For example the following code almost works, the problem is that if the type is undefined it's still required (test(undefined)).
const symbol =
function test<Type>(param: if Type something ? required : not) {}
Is this possible and how?
#How to make a parameter conditionally optional?
leaden token
How to make a parameter conditionally optional?
brisk mountain
overloads
solar heath
You can also use spread arguments:
plain siloBOT
declare function fn<T>(...args: T extends string ? [arg: 42] : []): void
fn<string>()
//
// Expected 1 arguments, but got 0.
fn<string>(42)
fn<number>()
fn<number>(42)
// ^^
// Expected 0 arguments, but got 1.
Thanks so much (had this problem so many times), I think overloads are easier to understand (it's for a library).
!resolved
I think I have to use spread arguments after all because it's for a class construtor.
solar heath
You can overload constructor as well.
leaden token
But the generic can't have a different type for each overload, at least this is how I did it:
const symbol = Symbol();
type Symbol = typeof symbol;
function test<Type extends Symbol>(param?: undefined): void;
function test<Type>(param: Type): void;
function test<Type = Symbol>(param: Type | undefined) {}
sure it can, only one of them is instantiated
but since the first overload doesn't use its generic, it isn't inferrable
leaden token
I don't understand, the last example I gave works, what I meant is that I don't think I can do the same with a class construtor.
leaden token
It works now with Burrito's answer, I would prefer to overload because it's easier to understand (for the library users) but I don't think it's possible because classes have the generics types and constructor parameters in different places so I can't change one based on the other like with functions.
solar heath
But the generic can't have a different type for each overload, at least this is ...
I'm not sure what this piece of code is supposed to do, but it seems to me it can just be:
function test<T>(param?: T) {}
But yeah, it does sound like an XY problem, you should probably explain what you are actually trying to achieve than your attempted solution.
leaden token
It's a router that has a state for each request that can be of a custom type, the default is Record<PropertyKey, unknown> and I want it so you don't have to pass an empty object in that case.
I got it working like this:
declare const DATA_BRAND: unique symbol;
type Default = Record<PropertyKey, unknown> & { [DATA_BRAND]: 'something' };
class Router<Data = Default> {
constructor(...[data]: Data extends Default ? [] : [Data]) {}
}
That doesn't work for the case I define a type but not an argument (Router<string>() should be invalid).