isIsomorphic.js

var isIsomorphic = function(s, t) {
    // Create two empty objects to store the mapping of characters
    var mapS = {};
    var mapT = {};

    // Iterate over the length of the first string
    for (var i = 0; i < s.length; i++) {
        // If the character in s doesn't exist in mapS, add it
        if (!mapS[s[i]]) {
            mapS[s[i]] = t[i];
        }
        // If the character in t doesn't exist in mapT, add it
        if (!mapT[t[i]]) {
            mapT[t[i]] = s[i];
        }
        // If the current character in s doesn't map to the same character in t, or vice versa, return false
        if (mapS[s[i]] !== t[i] || mapT[t[i]] !== s[i]) {
            return false;
        }
    }
    // If all characters map correctly, return true
    return true;
};