bacon.js

      var username = $(".add-user");
      var avatar = username.parent().find(".avatar");

      username
        .asEventStream("keyup")
        .map(function(event) { return $(event.target).val() })
        .debounce(500)
        .flatMapLatest(function(u) {
          return Bacon.fromPromise($.ajax("https://api.github.com/users/" + u));
        })
        .mapError(function() {avatar.attr("src", "")})
        .onValue(function(value) {avatar.attr("src", value.avatar_url);});