title: Understanding $watch in AngularJS?
body: <p>I am a little confused with the <code>$watch</code> syntax in AngularJS.</p>
  
  <p>Given a variable <code>$scope.foo</code>, with initial value of <code>[1,2,3]</code>, and update causes it to now equal <code>[1,2]</code>.  I want a function to be run whenever <code>$scope.foo</code> is updated, with <code>$scope.foo</code> as arg.</p>
  
  <p>How do I do this in AngularJS?</p>