flattenImplementation.js

function isFlattenable(value) {
  return Array.isArray(value)
}

function flattenImplementation (array, depth = Infinity, isStrict, result = []) {
      let index = -1
      let length = array.length

      while (++index < length) {
        var value = array[index]
        if (depth > 0 && isFlattenable(value)) {
          if (depth > 1) {
            flattenImplementation(value, depth - 1, isStrict, result)
          } else {
            result.push(value)
          }
        } else if (!isStrict) {
          result[result.length] = value
        }
      }
      return result
    }