I need a function that will only take a second argument when certain conditions are met.
let func = <T extends boolean>(arg1: T, arg2: T extends true ? void : string) => {};
func(true); // ERROR Expected 2 arguments, but got 1
func(true, undefined); // OK
func(false, ''); // OK
This behavior is strange, especially because typing a function argument as void does not require it to be passed in a call. For example:
let func1 = (arg: void) => {};
func1(); // OK
There is a design restriction that you can find more information about in microsoft/TypeScript#29131. The issue of treating a trailing function parameter as optional was addressed in microsoft/TypeScript#27522, but the checking is limited, especially when dealing with conditional types.
If you are looking for a workaround, you may consider utilizing tuples in rest parameter positions to achieve the desired behavior. Tuples allow for optional elements and even parameter name labels. You can try something like the following:
let fn = <T extends boolean>(
arg1: T,
...rest: (T extends true ? [arg2?: undefined] : [arg2: string])
) => { };
fn(true); // okay
fn(true, undefined); // okay
fn(true, ''); // error
fn(false); // error
fn(false, undefined); // error
fn(false, ''); // okay
For a more interactive experience, you can check out the code in the TypeScript Playground.
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