Explicit any type in for..in loop

For the reason of consistency, it should be possible to explicitly annotate the iterator variable at least with the “any” type in a for…in loop.

Edit from @sandersn: Fixes should apply to JSDoc too: https://github.com/microsoft/TypeScript/issues/43756, and perhaps allow annotations on for .. of too

[NemoStein]

In a for…in loop

for (const x in y) { ... }

I really think that x should be implicitly typed to keyof y instead of string (or any) considering that the loop will only assign enumerable non-Symbol properties of the y and nothing else can be assigned to x as it’s a const.

[Zarel]

I’m not sure if this is the right place to talk about this, but I have no idea how I’m supposed to safely use for..in in TypeScript.

type Stats = {atk: number, def: number};

function clearStats(stats: Stats) {
  for (var stat in stats) stats[stat] = 0;
}

[ts] Element implicitly has an ‘any’ type because type ‘{ atk: number; def: number; }’ has no index signature.

type StatName = 'atk' | 'def';
type Stats = {atk: number, def: number};

function clearStats(stats: Stats) {
  for (var stat: StatName in stats) stats[stat] = 0;
}

[ts] The left-hand side of a ‘for…in’ statement cannot use a type annotation.

type StatName = 'atk' | 'def';
type Stats = {[stat: StatName]: number};

function clearStats(stats: Stats) {
  for (var stat in stats) stats[stat] = 0;
}

[ts] An index signature parameter type must be ‘string’ or ‘number’.

Is it just me, or should there be a way to strongly type this?

I’ve been using

type StatName = 'atk' | 'def';
type Stats = {atk: number, def: number};

function clearStats(stats: Stats) {
  for (var stat in stats) stats[stat as StatName] = 0;
}

which works, but involves an unsafe cast. Is there a way to do this safely?