compose

function compose2(fn1, fn2) {
  return (...args) => fn1(fn2.apply(null, args));    
}

function compose(...fns) {
  let head = fns.shift();
  let tail = fns;
  
  if (head && tail.length === 0) {
    return head;
  }
  
  if (tail.length === 1) {
    return compose2(head, tail[0]);
  }
  
  return compose2(head, compose.apply(null, tail));
}

function add2(x) {
  return x + 2;
}

function sqr(x) {
  return x * x;
}


var l = console.log;

l(
  compose(sqr)(2)
);

l(
  compose(sqr, add2)(2)
);

l(
  compose(sqr, add2, add2)(2)
);

l(
  compose(sqr, add2, add2, add2)(2)
);

l(
  compose(sqr, add2, add2, add2, add2)(2)
);