diff --git a/lib/execjs/external_runtime.rb b/lib/execjs/external_runtime.rb
index f19ed63..cd2c784 100644
--- a/lib/execjs/external_runtime.rb
+++ b/lib/execjs/external_runtime.rb
@@ -9,17 +9,20 @@ module ExecJS
         @script  = ""
       end
 
-      def eval(source)
+      def eval(source, options = {})
         if /\S/ =~ source
           exec("return eval(#{"(#{source})".to_json})")
         end
       end
 
-      def exec(source)
-        @script << source
-        @script << "\n"
+      def exec(source, options = {})
+        if !options[:pure]
+          @script << source
+          @script << "\n"
+          source = @script
+        end
 
-        compile_to_tempfile(@script) do |file|
+        compile_to_tempfile(source) do |file|
           extract_result(@runtime.exec_runtime(file.path))
         end
       end
@@ -67,12 +70,12 @@ module ExecJS
 
     def exec(source)
       context = Context.new(self)
-      context.exec(source)
+      context.exec(source, :pure => true)
     end
 
     def eval(source)
       context = Context.new(self)
-      context.eval(source)
+      context.eval(source, :pure => true)
     end
 
     def compile(source)
diff --git a/lib/execjs/ruby_racer_runtime.rb b/lib/execjs/ruby_racer_runtime.rb
index 3c9bc0b..c16d5ac 100644
--- a/lib/execjs/ruby_racer_runtime.rb
+++ b/lib/execjs/ruby_racer_runtime.rb
@@ -5,13 +5,13 @@ module ExecJS
         @v8_context = ::V8::Context.new
       end
 
-      def exec(source)
+      def exec(source, options = {})
         if /\S/ =~ source
-          eval "(function(){#{source}})()"
+          eval "(function(){#{source}})()", options
         end
       end
 
-      def eval(source)
+      def eval(source, options = {})
         if /\S/ =~ source
           unbox @v8_context.eval("(#{source})")
         end
@@ -46,12 +46,12 @@ module ExecJS
 
     def exec(source)
       context = Context.new
-      context.exec(source)
+      context.exec(source, :pure => true)
     end
 
     def eval(source)
       context = Context.new
-      context.eval(source)
+      context.eval(source, :pure => true)
     end
 
     def compile(source)
diff --git a/lib/execjs/ruby_rhino_runtime.rb b/lib/execjs/ruby_rhino_runtime.rb
index 48923a3..a436fc5 100644
--- a/lib/execjs/ruby_rhino_runtime.rb
+++ b/lib/execjs/ruby_rhino_runtime.rb
@@ -5,13 +5,13 @@ module ExecJS
         @rhino_context = ::Rhino::Context.new
       end
 
-      def exec(source)
+      def exec(source, options = {})
         if /\S/ =~ source
-          eval "(function(){#{source}})()"
+          eval "(function(){#{source}})()", options
         end
       end
 
-      def eval(source)
+      def eval(source, options = {})
         if /\S/ =~ source
           unbox @rhino_context.eval("(#{source})")
         end
@@ -44,12 +44,12 @@ module ExecJS
 
     def exec(source)
       context = Context.new
-      context.exec(source)
+      context.exec(source, :pure => true)
     end
 
     def eval(source)
       context = Context.new
-      context.eval(source)
+      context.eval(source, :pure => true)
     end
 
     def compile(source)
diff --git a/test/test_execjs.rb b/test/test_execjs.rb
index 751a544..512b865 100644
--- a/test/test_execjs.rb
+++ b/test/test_execjs.rb
@@ -23,4 +23,9 @@ class TestExecJS < Test::Unit::TestCase
     assert_equal "bar", context.exec("return foo()")
     assert_equal "bar", context.eval("foo()")
   end
+
+  def test_pure_evaluation
+    context = ExecJS.compile("foo = function() { return \"bar\"; }")
+    assert_equal "bar", context.eval("foo()", :pure => true)
+  end
 end