Why can't my `KeysOfType<T, LeveledLogMethod>>` be assigned to a `LeveledLogMethod`? | TypeScript Community | Page 1

turbid rivetBOT Nov 11, 2023, 4:21 AM

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bawdyinkslinger#0

Playground Link

Preview:```ts
declare class Logger {
silent: boolean
exitOnError: Function | boolean
defaultMeta?: any
clear(): this
close(): this

// for cli and npm levels
error: LeveledLogMethod
warn: LeveledLogMethod
...```

slow hound Nov 11, 2023, 4:22 AM

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!ts

turbid rivetBOT Nov 11, 2023, 4:22 AM

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declare class Logger {
    silent: boolean;
    exitOnError: Function | boolean;
    defaultMeta?: any;
    clear(): this;
    close(): this;

    // for cli and npm levels
    error: LeveledLogMethod;
    warn: LeveledLogMethod;
    help: LeveledLogMethod;
    data: LeveledLogMethod;
    info: LeveledLogMethod;
    debug: LeveledLogMethod;
    prompt: LeveledLogMethod;
    http: LeveledLogMethod;
    verbose: LeveledLogMethod;
    input: LeveledLogMethod;
    silly: LeveledLogMethod;
}

type LeveledLogMethod = () => string;

type KeysOfType<Obj, Type> = {
    [Prop in keyof Obj]: Obj[Prop] extends Type ? Prop : never
}[keyof Obj]

function validateBeforeCall<T extends Logger, Prop extends KeysOfType<T, LeveledLogMethod>>(logger: T, logFunction: Prop): LeveledLogMethod {
    const x: LeveledLogMethod = logger[logFunction]
//        ^
// Type 'T[Prop]' is not assignable to type 'LeveledLogMethod'.
//   Type 'T[KeysOfType<T, LeveledLogMethod>]' is not assignable to type 'LeveledLogMethod'.
//     Type 'T[T[keyof T] extends LeveledLogMethod ? keyof T : never]' is not assignable to type 'LeveledLogMethod'.
//       Type 'T[keyof T]' is not assignable to type 'LeveledLogMethod'.
//         Type 'T[string] | T[number] | T[symbol]' is not assignable to type 'LeveledLogMethod'.
//           Type 'T[string]' is not assignable to type 'LeveledLogMethod'.
//        ^? - const x: LeveledLogMethod
    
    return () => "hi";
}```

slow hound Nov 11, 2023, 4:22 AM

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why can't x be a LeveledLogMethod?

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hm, I just noticed I can call validateBeforeCall with any key as the second param, that's not working right

vagrant oracle Nov 11, 2023, 4:36 AM

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It doesn't solve your problem, but just as a quick note, the Prop generic is not necessary:

- validateBeforeCall<T extends Logger, Prop extends KeysOfType<T, LeveledLogMethod>>(logger: T, logFunction: Prop)
+ validateBeforeCall<T extends Logger>(logger: T, logFunction: KeysOfType<T, LeveledLogMethod>)

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As a general rule of thumb, a generic type parameter should be used more than once.

slow hound Nov 11, 2023, 4:41 AM

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vagrant oracle As a general rule of thumb, a generic type parameter should be used more than on...

I solved my prob

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but I don't know why it works

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and it doesn't compile in the playground, but it does in my ide...

vagrant oracle Nov 11, 2023, 4:42 AM

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Run tsc to make sure it's not just your IDE failing to report errors.

turbid rivetBOT Nov 11, 2023, 4:43 AM

#

bawdyinkslinger#0

Playground Link

Preview:```ts
declare class Logger {
silent: boolean
exitOnError: Function | boolean
defaultMeta?: any
clear(): this
close(): this

// for cli and npm levels
error: LeveledLogMethod
warn: LeveledLogMethod
...```

slow hound Nov 11, 2023, 4:44 AM

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vagrant oracle Run `tsc` to make sure it's not just your IDE failing to report errors.

it does

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I'm not sure why this playground doesn't

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and I am using 5.2.2

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I guess I'm okay moving on. Unless, in your opinion, it makes sense that that error should occur

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oh!

vagrant oracle Nov 11, 2023, 4:48 AM

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Is your defaultMeta optional in your local code?

slow hound Nov 11, 2023, 4:48 AM

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lol no

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That's what I was about to say

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Once I removed it from the example it works the same way as in my IDE

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wait hold on

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wtf

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I misspoke

turbid rivetBOT Nov 11, 2023, 4:49 AM

#

bawdyinkslinger#0

Playground Link

Preview:```ts
class Logger extends NodeJSStream.Transform {
constructor(options?: LoggerOptions)

silent: boolean
format: logform.Format
levels: Config.AbstractConfigSetLevels
level: string
transports: Transport[]
exceptio

...```

slow hound Nov 11, 2023, 4:49 AM

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That is the unmodified class in its declaration file

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Okay I think I just have a looser tsconfig option

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in my project

vagrant oracle Nov 11, 2023, 4:51 AM

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Btw, the Prop generic type parameter isn't necessary.

vagrant oracle Nov 11, 2023, 4:52 AM

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vagrant oracle It doesn't solve your problem, but just as a quick note, the `Prop` generic is n...

Just like this.

slow hound Nov 11, 2023, 4:52 AM

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oh yeah, that makes sense

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Does that make a difference or was it merely unnecessarily verbose?

vagrant oracle Nov 11, 2023, 4:52 AM

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Your solution also doesn't do the same thing as the initial question, but well, if that's aceeptable then sure.

slow hound Nov 11, 2023, 4:53 AM

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vagrant oracle Your solution also doesn't do the same thing as the initial question, but well, ...

are you referring to the return type?

vagrant oracle Nov 11, 2023, 4:53 AM

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No

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Your initial code works for generic loggers, eg if you make a new logger subclass with a new log level method, you can use that in your initial code.

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That wouldn't work in your latter code.

slow hound Nov 11, 2023, 4:58 AM

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by "wouldn't work", you mean it wouldn't allow me to pass in the subclass's new method?

vagrant oracle Nov 11, 2023, 4:58 AM

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Yes.

#Why can't my `KeysOfType<T, LeveledLogMethod>>` be assigned to a `LeveledLogMethod`?