diff --git a/test/jstests/morespace.py b/test/jstests/morespace.py
index 1e238f419..327e46192 100644
--- a/test/jstests/morespace.py
+++ b/test/jstests/morespace.py
@@ -1,8 +1,6 @@
 from youtube_dl.jsinterp.jsgrammar import Token
 from youtube_dl.jsinterp.tstream import _ASSIGN_OPERATORS
 
-skip = {'i': 'Interpreting set field not yet implemented'}
-
 tests = [
     {
         'code': 'x =  2  ; return x;',
diff --git a/test/jstests/precedence.py b/test/jstests/precedence.py
index 7b8bf9bcc..8008ff589 100644
--- a/test/jstests/precedence.py
+++ b/test/jstests/precedence.py
@@ -1,7 +1,7 @@
 from youtube_dl.jsinterp.jsgrammar import Token
 from youtube_dl.jsinterp.tstream import _ASSIGN_OPERATORS, _OPERATORS
 
-skip = {'i': 'Interpreting get field not yet implemented'}
+skip = {'i': 'Interpreting built-in fields not yet implemented'}
 
 tests = [
     {
diff --git a/youtube_dl/jsinterp/jsinterp.py b/youtube_dl/jsinterp/jsinterp.py
index 9aebe128f..312bfeba4 100644
--- a/youtube_dl/jsinterp/jsinterp.py
+++ b/youtube_dl/jsinterp/jsinterp.py
@@ -12,7 +12,7 @@ class Context(object):
     def __init__(self, variables=None, ended=False):
         self.ended = ended
         self.no_in = True
-        self.local_vars = {}
+        self.local_vars = {'this': {}}
         if variables is not None:
             for k, v in dict(variables).items():
                 # XXX validate identifiers
@@ -58,6 +58,10 @@ class JSInterpreter(object):
         self._context = Context()
         self._context_stack = []
 
+    @property
+    def this(self):
+        return self._context.local_vars['this']
+
     def statements(self, code=None, pos=0, stack_size=100):
         if code is None:
             code = self.code
@@ -889,7 +893,22 @@ class JSInterpreter(object):
                 ref = self.interpret_expression(left)
             else:
                 # TODO handle undeclared variables (create propery)
-                leftref = self.interpret_expression(left)
+                try:
+                    leftref = self.interpret_expression(left)
+                except KeyError:
+                    lname = left[0]
+                    key = None
+                    if lname is Token.OPEXPR and len(left[1]) == 1:
+                        lname = left[1][0][0]
+                        if lname is Token.MEMBER:
+                            lid, args, tail = left[1][0][1:]
+                            if lid[0] is Token.ID and args is None and tail is None:
+                                key = lid[1]
+                    if key is not None:
+                        u = Reference(self.undefined, (self.this, key))
+                        leftref = self.this[key] = u
+                    else:
+                        raise ExtractorError('''Invalid left-hand side in assignment''')
                 leftvalue = leftref.getvalue()
                 rightvalue = self.interpret_expression(right).getvalue()
                 leftref.putvalue(op(leftvalue, rightvalue))
@@ -952,7 +971,7 @@ class JSInterpreter(object):
         elif name is Token.ID:
             # XXX error handling (unknown id)
             ref = (self._context.local_vars[expr[1]] if expr[1] in self._context.local_vars else
-                   self.global_vars[expr[1]])
+                   self.this[expr[1]] if expr[1] in self.this else self.global_vars[expr[1]])
         
         # literal
         elif name in token_keys: