Ceaser cipher

Usage

const ceaser = require('./ceaser.js');
let text = 'Hello world!';
let key = 3;
console.log(ceaser(text, key)); // Output: Khoor zruog!

According to wikipedia:

In cryptography, a Caesar cipher, also known as Caesar’s cipher, the shift cipher, Caesar’s code or Caesar shift, is one of the simplest and most widely known encryption techniques. It is a type of substitution cipher in which each letter in the plaintext is replaced by a letter some fixed number of positions down the alphabet. For example, with a left shift of 3, D would be replaced by A, E would become B, and so on. The method is named after Julius Caesar, who used it in his private correspondence.