The ES6 function below flattens an arbitrarily deep array.

flattenArbitrarilyDeepArray.js

function flattenArbitrarilyDeepArray(arr, cache = []) {
  return arr.reduce((acc, item) => {
    if (Array.isArray(item)) {
      acc.concat(flattenArbitrarilyDeepArray(item, acc))
    }
    else acc.push(item)

    return acc
  }, cache)
}

flattenArbitrarilyDeepArray([[1,2,[3]],4])                    // [1, 2, 3, 4]
flattenArbitrarilyDeepArray([[1,2,[3]],4, [5, [6, [7, 8]]]]) // [1, 2, 3, 4, 5, 6, 7, 8]