getBindings doesn't detect a closure's free variables

[infinity0]

> var recast = require("recast");
undefined
> recast.visit(recast.parse("var a, bb; bb = 3; var x = function() { bb = 4; };"), { visitIdentifier: function(path) {
  console.log(path);
  console.log(path.scope.getBindings());
  console.log(path.scope.getBindings().hasOwnProperty(path.node.name));
  console.log("====");
  this.traverse(path);
}});
[..]
[..]
[..]
====
NodePath {
  value: 
   Identifier {
     type: 'Identifier',
     name: 'bb',
     loc: { start: [Object], end: [Object], lines: Lines {}, indent: 0 } },
  parentPath: 
   NodePath {
     value: 
      AssignmentExpression {
        type: 'AssignmentExpression',
        operator: '=',
        [..]
  name: 'left',
  __childCache: null }
{}
false
====
{ program: 
  [..]
  type: 'File',
  comments: null }