Typescript: Use of spread operator when at least one parameter is required

Question

I am learning how to use the rest parameter/spread operator in typescript. Now, I need to write a function that takes an array as a parameter and I want to use the rest operator. This is the function:

insert(elem: number, ...elems: number[])

The elem parameter is there because I need the array to have at least one element.

So, given an array like this:

const numArray = [1, 2, 3, 4]

How can I pass the array to the function? I've tried the following but it gave me an error:

insert(...numArray)

I understand the error, because numArray may have from 0 to N elements, and the function needs at least one element, but I don't know the best solution to it.

Is there any way to achieve this?

Note: The insert function is part of a library that I'm developing, so I need to make it as usable as can be, and not depending on how the user will make use of it

Answer 1

You can create an array type of at least one (or more) items and spread it:

function insert(...elem: [number, ...number[]]) { 
  // ...
}

You could even write a helper type e.g.:

type AtLeastOne<T> = [T, ...T[]]

function insert(...element: AtLeastOne<number>) {
  //...
}