Why do Double Quotes Vs. Single Quotes matter here?

//Handle HTTP route GET / and POST / i.e. Home
function home(request, response) {
  //if url == "/" && GET
  if (request.url === "/") {
    //show search
    response.writeHead(200, { 'Content-Type': 'text/plain' });
    response.write("Header\n");
    response.write("Search\n");
    response.end('Footer\n');
  }
  //if url == "/" && POST
  //redirect to /:username
}

//Handle HTTP route GET /:username i.e. /chalkers
function user(request, response) {
  //if url == "/...."
  var username = request.url.replace("/", "");
  if (username.length > 0) {
    response.writeHead(200, { 'Content-Type': 'text/plain' });
    response.write("Header\n");
    response.write(username + "\n");
    response.end('Footer\n');


    //get json from Treehouse
    //on "end"
    //show profile
    //on "error"
    //show error
  }
}

module.exports.home = home;
module.exports.user = user;