Using TypeScript generics to define function parameters

I'm currently working on opening a typescript method that utilizes generics. The scenario involves an object with different methods, each with specified types for function parameters.

const emailTypes = {
  confirmEmail: generateConfirmEmailOptions, // { a: string, b: string;}
  restorePassword: generateRestorePasswordOptions, // { a: string, b: number;}
};

The type with keys from this object:

export type IEmailTypes = typeof emailTypes;

and here is the method itself:

export const getEmailOptions = <T extends keyof IEmailTypes>(emailType: T, emailData: Parameters<IEmailTypes[T]>[0]) =>
  emailTypes[emailType](emailData);

I am encountering an issue specifically with (emailData), and I cannot figure out why. I believed I was following the correct approach by passing the key names to T and then extracting the function parameters based on this key. However, it seems there are discrepancies in types.

My intention is to have TypeScript provide clear guidance on what emailData should be when inputting, for example, confirmEmail into emailType.

If anyone can point out where I might be going wrong, I would greatly appreciate it.

For better clarity, I've included a screenshot.

*methods in emailTypes have parameter types

typescripttypescript-typingstypescript-generics

Answer №1

Take a look at @jcalz's response here.

You're faced with the decision to either compromise type safety by casting, or modify the code to assist the compiler. Check out the suggested changes here:

declare const generateConfirmEmailOptions: (opts: { a: string; b: string }) => void
declare const generateRestorePasswordOptions: (opts: { a: string; b: number }) => void

interface Args {
  confirmEmail: Parameters<typeof generateConfirmEmailOptions>[0]
  restorePassword: Parameters<typeof generateRestorePasswordOptions>[0]
}

type Mapping = { [K in keyof Args]: (opts: Args[K]) => void }

const emailTypes: Mapping = {
  confirmEmail: generateConfirmEmailOptions, 
  restorePassword: generateRestorePasswordOptions, 
};

const getEmailOptions = <K extends keyof Args>(emailType: K, emailData: Args[K]) => {
  const fn = emailTypes[emailType] 
  return fn(emailData);
}

It's unfortunate that there is some code duplication involved, but for now, this seems to be the only completely type-safe approach. We'll have to wait and see what improvements future versions of TypeScript bring.

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