As written, type OptionalArgBroken<Arg> = Arg extends undefined ? () => void : (arg: Arg) => void; is a distributive conditional type because the type being checked, Arg, is a naked generic type parameter. “Distributive” means that if the Arg passed in is a union, then the type will be evaluated for each member of … Read more
You asked for it. Let’s do some type manipulation to detect if a given type is a union or not. The way it works is to use the distributive property of conditional types to spread out a union to constituents, and then notice that each constituent is narrower than the union. If that isn’t true, … Read more
I think what you are actually looking for is a discriminated union. IObject should itself be a union: export type IObject = { type: “checked” parameters: IParametersCheck } | { type: “counter” parameters: IParametersCounter } export type ObjectType = IObject[‘type’] //// in case you need this union export type ObjectParameters = IObject[‘parameters’] //// in case … Read more
Update 2018-10: @MattMcCutchen has figured out that it is possible to detect readonly fields (invalidating the struck-out passage below), as shown in this answer. Here is a way to build it: type IfEquals<X, Y, A=X, B=never> = (<T>() => T extends X ? 1 : 2) extends (<T>() => T extends Y ? 1 : … Read more