diff --git a/src/jasinta.py b/src/jasinta.py
--- a/src/jasinta.py
+++ b/src/jasinta.py
@@ -1,4 +1,4 @@
-from std import lib as stdlib, Number
+from std import lib as stdlib, Number, String
 
 
 class Undefined:
@@ -12,6 +12,9 @@ def evaluate(node, context):
 	node_type = node["type"]
 	# retrieve an already existing object from the context
 	if node_type == "Identifier":
+		if hasattr(context[-1], node["name"]):
+			return getattr(context[-1], node["name"])
+
 		for ctx in reversed(context):
 			if node["name"] in ctx:
 				return ctx[node["name"]]
@@ -68,7 +71,10 @@ class AssignmentExpression:
 	def eval(self, context):
 		left = evaluate(self.node["left"], context)
 		right = evaluate(self.node["right"], context)
-		if self.node["operator"] == "+=":
+		op = self.node["operator"]
+		if op == "=":
+			left.val = right.val
+		elif op == "+=":
 			left.val += right.val
 		else:
 			raise NotImplementedError
@@ -84,7 +90,7 @@ class BinaryExpression:
 
 		# in fact this doesn't follow JavaScript "+" semantics
 		if self.node["operator"] == "+":
-			return Number(left.val+right.val)
+			return left+right
 		else:
 			raise NotImplementedError
 
@@ -133,7 +139,7 @@ class Literal:
 			else:
 				return Number(val)
 		else:
-			raise NotImplementedError
+			return String(val)
 
 
 node_index = {cls.__name__: cls for cls in [
diff --git a/src/std.py b/src/std.py
--- a/src/std.py
+++ b/src/std.py
@@ -2,7 +2,45 @@ class Number:
 	def __init__(self, x):
 		self.val = x
 
+	def __add__(self, other):
+		return Number(self.val+other.val)
+
+
+class String:
+	def __init__(self, s):
+		self.val = s
+
+	@staticmethod
+	def fromCharCode(x):
+		return String(chr(x.val))
+
+	def __add__(self, other):
+		return String(self.val+other.val)
+
+	def charAt(self, i=None):
+		i = (i or Number(0)).val
+
+		if 0 <= i < len(self.val):
+			return String(self.val[i])
+		else:
+			return String("")
+
+	def substr(self, start=None, length=None):
+		start = (start or Number(0)).val
+		length = length.val if length is not None else len(self.val)
+
+		if length < 0:
+			return String("")
+
+		return String(self.val[start:start+length])
+
+	def slice(self, start=None, end=None):
+		start = (start or Number(0)).val
+		end = end.val if end is not None else None
+		return String(self.val[start:end])
+
 
 lib = {
+	"String": String,
 	"document": {"write": lambda x: print(x.val)}
 }
diff --git a/tests/test_jasinta.py b/tests/test_jasinta.py
--- a/tests/test_jasinta.py
+++ b/tests/test_jasinta.py
@@ -6,9 +6,9 @@ from pyjsparser import parse
 from jasinta import interpret
 
 
-class TestBasicAddition(TestCase):
+class TestBasics(TestCase):
 	@patch("builtins.print")
-	def test_interpret(self, mock_print):
+	def test_addition(self, mock_print):
 		s = """var a=3;
 var b=1;
 var c=a+b;
@@ -17,10 +17,8 @@ document.write(c);"""
 		interpret(parse(s))
 		mock_print.assert_called_with(4)
 
-
-class TestAssignment(TestCase):
 	@patch("builtins.print")
-	def test_interpret(self, mock_print):
+	def test_assignment(self, mock_print):
 		s = """var a=1;
 var b=1;
 a+=b;
@@ -29,3 +27,18 @@ document.write(b);"""
 
 		interpret(parse(s))
 		mock_print.assert_called_with(3)
+
+
+class TestString(TestCase):
+	@patch("builtins.print")
+	def test_assignment(self, mock_print):
+		s = """var s = "abcdefgh";
+var a = s.charAt(0);
+var bcd = s.slice(1, 4);
+var efg = s.substr(4, 3);
+var h = String.fromCharCode(104);
+document.cookie = a+bcd+efg+h;
+document.write(document.cookie);"""
+
+		interpret(parse(s))
+		mock_print.assert_called_with("abcdefgh")