strictNullChecks blocks safe typeof capture

[dylanscott]

Sorry if this is a dupe - I tried searching but github's search isn't great.

TypeScript Version: 2.3.1

Code

interface Nested {
    child: {
        prop: string
    }
}

declare function maybeGetNestedValue(): Nested | null

const nested = maybeGetNestedValue()

const child: typeof nested.child = {
    prop: 'value'
}

TypeScript Playground link
Note: You need to manually enable strictNullChecks under the options for this link. If I enable it and try to share it says the text buffer is too big to share.

Expected behavior: I would expect to be able to use typeof to capture the type of this nested field, since as you can see in the emitted code in the TypeScript Playground link, there is no dereference of the nested field emitted, so the code should always be safe.

Actual behavior: The strictNullChecks flag raises an error because the nested field may be null.

[dylanscott]

Also note: It appears you cannot use the non-null assertion operator to appease the compiler here. typeof nested!.child doesn't appear to even be syntactically valid.

[aluanhaddad]

That error message is indeed strange in type position.

What type would you expect the child variable to have?
It cannot be

{
    prop: string
}

[dylanscott]

Ah, I suppose it can't. I'm trying to capture the type { prop: string } but you're right that I was thinking about it wrong. typeof nested.child isn't well-defined (or maybe it is? maybe { prop: string } | never). In this case I guess you could just do Nested['child'], but that doesn't work if you have further layers of nesting with potentially null fields.

[aluanhaddad]

The simplest fix is to create an extra local which is obviously not ideal or, as you need to test nested itself before use, you could just test it earlier

declare function maybeGetNestedValue(): Nested | null;

const nested = maybeGetNestedValue();
const notNullNested = nested!;
const child: typeof notNullNested.child = {
    prop: 'value'
};

You can also use Nested["child"].

[aluanhaddad]

@Andy-MS right, thank you! I keep forgetting about that syntax. It's definitely the way to go here.

[RyanCavanaugh]

Feels like property access in typeof expressions should implicitly be getting x!.y. Unclear what value there is warning someone about a possibly-null x in that case