CODEWITHSUNDEEP
typescripttypescript-typings
Adam
Adam

Reputation: 3518

Typescript make param optional

Here is playground

I am trying to make a parameter optional, but still make typescript understand what will get returned from function.

If param is defined, then type of param is returned. Otherwise - if no param is provided, function will return undefined.

const meh = <T>(initialData?: T): { data: T | undefined} => {
    if (initialData) {
        return { data: initialData }
    }

    return {
        data: undefined
    }
}

const res1: undefined = meh().data // is ok:

type ResType = {
    hello: string
}

const res2: ResType = meh<ResType>({ hello: 'hello'}).data.hello // TS error: Object is possibly undefined

Which makes sense. Though, I found conditional types and I though I could do something like

const meh = <T>(initialData?: T): { data: T ? T : undefined} => {

, but it gives syntax error.

Upvotes: 0

Views: 136

Answers (1)

pushkin
pushkin

Reputation: 10209

Could you use function overloads?

function meh(): { data: undefined };
function meh<T>(initialData: T): { data: T };

function meh<T>(initialData?: T): any {
    if (initialData) {
        return { data: initialData }
    }

    return {
        data: undefined
    }
}

type ResType = {
    hello: string
}

const res1: ResType = meh<ResType>({ hello: 'hello'}).data
const res2: undefined = meh().data

Upvotes: 1

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