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

With TypeScript 4.0 variadic tuple types, adding an item I to a tuple type T gets much easier:

type Prepend = [I, ...T]
type Append = [...T, I]
type AddBetween = [...T, I, ...U]

type PrependTest = Prepend 
// [boolean, string, number]

type AppendTest = Append 
// [string, number, boolean]

type AddBetweenTest = AddBetween 
// [string, boolean, number]

Playground sample code