Execute Node.js Online

function rep() {
    var str = 'Hello World';
    str = setCharAt(str,4,'a');
    console.log("replaced string is",str);
}

function setCharAt(str,index,chr) {
    if(index > str.length-1) return str;
    return str.substr(0,index) + chr + str.substr(index+1);
}
rep();