Function with properties type inferred only at top level

[danvk]

TypeScript Version: 3.5.1

Search Terms:

• 2739
• function properties block
• top level function

Code

interface Person {
  first: string;
  last: string;
}

const dice = () => Math.floor(Math.random() * 6);
dice.first = 'Rando';
dice.last = 'Calrissian';
const diceP: Person = dice;  // OK

// but inside a block...
{
  const dice = () => Math.floor(Math.random() * 6);
  dice.first = 'Rando';  // Property 'first' does not exist on type '() => number'. 
  dice.last = 'Calrissian';  // Property 'last' does not exist on type '() => number'.
  const diceP: Person = dice;  // Type '() => number' is missing the following properties from type 'Person': first, last
}

Expected behavior:

I would expect the same behavior both inside and outside the block.

Actual behavior:

Assigning properties to a function at the top level is allowed, but not inside a block. Frankly, I'm impressed that the first example works at all. But I don't see why they would be different!

Playground Link: link

Related Issues:

• expando fields not added to symbol exports  #31778

[RyanCavanaugh]

This is the intended behavior - you can "declare" new properties "locally" with bare assignments, but you can't just tack on new properties anywhere

[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.

[weswigham]

Second look: Probably a bug. Moving the same code into a block shouldn't really affect it's validity, as is shown here.

[weswigham]

cc @sandersn there's some sort of check we have for top-level blocks, but not nested blocks, which seems quite unfortunate here.

[sandersn]

Check the binder in bindPropertyAssignment; it's where the most complex top-level check is, and is probably the culprit. I don't think there are any top-level checks in the checker, but I could be wrong.