Compose

compose.js

var compose = function (f, g) {
  return function (x) {
    return f(g(x));
  };
};

var toUpperCase = x => x.toUpperCase();
var exclaim     = x => x + '!';
var shout       = compose(exclaim, toUpperCase);

shout('hello world');
// "HELLO WORLD!"

var head    = x => x[0];
var reverse = reduce((acc, x) => [x].concat(acc), []);
var last    = compose(head, reverse);

last(['hello', 'world']);
// 'world'

var loudLastUpper = compose(shout, last);

loudLastUpper(['hello', 'world']);
// 'WORLD!'