Flattening an array is easy if you know recursion and map/reduce!

flatten.js

// I prefer NOT to monkey-patch, also you can't do lazy operations
// if we stick this on the array prototype!
function flatten (arr, depth=Infinity) {
  if (depth === 0) return arr

  return arr.reduce( (acc, v) => {
    // normally I would ternary something like this, but the spread operator
    // prevents it :)
    if (Array.isArray(v)) {
      acc.push(...flatten(v, depth-1))
    } else {
      acc.push(v)
    }
    return acc
  }, [])
}

module.exports = flatten