Composing Functions

compose.js

function increment(x) { return x + 1; }
function decrement(x) { return x - 1; }
function double(x) { return x * 2; }
function half(x) { return x / 2; }

function compose(...fns) {
  return pipe(...fns.reverse());
}
function pipe(...fns) {
  return function(input) {
    return fns.reduce((value, fn) => {
      return fn(value);
    }, input);
  }
}

var f = compose(decrement,double,increment,half);
var p = pipe(half,increment,double,decrement);

console.log(f(3) === 4);
// true

console.log(f(3) === p(3));
// true