can't narrow from unknown to "not a number or bigint"

[ljharb]

🔎 Search Terms

number bigint narrow

🕗 Version & Regression Information

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

⏯ Playground Link

https://tsplay.dev/wX09ow

💻 Code

function nonNumeric(x: null | undefined | string | boolean | symbol | object, y: unknown) {
	return true;
}

function foo(x: unknown, y: unknown) {
	if (typeof x === 'number' || typeof x === 'bigint') {
		return false;
	}
	return nonNumeric(x, y);
}

🙁 Actual behavior

errors with Argument of type 'unknown' is not assignable to parameter of type 'string | boolean | symbol | object | null | undefined'.(2345)

🙂 Expected behavior

passes, because the typeof check ensures that unknown is properly narrowed to string | boolean | symbol | object | null | undefined.

Additional information about the issue

No response

[MartinJohns]

I would say this is a design limitation.

unkown is not a union of the types you listed, so the compiler can't narrow the type in the else branch. This would require a type "unknown, except number", which would first require negated types (#4196).

[ljharb]

Why isn't unknown a union of all possible types? I would expect it to be a named shorthand for null | undefined | string | boolean | number | bigint | symbol | object since that includes every possible JS value.

[MartinJohns]

See: #39498 (comment)

[ljharb]

oof, ok - that issue is actually my precise need for that exact same library. That's very unfortunate.

[craigphicks]

Consider this working code

function nonNumericTypeof(typeofx: "string" | "boolean" | "symbol" | "undefined" | "object" | "function") {
	return true; 
}

function foo(x: unknown) {
    const t = typeof x;
	if (t === 'number' || t === 'bigint') {
		return false;
	}
    t; // const t: "string" | "boolean" | "symbol" | "undefined" | "object" | "function"
    return nonNumericTypeof(t);
}

Conceivably, type information could embed the results of typeof comparisons (both true and false branches because typeof range is finite) . However, there are reasons why that very hard to integrate into current TypeScript, one of which is that number, string, etc. can be promoted to objects for type comparison.

[ljharb]

How difficult it would be to achieve is entirely separate and not something I have knowledge of.

However, I can't conceive of any code where this narrowing would be incorrect, including your example.

[typescript-bot]

This issue has been marked as "Duplicate" and has seen no recent activity. It has been automatically closed for house-keeping purposes.