Encoder with generator

encoder.js

Number.prototype.mod = function(n) {
  return ((this % n) + n) % n;
};

function encode(string, generator, cycle=10, reverse=false) {
  let sequence = generator();
  let start = ' '.charCodeAt();
  let end = '~'.charCodeAt();
  
  return Array.from(string).map((c, i) => {
    return (
      (
        c.charCodeAt() - start + (reverse ? -1 : 1) * sequence.next(i % cycle === 0).value
      ).mod(end - start)
    ) + start;
  }).map(n => String.fromCharCode(n))
  .join('');
}

// Usage:

//1. generator
function* fibonacci () {
  let fn1 = 0;
  let fn2 = 1;
  while (true) {  
    var current = fn1;
    fn1 = fn2;
    fn2 = current + fn1;
    var reset = yield current;
    if (reset) {
        fn1 = 0;
        fn2 = 1;
    }
  }
}

//2. cycle
let cycle = 10;

//3. encode
encode('Hello World!', fibonacci, cycle); //outputs: Hfmnr%_|)0d!

//4. decode - reverse of encode
encode('Hfmnr%_|)0d!', fibonacci, cycle, true); //outputs: Hello World!