New definition of PropertyKey does not observe the `keyofStringsOnly` setting

[pelotom]

TypeScript Version: 2.9.2

Search Terms: PropertyKey keyofStringsOnly

Code

In lib.es5.d.ts, we have now

declare type PropertyKey = string | number | symbol;

however this seems incorrect if "keyofStringsOnly": true. So as to observe that setting, it seems like it should be

declare type PropertyKey = keyof any;

[mhegazy]

PropertyKey type alias has nothing to do with keyof behaviour. --keyofStringsOnly controls the behavior of keyof to choose whether to use TypeScript 2.9 or pre-TypeScript 2.9 behaviour.

For instance, Object.hasOwnProperty accepts a symbol as its input regardless of --keyofStringsOnly.

[pelotom]

Ok, but PropertyKey’s definition was changed in 2.9, coincidentally with the keyof change, and it is a breaking change. --keyofStringsOnly is supposed to prevent breaking changes in 2.9, so it seems like it should cover this as well.

[mhegazy]

--keyofStringsOnly is supposed to prevent breaking changes in 2.9,

it is not a catch all flag. it is meant for a specific use case.

[pelotom]

So PropertyKey's definition was just coincidentally changed to string | number | symbol at the same time as the keyof change? Why?

[mhegazy]

I am not sure i follow. PropertyKey was just moved from lib.es6.d.ts to lib.d.ts to consolidate definitions of Object methods. see 68ce69a#diff-a6b488d9bd802977827b535a3011c1f3

[pelotom]

I see. I'm not sure why this is suddenly breaking my code in 2.9 then, but oh well 🤷‍♂️. Thanks for the info.

[nalply]

Let me add to this issue and point out something I just discovered today.

The definition of PropertyKey is a bit too loose for some definitions in Reflect, Proxy and probably a few more. Numbers will never be encountered in these places. This is not a bug but some surprising hidden JavaScript weirdness: keys are coerced to string even for arrays.

Example: Reflect.ownKeys([42]) gives [ '0', 'length' ].

Perhaps TypeScript should introduce a second, more strict definition like this: type PropertyKeyOut = string | symbol?

So that the return type of Reflect.ownKeys() in lib.es2015.reflect.d.ts can be defined as PropertyKeyOut[]?

[monfera]

As luck would have it, I almost filed a TS issue today (ES2015 explicitly says that Reflect.ownKeys can only ever return an array of string and symbol elements) but one existed already and the fix got merged a few hours ago 🎉