Here are my solutions to the FCC Basic JavaScript Algorithms. Happy to report all algorithms can be
written in one (albeit sometimes long) line.
//================================================================================================//
Reverse a String:

const reverseString = (str) => str.split('').reverse().join('');
//================================================================================================//
Factorialize a Number

const factorialize = (num) => num > 1 ? num * factorialize(num-1) : 1;
//================================================================================================//
Check for Palindromes

const palindrome = (str) => str.replace(/\W|_/g, "").toLowerCase() == str.replace(/\W|_/g, "").toLowerCase().split("").reverse().join("");

better:
function palindrome(str) {
  let cleanStr = str.replace(/\W|_/g, "").toLowerCase();
  return cleanStr === cleanStr.split("").reverse().join("");
}
//================================================================================================//
Find the Longest Word in a String

const findLongestWord = (str) => Math.max(...str.split(" ").map(a => a.length));
const findLongestWord = (str) => str.split(' ').reduce((a,b)=> b.length > a ? b.length : a, 0);
//================================================================================================//
Title Case a Sentence

const titleCase = (str) => str.split(' ').map(v => v[0].toUpperCase() + v.slice(1).toLowerCase()).join(' ');
//================================================================================================//
Return Largest Numbers in an Array

const largestOfFour = (a) => a.map(b => Math.max(...b));
//================================================================================================//
Confirm the Ending

const confirmEnding = (str, target) => str.slice(-target.length) == target;
//================================================================================================//
Repeat a string repeat a string

const repeatStringNumTimes = (str, num) => num > 0 ? str.repeat(num) : "";
//================================================================================================//
Truncate a string

//quick and dirty:
const truncateString = (str, num) => num >= str.length ? str : num<=3 ? str.slice(0,num)+"..." : str.slice(0, num-3) + "...";

//longer and easier to read:
function truncateString(str, num) {
  if (num >= str.length){
    return str;
  } else if (num <= 3){
    return str.slice(0, num) + "...";
  } else {
    return str.slice(0,num-3) + "...";
  }
}
//================================================================================================//
Chunky Monkey

clever:
const chunkArrayInGroups = (arr, size) => arr.reduce((a,b,i,arr) => i%size == 0 ? a.concat([arr.slice(i,i+size)]) : a, []);

classic:
function chunkArrayInGroups(arr, size) {
  var result = [];
  for (var i=0; i<arr.length; i+=size){
    result.push(arr.slice(i, i+size));
  }
  return result;
}
//================================================================================================//
Slasher Flick

const slasher = (arr, howMany) => arr.slice(howMany);
//================================================================================================//
Mutations

const mutation = (arr) => arr[1].toLowerCase().split('').every(v => arr[0].toLowerCase().includes(v));
//================================================================================================//
Falsy Bouncer

const bouncer = (arr) => arr.filter(Boolean);
const bouncer = (arr) => arr.filter(v => v);
//================================================================================================//
Seek and Destroy

const destroyer = (arr,...args) => arr.filter(n => args.includes(n));
//================================================================================================//
Where do I belong?

const getIndexToIns = (arr, num) => arr.filter(a => a < num).length;
//================================================================================================//
Caesars Cipher

const rot13 = (str) => str.replace(/[A-Z]/g, l => String.fromCharCode(l.charCodeAt()%26 + 65));
function rot13(str) {
  return str.split('').map(l => {
     let i = l.charCodeAt(0);
     return (i < 65 || i > 90) ? l : String.fromCharCode(i%26+65);
  }).join('');
}
//================================================================================================//