationships between `{}` type, `object`, and primitives

🔎 Search Terms

empty object type, {}, type safety violation, unsound

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about object types and structural typing

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=5.8.0-dev.20241124#code/GYVwdgxgLglg9mABFAhgawKYGcDyAjAKw2gAoAPALkTkOKgEpEBvAKEXcQHpPEAVACxhZEAJxgBzflAA2AT0RDEKPgGVEGESLgiAdGw4xgicogC85xAAZGrDncQQEWONIw6NWkSQDkAxXhEUSH5ELH44EGkAE0Q8DERwEQwUCH4UPFdvegBufXYAXxZClkcwLChqKiZ8sytclm5EACEQCqhBYSEwbygAGiUwGLCI6Ni3FlRMXFpSOBygA

💻 Code

function takesObject(x: object) {
    // This rightly is a TS error.
    if (x === 0) {
        console.error('This branch should be unreachable');
    }
}

const o: {} = 0;

// But this isn't, and should be.
takesObject(o);

🙁 Actual behavior

No error on takesObject(o). {} can be assigned to type object, despite {} meaning "all nonnullish values", whereas object means only JS object types.

🙂 Expected behavior

Error on takesObject(o). {} is a wider type than object.

Additional information about the issue

This stems from a typescript-eslint investigation into making no-unnecessary-condition be more correct around possibly-falsy "object types", such as {}, or even { toFixed(): string} (to which number may be assigned). See https://github.com/typescript-eslint/typescript-eslint/pull/10378. This also relates to previous (controversial) conversations about whether to flag the {} type with the linter, see, e.g. https://github.com/typescript-eslint/typescript-eslint/issues/8700.

I'm wondering if this was simply an oversight in https://github.com/microsoft/TypeScript/pull/49119, which aimed to fix soundness holes with {}?

Reproduce the assignability cases in the linked TypeScript Playground, then read the referenced TypeScript PR #49119 and the typescript-eslint investigation in PR #10378 for context on the {}/object boundary. The issue is complete when the reported takesObject(o) case produces the expected diagnostic and related primitive cases are checked for regressions.