Bad type checking inside typeguarded block

[lonewarrior556]

TypeScript Version: 3.5.1

Search Terms: Type Guard

interface FOO {
    [key:string]: number
}

declare var foo: FOO | null

if(foo) {
    var a =  foo['string'] // good
    Object.keys(foo).map(key => {
        var b =  foo[key]  // foo cannot possibly be null
    })
}

//related
foo && foo['string'] && Object.keys(foo).map(key => {
    var a = foo[key] // foo cannot possibly be null
})

Expected behavior:
Should know Object is not 'null'

Actual behavior:
Thinks Object is possibly 'null'

Playground Link: ts-playground

Related Issues:

[AnyhowStep]

It cannot know that foo won't become null inside the function, in the general case.

In this specific instance, you can say that the callback is invoked immediately and never invoked again after .map(), so, it cannot be null.

But the compiler can't know that for sure yet

[AnyhowStep]

#11498

[fatcerberus]

Also see #9998.

[typescript-bot]

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