Flattens an array of integers, throws if invalid input.

flatteningarraysofintegers.js

// DEMO: https://observablehq.com/@richardvanbergen/flattening-arrays-of-things

function flatten(arr) {
  let flatArr = []
  for (let num of arr) {
    if (!Array.isArray(num) && !Number.isInteger(num)) {
      throw new Error(`Input '${num}' is not an integer!`)
    }

    if (Array.isArray(num)) {
      flatArr = [...flatArr, ...flatten(num)]
    } else {
      flatArr.push(num)
    }
  }
  return flatArr
}

flatten([1,[2],[3,[4,5]],6]) // [1, 2, 3, 4, 5, 6]
flatten([1, 2]) // [1, 2]
flatten([6,[5,[4],3],[2],1]) // [6, 5, 4, 3, 2, 1]
flatten([6.1]) // throws
flatten([{},['string',[],3],[NaN],-Infinity]) // throws