'typeof' doesn't return real type in switch control flow(type guard)

[Messiahhh]

Bug Report

🔎 Search Terms

type guard、narrowing

🕗 Version & Regression Information

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

⏯ Playground Link

Playground link with relevant code

💻 Code

const value: number | string = 100;

function fn(): number | undefined {
    switch (typeof value) {
        case 'number':
            const v = typeof value;
            return v;
        default: 
            return 
    }
}

🙁 Actual behavior

the type of 'v' is not number

🙂 Expected behavior

the typeof of 'v' is number

[Josh-Cena]

The operator typeof has two meanings. When it's used in the context of a type, it is a TS syntax that gets the type of a value; when it's used in the context of a value, it is a JS syntax, which always returns a string. You can check the MDN docs and the handbook

[Messiahhh]

The operator typeof has two meanings. When it's used in the context of a type, it is a TS syntax that gets the type of a value; when it's used in the context of a value, it is a JS syntax, which always returns a string. You can check the MDN docs and the handbook

I already know about this, but typeof of value in 'case "number"' statement is number, while typeof value doesn't return number

[Messiahhh]

The operator typeof has two meanings. When it's used in the context of a type, it is a TS syntax that gets the type of a value; when it's used in the context of a value, it is a JS syntax, which always returns a string. You can check the MDN docs and the handbook

[Josh-Cena]

const a: typeof v = 1;
      // ^ `typeof` is used in type-space: it is a TS syntax,
      // will be erased when you compile to JS.
      // This will make `a` have the type `number`.
const a = typeof v;
       // ^ `typeof` is used in value-space: it is a JS syntax.
       // This will make `a` have the value `"number"`, which is a string.

[Messiahhh]

The operator typeof has two meanings. When it's used in the context of a type, it is a TS syntax that gets the type of a value; when it's used in the context of a value, it is a JS syntax, which always returns a string. You can check the MDN docs and the handbook

@Josh-Cena my example code is not exactly correct, but look at this picture, the type of v

[Josh-Cena]

The type of value is number, but typeof value returns a string. typeof value is an actual JS value, which is a string "number".

[whzx5byb]

@Messiahhh It seems that you are confused when value is number but typeof value is "string" | "number" | etc..., right? The behavior is discussed in #46399.

[typescript-bot]

This issue has been marked as 'Question' and has seen no recent activity. It has been automatically closed for house-keeping purposes. If you're still waiting on a response, questions are usually better suited to stackoverflow or the TypeScript Discord community.