index.js

module.exports = {
    encrypt: function encrypt(shift, plaintext) {
        return caesar(shift, plaintext)
    }
    ,
    decrypt: function decrypt(shift, ciphertext) {
        return caesar((26 - shift) % 26, ciphertext)
    }
}

function caesar(shift, input) {
    if (!/^-?\d+$/.test(shift)) console.error('Shift is not an integer')
    if (shift < 0 || shift >= 26) console.error('Shift is out of range')
	
    var output = '', len = input.length

	for (var i = 0; i < len; i++) {

		var c = input.charCodeAt(i);

        // Small fix for cedilla
        if (c == 231)
            c = 99;
        if (c == 199)
            c = 67;

		if (c >= 65 && c <=  90) { 
            // upcase
            output += String.fromCharCode((c - 65 + shift) % 26 + 65)
        }  
		else if (c >= 97 && c <= 122) {  
            // downcase
            output += String.fromCharCode((c - 97 + shift) % 26 + 97) 
        }
		else {
            // copy thru
            output += input.charAt(i)
        }

	}
	return output
}