JSFiddle - React, Tailwind, and code Playground

JavaScript

function substring(str) {
    var myString = str;
    var count = 0;
    var returnStr = "";
    for (var i = 0; i <= myString.length - 2; i++) {
        var tempPattern = myString.substr(i, 2);
        if (tempPattern.charAt(0) == tempPattern.charAt(1)) {
            continue;
        } else {
            var pattern = new RegExp(tempPattern, "g");
            var tempCount = 0;
            var tempReturnStr = [];
            while (pattern.test(myString)) {
                tempCount++;
                tempReturnStr.push(tempPattern);
            }

            if (tempCount >= count) {
                count = tempCount;
                returnStr = tempReturnStr.join("");
            }
        }
    }
    return returnStr;
}

alert(substring("abcdef"));