caesars-cipher.js

const CYPHER_LIMIT = 78
const A = 65
const Z = 90

const add = ( c ) => ( s ) => s.concat( c )
const getCharCode = String.fromCharCode
const isSpace = ( x ) => ( x === 32 )

const isInRange = ( min, max ) => ( x ) => 
  ( ( x >= min ) &&  ( x <= max ) )

const toCharCode = ( letter, i, str ) => 
  str.join( '' ).charCodeAt( i )

const getPosition = ( CYPHER_LENGTH ) => ( x ) =>
  ( x >= CYPHER_LIMIT ) ? x - CYPHER_LENGTH  : x + CYPHER_LENGTH 

const getCharCodeFromCypher = ( CYPHER_LENGTH ) => ( x ) => 
  ( isInRange( A, Z )( x ) ) ? getPosition( CYPHER_LENGTH )( x ) : x

const cypherThis = ( CYPHER_LENGTH ) => ( x, isSpace ) =>
  getCharCode( isSpace ?  x : getCharCodeFromCypher( CYPHER_LENGTH )( x ) )  

const toCypher = ( CYPHER_LENGTH ) => ( result, x, i ) => 
  add ( cypherThis( CYPHER_LENGTH )( x ), isSpace( x )  )( result )

const rot = ( CYPHER_LENGTH ) => ( str ) => 
  str.toUpperCase()
      .split( '' )
      .map( toCharCode )
      .reduce( toCypher( CYPHER_LENGTH ), '' )

const rot13 = rot( 13 )

console.log('rot13 LBH QVQ VG!', rot13( 'LBH QVQ VG!' ) )
console.log('rot13 suissa', rot13( 'suissa' ) )