Test

function stringRev(str){
    if (typeof str === 'string'){
        var resultstring = '';
        if (str.length % 2 === 0) {
            for (let i=0; i<str.length; i=i+2){
                let currentBlock = str.slice(i, i+2).split("").reverse().join("");
                resultstring+=currentBlock;
            }
        } else {
            for (let i=0; i<str.length-1; i=i+2){
                let currentBlock = str.slice(i, i+2).split("").reverse().join("");
                resultstring+=currentBlock;
            }
            resultstring+=str[str.length-1];
        }
        return resultstring;
    } else {
        return 'Wrong string';   
    }
}

console.log(stringRev('Helts'));