compose.js

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

var increment = function(x) {
  return x + 1;
}
var decrement = function(x) {
  return x - 1;
}

// increment のあとに decrement する関数を作る
var do_nothing = compose(increment, decrement);

console.log(do_nothing(10)); // => 10