Sum with variable args from just numbers to arrays of arrays of numbers

recursiveSum.ts

type Sum<T> = Array<T> | Array<Sum<T>>

function sum<T extends number>(...list: Sum<T>): number {
  const innerSum = (a: number, b: number | Array<number>) => (
    a + (b instanceof Array ? sum(...b) : b));

  return (list as number[]).reduce(innerSum, 0);
}