Or better:

    type NonEmptyArray = [unknown, ...unknown[]];
    const nea: NonEmptyArray = [];
    //    ^^^ Type '[]' is not assignable to type 'NonEmptyArray'.
    //        Source has 0 element(s) but target requires 1.

Can you map over this and preserve the NonEmptyArray type?

Do you mean pass in a type?

    type NonEmptyArray<T> = [T, ...T[]];

I mean if I have a NonEmptyArray say xs, and I then xs.map(x=>2*x), will the result of this expression be a NonEmptyArray ?

No, unfortunately it will just be number[] as the return type.