function ceaser_cipher(){
    this.alphabets=['a','b','c','d','e','f','g','h','i','j','k','l','m','n','o','p','q','r','s','t','u','v','w','x','y','z'];
}
ceaser_cipher.prototype.encrypt=function(s,n){
var plain_text=[...s];
for(var i = 0, length1 = plain_text.length; i < length1; i++){
		plain_text[i]=this.alphabets[this.alphabets.indexOf(plain_text[i])+n];
	}	
	return plain_text.toString();
};
ceaser_cipher.prototype.decrypt=function(s,n){
var cipher_text = [...s];
	for(var i = 0, length1 = cipher_text.length; i < length1; i++){
		cipher_text[i]=this.alphabets[this.alphabets.indexOf(cipher_text[i])-n];
	}
	return cipher_text.toString();
};

var secret_msg = new ceaser_cipher();
var cipher_text = secret_msg.encrypt("abc",3);
var plain_text = secret_msg.decrypt("def",3);
console.log('The encoded one : ' + cipher_text);
console.log('The decoded one : ' + plain_text);