extract the types of nodejs callback style function

[stable gorge]

Having problems getting the correct parameter:
https://www.typescriptlang.org/play?#code/JYWwDg9gTgLgBAbygUwIYBMBiwA2yC+cAZlBCHAEQB2E6yAXEQM4UBQ7MAnmMnAIJQA5gEYAPJioA+OAF5WASAlxkADxjIq6JnAAUC+cCpgArjHpxDRZFDgANADRwA8mBhNzqKp0cBhAEIeXgoAlLLSAG4QwOhwAPwKtnD0ClTI4dYc3LwubuJSsgpKquqa2nryBkamgd7Oru4WVFY2DnD+NSFhcJHRcQlJKWkZrHjwRLgM-EJiXDwQRHAoGNh40uwA9OtwAJIA5OSCyDAwhoJwMAAWvADGqDg4AEao1wDW51m6AMxQMWCoUKgQEdrMFHDR4JdeBBXMAIFRtLNeDoAEyaOB-AFA9RQYIjI5waFucw5JiiRHzRZoLATaRAA

[tacit lantern]

@stable gorge Here's a shortened URL of your playground link! You can remove the full link from your message.

ricmed#0

Playground Link

Preview:```ts
import {readFile} from "node:fs"

type Arg1<Fn> =
Fn extends (
input: infer X, Opts: any, CB: any
) => void ?
X :
never

type Opts<Fn> =
Fn extends (
input: any, Opts: infer X, CB: any
) => void ?
X :
never

let file: Arg1<typeof readFile>

// I'm getting the callback type (3rd parameter), not the options type (2nd parameter)
...```

You can choose specific lines to embed by selecting them before copying the link.

[glass thunder]

it's not possible to extract the signature of a function that uses overloads

readfile has 4 possible overloads

while the 1st parameter is always the same (path: PathOrFileDescriptor) there are no problems

but as soon as the overloads have a different type for the other parameters, you'll always get the type of the last overload

https://www.typescriptlang.org/docs/handbook/2/conditional-types.html#inferring-within-conditional-types

When inferring from a type with multiple call signatures (such as the type of an overloaded function), inferences are made from the last signature (which, presumably, is the most permissive catch-all case). It is not possible to perform overload resolution based on a list of argument types.

[stable gorge]

thanks!