React

http://reactfordesigners.com/labs/reactjs-introduction-for-people-who-know-just-enough-jquery-to-get-by/

by Anton

HTML

<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/css/bootstrap.min.css">
<script src="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/js/bootstrap.min.js"></script>
<div class="well clearfix">
  <textarea class="form-control"></textarea><br/>
  <button class="btn btn-primary pull-right">Tweet</button>
</div>

JavaScript

// Initially disable the button
$("button").prop("disabled", true);

// When the value of the text area changes...
$("textarea").on("input", function() {
  // If there's at least one character...
  if ($(this).val().length > 0) {
    // Enable the button.
    $("button").prop("disabled", false);
  } else {
    // Else, disable the button.
    $("button").prop("disabled", true);
  }
});