const steepArr = [[1, 2, [3]], 4]
const simpleArr = [1, 2, 3, 4]
const easyArr = [[1], [2], [3], [4]]
const stringArr = [["$1"], ["$2"], ["$3"], ["$4"]]

const flatten = (arr, result = []) => {
  for(let item of arr){
    if (Array.isArray(item)) {
      flatten(item, result)
    } else {
      result.push(item)
    }
  }
  return result
}

console.log('*** Results for steepArr: ', flatten(steepArr));
console.log('*** Results for simpleArr: ', flatten(simpleArr));
console.log('*** Results for easyArr: ', flatten(easyArr));
console.log('*** Results for stringArr: ', flatten(stringArr));