JSFiddle - React, Tailwind, and code Playground

JavaScript

const caesarCipher = (string, number) => {
    const alphabet = "abcdefghijklmnopqrstuvwxyz".split("");
    const input = string.toLowerCase();
    let output = "";

    for (let i = 0; i < input.length; i++) {
        const letter = input[i];

        if (alphabet.indexOf(letter) === -1) {
            output += letter;
            continue;
        }

        let index = alphabet.indexOf(letter) + number % 26;
        if (index > 25) { console.log(1); index -= 26; }
        if (index < 0) { console.log(2); index += 26; }

        output += string[i] === string[i].toUpperCase() ? alphabet[index].toUpperCase() : alphabet[index];
    }

    return output;
};

console.log(caesarCipher("I love JavaScript!", -2))