Random FP examples illustrating syntactic sugar from ES2015. Toy problems, not optimized.

reverse.js

function empty(coll) { return 0 === coll.length; }

/**
 * These one-liners are equivalent...
 * But they're hard to read, and both are undefined on empty collections. :(
 */

function reverse([h, ...t]) { return empty(t) ? h : reverse(t).concat(h); }

let reverse = ([h,...t]) => empty(t) ? h : reverse(t).concat(h);

/**
 * Less pretty, but actually works on empty collections.
 */

function reverse(coll) {
  if (empty(coll)) return coll;
  let [h,...t] = coll;
  return empty(t) ? h : reverse(t).concat(h);
}

Here's a different implementation, building on the one here:

function reverse (arr) {
  return arr.reduce((acc, x) => [x].concat(acc), []);
}

This is a nice and non-recursive solution for arrays, but reduce isn't a method of Str!

Even if we extended the prototype of Str, we would need a different implementation of this reverse function in order to pass an empty string, "", instead of [] as initialValue.

And, type safety aside, if our function can't reverse strings, why not just call arr.reverse()?