code: function _htob (hex) {
     if (hex) {
        if (typeof hex !== 'string') {
           throw new TypeError('Expected input to be a string')
        }
  
        if ((hex.length % 2) !== 0) {
           throw new RangeError('Expected string to be an even number of characters')
        }
  
        var view = new Uint8Array(hex.length / 2)
  
        for (var i = 0; i < hex.length; i += 2) {
           view[i / 2] = parseInt(hex.substring(i, i + 2), 16)
        }
  
        return view
     } else {
        return null;
     }
  }
docs: / Convert hex string to array of bytes.