Typescript infer type from default value

Question

How do I infer the type from a passed argument or default parameter

export type ParamList = {
  SignIn: { warningMessage?: string } | undefined
  ResetOrCreatePassword: { email: string; hasPasswordAssociated: boolean }
}

type Props = {
  text: string
  fallbackScreen?: keyof ParamList
  params?: ParamList[keyof typeof fallbackScreen] // not valid but shows the goal
}

function MenuLink({fallbackScreen = 'SignIn', params = undefined, ...rest}: Props) {
  // impl not important
}

// Cases that should work
MenuLink({ text: "Go to sign in" })

MenuLink({
  text: "Reset password",
  fallbackScreen: 'ResetOrCreatePassword', 
  params: {
    email: '[email protected]',
    hasPasswordAssociated: true
  }
}) 

// TS should warn about missing params 👇
MenuLink({
  text: "Create password",
  fallbackScreen: 'ResetOrCreatePassword'
})

Your first case doesn't make any sense since the only parameter in MenuLink is required. — k.tten
– k.tten, Commented Sep 15, 2022 at 15:40
Didn't fix the above comment's remark, so I still don't know your intent. — k.tten
– k.tten, Commented Sep 15, 2022 at 17:59
You are right, Good point - I had the function boiled down to the minimum to show my attention. Hope I have it right now :) — Norfeldt
– Norfeldt, Commented Sep 15, 2022 at 18:16

k.tten · Accepted Answer · 2022-09-15 18:23:04Z

1

It is possible to achieve the desired behavior through the use of mapped types to generate the possible props:

type WithParams<T> = {
    [K in keyof T]: {
        text: string;
        fallbackScreen: K;
        params: T[K];
    }
}[keyof T];

type Props = {
    text: string;
    fallbackScreen?: never;
    params?: never;
} | WithParams<ParamList>;

This will give you something like this:

type Props = {
    text: string;
    fallbackScreen?: never;
    params?: never;
} | {
    text: string;
    fallbackScreen: "SignIn";
    params: { warningMessage?: string }
} | { ... }

You can see in the below playground that it also provides helpful errors.

Playground

answered Sep 15, 2022 at 18:23

k.tten

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