es6 and recursion, examples from http://raganwald.com/2015/02/02/destructuring.html

es6

const length = ([first, ...rest]) =>
  first === undefined
    ? 0
    : 1 + length(rest);


const flatten = ([first, ...rest]) => {
  if (first === undefined) {
    return [];
  }
  else if (!Array.isArray(first)) {
    return [first, ...flatten(rest)];
  }
  else {
    return [...flatten(first), ...flatten(rest)];
  }
}


const squareAll = ([first, ...rest]) =>
  first === undefined
  ? []
  : [first * first, ...squareAll(rest)];
      
const truthyAll = ([first, ...rest]) =>
  first === undefined
    ? []
    : [!!first, ...truthyAll(rest)];


const mapWith = (fn, [first, ...rest]) => 
  first === undefined
    ? []
    : [fn(first), ...mapWith(fn, rest)];
   

const foldWith = (fn, terminalValue, [first, ...rest]) =>
  first === undefined
    ? terminalValue
    : fn(first, foldWith(fn, terminalValue, rest));
foldWith((number, rest) =>
  number * number + rest, 0, [1, 2, 3, 4, 5])


// console.log(mapWith((x) => x * x, [1, 2, 3, 4, 5]));  
// console.log(mapWith((x) => !!x, [null, true, 25, false, "foo"]));

 
//console.log(squareAll([1, 2, 3, 4, 5]));
//console.log(flatten(["foo", [3, 4, []]]));
//console.log(length([1,2,3,5]));
//console.log(truthyAll([null, true, 25, false, "foo"]));