Cipher Delta letter substitution algorithm (only English letters)

substitution-ciper.js

function reverse(s) {
    return s.split("").reverse().join("");
}
const engAlphabets = "abcdefghijklmnopqrstuvwxyz".repeat(3);
const reverseEngAlphabets = reverse("abcdefghijklmnopqrstuvwxyz").repeat(3);
const engAlIndexes = {
    a: 0,
    b: 1,
    c: 2,
    d: 3,
    e: 4,
    f: 5,
    g: 6,
    h: 7,
    i: 8,
    j: 9,
    k: 10,
    l: 11,
    m: 12,
    n: 13,
    o: 14,
    p: 15,
    q: 16,
    r: 17,
    s: 18,
    t: 19,
    u: 20,
    v: 21,
    w: 22,
    x: 23,
    y: 24,
    z: 25,
};
const revEngAlIndexes = {
    z: 0,
    y: 1,
    x: 2,
    w: 3,
    v: 4,
    u: 5,
    t: 6,
    s: 7,
    r: 8,
    q: 9,
    p: 10,
    o: 11,
    n: 12,
    m: 13,
    l: 14,
    k: 15,
    j: 16,
    i: 17,
    h: 18,
    g: 19,
    f: 20,
    e: 21,
    d: 22,
    c: 23,
    b: 24,
    a: 25,
};
//string reverse if negative number
/**
 * Returns substituted cipher version
 *
 * @remarks
 * This method is part of the {@link core-library#Statistics | Statistics subsystem}.
 *
 * @param content - The content you want to wrap
 * @param y - How many words you want to skip, default 1
 * @returns returns the cipher
 *
 * @beta
 */
const substituteContent = (userContent, skip = 1) => {
    let substitutedContent = "";
    for (let i = 0; i < userContent.length; i++) {
        const char = userContent[i];
        //if user wants to skip forward
        if (skip >= 0) {
            //finding the char index on the list
            const charIndex = engAlIndexes[char.toLowerCase()];
            if (charIndex >= 0) {
                //if the user word is uppercase, keeping the letter casing
                if (char === char.toUpperCase()) {
                    //if user wants to skip forward
                    substitutedContent = `${substitutedContent}${engAlphabets[charIndex + skip].toUpperCase()}`;
                }
                //if the user word is lowercase
                else {
                    substitutedContent = `${substitutedContent}${engAlphabets[charIndex + skip]}`;
                }
            }
            //if the char doesn't exists in the alphabet list
            else {
                substitutedContent = `${substitutedContent}${char}`;
            }
        }
        //if user wants to skip backward
        else if (skip >= -25) {
            //finding the char index on the list
            const charIndex = revEngAlIndexes[char.toLowerCase()];
            if (charIndex >= 0) {
                //if the user word is uppercase, keeping the letter casing
                if (char === char.toUpperCase()) {
                    //if user wants to skip forward
                    substitutedContent = `${substitutedContent}${reverseEngAlphabets[charIndex + Math.abs(skip)].toUpperCase()}`;
                }
                //if the user word is lowercased
                else {
                    substitutedContent = `${substitutedContent}${reverseEngAlphabets[charIndex + Math.abs(skip)]}`;
                }
            }
            //if the char doesn't exists in the alphabet list
            else {
                substitutedContent = `${substitutedContent}${char}`;
            }
        }
    }
    return substitutedContent;
};