Typescript Generics and not assignable error-CodePudding

I seem to have a understanding problem of how typescript types works. I have the following situation. I have a class which provides a function which can be overriden:

    /**
     * The current log method. Can be overridden to redirect output.
     */
    log: (...args: any[]) => void;

I have the following type

type AzureTransport = {
  (...args: any[]): void
}

No I have a kind of facade which looks like:

export interface DataChampLogger {
  info(...args: any[]): void
  error(...args: any[]): void
  warn(...args: any[]): void
  verbose(...args: any[]): void
  setLogLevel(level: DataChampLogLevel): void
  setTransport<T>(transport: T): void

// implementation
class .... {
 setTransport<AzureTransport>(transport: AzureTransport): void {
    AzureLogger.log = transport
  }
}
}

this does not work, I get the following error Type 'AzureTransport' is not assignable to type '(...args: any[]) => void'.ts(2322)

when I refactor the code to:

export interface DataChampLogger {
  info(...args: any[]): void
  error(...args: any[]): void
  warn(...args: any[]): void
  verbose(...args: any[]): void
  setLogLevel(level: DataChampLogLevel): void
  setTransport(transport: unknown): void

// implementation
class .... {
 setTransport(transport: AzureTransport): void {
    AzureLogger.log = transport
  }
}
}

everything works fine, but it is not really clear to why I get an error in the first version of the implementation.

[edit] Playground Link

Thanks for your time and help

Regards Mathias

CodePudding user response：

It's because Typescript cannot know that the AzureTransport type that you use in the class will be assignable to T, since T could be something else.

You can add a generic constraint like that:

export interface DataChampLogger {
  setTransport<T extends AzureTransport>(transport: T): void; // "T extends AzureTransport" is the constraint
}

class MyClass implements DataChampLogger {
  setTransport(transport: AzureTransport): void {
  }
}

Alternatively, you can move the generic to the interface itself, like that:

export interface DataChampLogger<T> // generic is here  {
  setTransport(transport: T): void;
}

class MyClass implements DataChampLogger<AzureTransport> {
  setTransport(transport: AzureTransport): void { // no complaints since T is AzureTransport
  }
}

On a side note, this

type AzureTransport = {
  (...args: any[]): void
}

is the same as

type AzureTransport = (...args: any[]) => void;

You don't need the curly brackets.