A function to display facebook like reactions '(You, Marie and 3 more reacted to this)'

DisplayFacebookReactions.js

// Some helper functions. Self-explanatories
const equal = (a, b) => a === b
const length = a => a.length
const not = a => !a
const lengthEqual = (arr, val) => equal(length(arr), val)
const lastIndex = arr => length(arr) - 1

/**
 * This function returns a string based on a names array composing the 'People reacted to this' from facebook.
 * It takes in cosideration if: You reacted, no one reacted, only one people reacted, you were the only one reacting
 * two people reacted and last but certently not least, if X number of people reacted. On this last case two variables
 * will be taken into consideration: `maxNamesToShow` and if `youLiked` in order to display the amount of 'other people'
 * had reacted
 * @param names {string[]} A list of names to display. If the name on variable YOU is in the list, it shows 'you' first 
 * @param maxNamesToShow {number} the amount of names (plus one) you want to be max displayed before it appends 'and [x] more'
 * @example console.log(displayReactions([])) // 'Be the first to react to this'
 * @example console.log(displayReactions(['Peter'])) //'Peter reacted to this'
 * @example console.log(displayReactions(['Peter', 'Mary'])) // 'Peter and Mary like this'
 * @example console.log(displayReactions(['Peter', 'Mary', 'Julie'])) //  Peter, Mary and Julie like this
 * @example console.log(displayReactions(['Peter', 'Mary', 'Julie', 'Marcus', 'Misael', 'Caesar'], 5)) //  You, Peter, Mary, Julie and 2 more reacted to this
 */
const displayReactions = (names = [], maxNamesToShow = 3) => {
  const BaseString = 'reacted to this'
  const YOU = 'Misael'
  if(not(length(names))) return `Be the first to react to this`
  if(lengthEqual(names, 1)) return `${equal(names[0], YOU)? 'you' : names[0]} ${BaseString}`
  let namesToDisplay
  let otherNamesTotal
  let namesList = names;
  let maxToShow = maxNamesToShow
  let youReactedMessage = ''
  if (names.includes(YOU)) { // you liked
    namesList = names.filter(name => name !== YOU)
    maxToShow = maxNamesToShow - 2
    youReactedMessage = ' You'
  }

  namesToDisplay = namesList.slice(0, maxToShow)
  otherNamesTotal = length(namesList.slice(maxToShow))

  youReactedMessage += length(namesToDisplay) ? ', ' : ''
  const finalString = namesToDisplay.map((name, idx, arr) => {
    if (equal(idx,  lastIndex(arr))) return name
    if (equal(idx,  length(arr) -2) && not(otherNamesTotal)) return `${name} and`
    return `${name},`
  }).join(' ')

  return otherNamesTotal ? `${youReactedMessage}${finalString} and ${otherNamesTotal} more ${BaseString}` : `${finalString} ${BaseString}`
}