typeof operator not using undefined guard in when used in a loop nested in the guard

[lesiak]

Bug Report

🔎 Search Terms

typeof, type guard, undefined guard, loop

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about Type System Behaviour and Type Guards

⏯ Playground Link

Playground link

💻 Code

interface IProperties {
    foo?: {
        aaa: string
        bbb: string
    }
}

function init(properties: IProperties) {
    if (properties.foo) {
        type FooOK = typeof properties.foo;  
        properties.foo; // type is { aaa: string; bbb: string; }
        for (const x of [1, 2, 3]) {
          properties.foo; // type is { aaa: string; bbb: string; }
          type FooOrUndefined = typeof properties.foo; //type is { aaa: string; bbb: string; } | undefined
        }
    } 
}

🙁 Actual behavior

Under a loop nested in undefined type guard for variable properties.foo,

• properties.foo is inferred to be { aaa: string; bbb: string; }
• typeof properties.foo is inferred to { aaa: string; bbb: string; } | undefined
  This happens both for for...of and while loops.

🙂 Expected behavior

typeof takes type guard into account:

• typeof properties.foo is inferred to { aaa: string; bbb: string; }

[MartinJohns]

I'm pretty sure this is related: #37516

[RyanCavanaugh]

I'm legitimately curious how you even noticed this

[lesiak]

@RyanCavanaugh Indeed, this code was not written intentionally. I was playing with a question on SO and pasted the check from someones answer in the wrong place.
https://stackoverflow.com/questions/66814919/scoping-rules-for-undefined-type-guard-in-typescript