Linear data flow with Box

// Code goes here

const Box = x =>
({
  map: f => Box(f(x)),
  fold: f => f(x),
  inspect: () => `Box(${x})`
})

const nextCharForNumberString = str =>
  Box(str)
  .map(s => s.trim())
  .map(r => parseInt(r))
  .map(i => i + 1)
  .map(i => String.fromCharCode(i))
  .fold(c => c.toLowerCase())

const result = nextCharForNumberString('  64 ')

console.log(result)

<!DOCTYPE html>
<html>

  <head>
    <link rel="stylesheet" href="style.css">
  
   

  </head>

  <body>
    <!-- // make console.log write to the page for better in-browser experience -->
    <script>
      (function () {
    var body = document.querySelector('body');
    body.style['fontFamily'] = 'monospace';
    body.style['fontSize'] = '2em';
    console.log = function ( x) { body.innerText += x + '\n'; };
    }());
  </script>
     <script src="script.js"></script>
  </body>

</html>