Flatten an array

flatten.spec.js

const tap = require("tap")

/**
 * This function takes an array of items and flattens them.
 * It can also handle multiply arguments.
 * 
 * @param {array} items that can contain nested array items
 * @return {array} flat array of input items or an empty array
 */
function flatten(items = []) {
  items = Array.isArray(items) ? items : Array.prototype.slice.call(arguments)
  return items.reduce((result, item) => {
    if (Array.isArray(item)) {
      result = result.concat(flatten(item))
    } else {
      result.push(item)
    }
    return result
  }, [])
}

tap.same(flatten([[1,2,[3]],4]), [1,2,3,4])
tap.same(flatten([1,2,3,4]), [1,2,3,4])
tap.same(flatten(1,2,3,4), [1,2,3,4])