Comments (7)
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.
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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).
from typescript.
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.
from typescript.
See: #39498 (comment)
from typescript.
oof, ok - that issue is actually my precise need for that exact same library. That's very unfortunate.
from typescript.
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.
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This issue has been marked as "Duplicate" and has seen no recent activity. It has been automatically closed for house-keeping purposes.
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