Static issue with user-defined type guard | TypeScript Community | Page 1

jolly carbon Mar 25, 2023, 8:50 AM

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export type Entity = {
   hasComponent<T>(componentName: string): this is T;
}

Hello- I'm getting an error here:
A 'this' type is available only in a non-static member of a class or interface.

How is the hasComponent method static? Doesn't hasComponent implicitly have this as an argument?
Or am I missing something entirely?

zinc shard Mar 25, 2023, 8:54 AM

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jolly carbon ```ts export type Entity = { hasComponent<T>(componentName: string): this is ...

this doesn't work on types, only in interfaces

jolly carbon Mar 25, 2023, 8:54 AM

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Oh, right

zinc shard Mar 25, 2023, 8:54 AM

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and functions have a this parameter, not a this type

jolly carbon Mar 25, 2023, 8:54 AM

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yep 👍

zinc shard Mar 25, 2023, 8:54 AM

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zinc shard `this` doesn't work on types, only in interfaces

because how this works, is that it's whatever the subclass is

jolly carbon Mar 25, 2023, 8:55 AM

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However the this is T idea would still work appropriately, right?

zinc shard Mar 25, 2023, 8:55 AM

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i guess the idea is that usually for types you dont really care about inheritance

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if you convert it to an interface it should work fine

jolly carbon Mar 25, 2023, 8:56 AM

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awesome, thanks a lot for the help :)

zinc shard Mar 25, 2023, 8:56 AM

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you could try manually adding a this parameter, which might make typescript recognize that its a parameter name not the special this type?

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i wouldn't know, i've never tried

jolly carbon Mar 25, 2023, 8:57 AM

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right, yeah I was just about to ask that.

zinc shard Mar 25, 2023, 8:57 AM

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jolly carbon ```ts export type Entity = { hasComponent<T>(componentName: string): this is ...

!pg

vocal onyxBOT Mar 25, 2023, 8:57 AM

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View in Playground

jolly carbon Mar 25, 2023, 8:57 AM

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I'm wondering what happens if I mix and match types and interfaces. I'll have a play around :)

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again- thanks a lot for the help

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!resolved

vocal onyxBOT Mar 25, 2023, 8:57 AM

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n_n#2622

Playground Link

zinc shard Mar 25, 2023, 8:57 AM

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nope :(

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!ts

vocal onyxBOT Mar 25, 2023, 8:57 AM

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export type Entity = {
   hasComponent<T>(this: Entity, componentName: string): this is T;
//                                                       ^^^^
// A 'this' type is available only in a non-static member of a class or interface.
}

jolly carbon Mar 25, 2023, 8:58 AM

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huh weird

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surely if you rename the this though

zinc shard Mar 25, 2023, 8:58 AM

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this is a special parameter

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it doesnt actually exist

jolly carbon Mar 25, 2023, 8:58 AM

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I suspect the this won't be picked up as the special parameter then

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yeah

zinc shard Mar 25, 2023, 8:58 AM

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yea

jolly carbon Mar 25, 2023, 8:59 AM

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I come from a lua background, so where I come from we don't really worry about this or self, everything is just a static function :p

zinc shard Mar 25, 2023, 8:59 AM

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i mean

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you can still define methods

jolly carbon Mar 25, 2023, 8:59 AM

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In a type you mean?

zinc shard Mar 25, 2023, 9:00 AM

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function obj:method () end

jolly carbon Mar 25, 2023, 9:00 AM

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oh yeah yeah

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but it's functionally equivalent to function obj.method(self) end

zinc shard Mar 25, 2023, 9:00 AM

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i do the very idiosyncratic local f = function () end though

jolly carbon Mar 25, 2023, 9:00 AM

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hah, yeah classic

zinc shard Mar 25, 2023, 9:01 AM

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in an effort to minimize the number of syntax constructs im using

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not like its harder to maintain or anything so might as well, right?

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and not like it's different semantically unlikecough JS

jolly carbon Mar 25, 2023, 9:01 AM

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haha yeah sure, each to their own! :)

jolly carbon Mar 25, 2023, 9:02 AM

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zinc shard i do the very idiosyncratic `local f = function () end` though

My past codebase had this kindof thing all over the place

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in hindsight it hurts readibility a tiny bit, but like, ~~does it really matter that much?~~

#Static issue with user-defined type guard