Caesar Cipher / Character Shifter

caesar-cipher.js

// Shifts an alphabetic string (e.i. 'a' --> 'b')
// The 'str' argument is the string to pass in
// 'n' is the number of characters that should be shifted
// Supports shifting either forwards or backwards any amount
// Example:
// Input: shift('Ebiil, Tloia!', 3);
// Expected Output: "Hello, World!"
// Takes into account capital letters, lowercase letters, ignores non-alphabetic characters

function shift(str, n) {
	var shifted = '';
	n %= 26;
	for (var i = 0; i < str.length; i++) {
		let code = str[i].charCodeAt();
    let capital = (code > 64 && code < 91) ? true : false;
    if (code < (capital?65:97) || code > (capital?90:122) || n == 0) {
      shifted += str[i];
      continue;
    }
    if (n > 0) {
      if (code > (capital?90:122)-n) {
  			code = n + code - 26;
  		} else {
  			code += n;
  		}
    } else {
      if (code < (capital?65:97)-n) {
  			code = code + n + 26;
  		} else {
  			code += n;
  		}
    }
		shifted += String.fromCharCode(code);
	}
	return shifted;
}

// Below is the same function in prototype form aka 'string'.shift(n) as opposed to shift('string', n)

String.prototype.shift = function(n) {
  var shifted = '';
  n %= 26;
	for (var i = 0; i < this.length; i++) {
		let code = this[i].charCodeAt();
    let capital = (code > 64 && code < 91) ? true : false;
    if (code < (capital?65:97) || code > (capital?90:122) || n == 0) {
      shifted += this[i];
      continue;
    }
    if (n > 0) {
      if (code > (capital?90:122)-n) {
  			code = n + code - 26;
  		} else {
  			code += n;
  		}
    } else {
      if (code < (capital?65:97)-n) {
  			code = code + n + 26;
  		} else {
  			code += n;
  		}
    }
		shifted += String.fromCharCode(code);
	}
	return shifted;
}