How to conditionally make a parameter optional

[prime creek]

I would like to have the 4th parameter only when Timing is "after" otherwise id like to have it to be left away

[vapid tundra]

... : void

Or use overloads

[prime creek]

@vapid tundra when doing it void it still wants a 4th parameter by not making it optional

what are overloads?

[vapid tundra]

!hb overloads

[random escarp]

Declaration Files / Do's and Don'ts

Overloads and Callbacks

[vapid tundra]

... bad link

https://www.typescriptlang.org/docs/handbook/declaration-files/by-example.html#overloaded-functions

https://www.typescriptlang.org/docs/handbook/2/functions.html#function-overloads is probably better

Huh, I thought I'd seen someone using void with a conditional type to do that...

[prime creek]

sadly not

its complaining that it needs mor params

[random escarp]

Gerrit0#7591

Playground Link

Preview:```ts
function foo<T extends "now" | "after">(
timing: T,
...[duration]: T extends "after" ? [string] : [void]
) {}

foo("now")
foo("after", "")```

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

[vapid tundra]

Very ugly, but...

[prime creek]

@vapid tundra Oh it has to be a rest parameter

Ahhh

genius

when you use extends in a type it for some reason loses context of which propeties actually exist on the object

even if both types just have one propety difference

wreidly

uhhm

[vapid tundra]

Could probably use [] instead of [void], more semantically correct anyways

[prime creek]

@vapid tundra that was it, thank you!

!solved