Typescript: How to add an item to a tuple?

Question

Why this produces an error \"A rest element type must be an array type.\"?

type QWE = [boolean, ...T]
                              


        

Answer 1

Before 3.0 we could not spread tuples or generic type arguments to a function and this had to be implemented. Similarly, rest in tuples currently only support arrays, nobody implemented the ability to spread other tuples into the rest of a given tuple, and I'm guessing implementing it would require significant effort and complexity.

To add a tuple at the end of another known tuple we can use the ability to spread a tuple tu a function and then extract the argument types as a tuple.

type ArgumentTypes any> =
    T extends (...a: infer A) => any ? A : never;
type QWE = 
    ArgumentTypes<(a: boolean, ...r: T) => void>

type R = QWE<[number, string]>

Adding the tuple at the start is more problematic, I believe there are some very unrecommended hacks that can achieve this using recursive type aliases. You can also define multiple conditions to support up to a number of elements in a tuple, but I would avoid it if possible