for...in+if...hasOwnProperty is typed differently from Object.keys()

[MilesBHuff]

THIS IS NOT THE USUAL Object.keys TYPING "BUG".

TypeScript Version: 3.8.3

Search Terms: Object.keys

Code

function foo<T extends object>(bar: T): void {

    for(const key of Object.keys(bar)) {
        console.log(bar[key]); // TypeError
    }

    for(const key in bar) {
        if(bar.hasOwnProperty(key)) {
            console.log(bar[key]); // No error
        }
    }
}

Issue

The two for-loops above should be functionally identical (with the exception that the second for-loop is less-efficient.) Accordingly, key should have the same type in each.

However, in the former, key is a string, per the logic given by @RyanCavanaugh on Stack Overflow; while, in the latter, key is Extract<keyof T, string>. Correct me if I'm understanding this wrong; but if keyof isn't safe in the former, it seems like it wouldn't be safe in the latter, either.

Related Issues:

#12253, #30314, #13254. #30228, #28899, #28284, #26901, #30749, #31087, #32321, #34498, #35145.

[RyanCavanaugh]

There's actually special treatment to allow this that predates keyof T even existing. This is the intended behavior because for/in is a commonish way to iterate through things like arrays.

[typescript-bot]

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