FRP username validation

.js

// username is a input element in the dom

let userNameVal = Kefir.fromEvents(username, "blur")
    .map(e => $(e.target)
      .val());

let kUsernameValidation = username => {
  try {
    if (usernameValidation(username)) {
      return username;
    }
  } catch (e) {
    return Kefir.constantError(e.message);
  }
}


let validUsername = userNameVal
  .flatMapLatest(kUsernameValidation)
  .onError(function(error) {
    showErrors(helpText, error); // How do i end the stream here? is there a better way to write this?
  });
  
let usernameAvail = validUsername
    .flatMapLatest(isUserNameTaken) // does xhr to server
    .map(function(value) {
      hideErrors(helpText);
      // username exists;
      if (value) {
        showInputUnAvailable(username, "username is already taken.")
      } else {
        showInputAvailable(username);
      }
      return value;
    }).onError(function() {
      window.alert("something went wrong");
    });