Function with return type depending on argument with default value

[tiny oak]

How could I realize a function that

• takes an array as argument
• the return type is dependent on the array, for example just returning the same array again
• the argument has a default value

I tried doing this, but am failing at the theoretical possibility of the user specifying a generics argument for the function but not the optional parameter

'[1, 2, 3]' is assignable to the constraint of type 'T', but 'T' could be instantiated with a different subtype of constraint 'any[]'

[craggy kernel]

leumasme#0

Playground Link

Preview:```ts
function getArr<T extends any[]>(
arr: T = [1, 2, 3] as const
): T {
// ...
throw new Error("Not Implemented")
}

// Expected type: [1,2,3]
let a = getArr()
// Expected type: ["a","b","c"]
let b = getArr(["a", "b", "c"] as const)```

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

[glad tulip]

you kinda can't

uh i would like to retract my statement it is badly phrased

you can't with optional/defaulted parameter as the signature

because what if you call getArr<string[]>()? that's going to return [1, 2, 3] but says it returns string[]

so you would have to split it with overloads or just split the functions entirely

[tiny oak]

Hmm yeah that was what I suspected, just wanted to see if someone here knew about some magic that I didn't ^^ Thanks though!

!resolved