function login() {
        // get form username and password
        // post request to login
        // if success, set localStorage token and redirect
        // else show error
        document.getElementById('loginForm').addEventListener("submit", function(e){
            e.preventDefault();
            var form = e.target;
            var request = new XMLHttpRequest();
            var data = {};
            data.username = form.elements["username"].value;
            data.password = form.elements["password"].value;
            request.open(form.method, form.action);
            request.setRequestHeader("Content-Type", "application/json");
            request.send(JSON.stringify(data));
            request.onload = function(){
                var responseJSON = JSON.parse(request.responseText);
                if(responseJSON.code != 200 ) {
                    console.log("back-end error - login failed");
                    console.log(responseJSON);
                    localStorage.removeItem("tendresse_jwt");
                    return;
                }
                var jwt = responseJSON.data.token;
                if(jwt != null){
                    localStorage.setItem("tendresse_jwt",jwt);
                    write("login success");
                    initws();
                    // window.location.href = 'index.html';
                } else {
                    console.log("request success but JWT received is invalid.");
                    console.log(request);
                }
            };
            request.onerror = function(){
                console.log("front-end error | could not send the login request");
                console.log(request);
            };
        })
    }