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<body>
  <input id="username"/>
  <input id="password" type="password"/>
  <button onclick="login()">login</button>
  <script>
    const username = document.getElementById("username");
    const password = document.getElementById("password");
    function login() {
      fetch("/api/login", {
        method: "post",
        headers: {
          "content-type": "application/json"
        },
        body: JSON.stringify({
          username: username.value,
          password: password.value
        }).then(function (response) {
          return response.json();
        }).then(function (result) {
          // TODO: show result to user
        }).catch(function (err) {
          // TODO: show error to user
        });
      });
    }
  </script>
</body>