diff --git a/src/compiler/diagnosticInformationMap.generated.ts b/src/compiler/diagnosticInformationMap.generated.ts
index 739c2ea460f..9fc4248c519 100644
--- a/src/compiler/diagnosticInformationMap.generated.ts
+++ b/src/compiler/diagnosticInformationMap.generated.ts
@@ -492,15 +492,15 @@ module ts {
         implements_clauses_can_only_be_used_in_TypeScript: { code: 8005, category: DiagnosticCategory.Error, key: "'implements clauses' can only be used in TypeScript." },
         interface_declarations_can_only_be_used_in_TypeScript: { code: 8006, category: DiagnosticCategory.Error, key: "'interface declarations' can only be used in TypeScript." },
         module_declarations_can_only_be_used_in_TypeScript: { code: 8007, category: DiagnosticCategory.Error, key: "'module declarations' can only be used in TypeScript." },
-        type_aliases_can_only_be_used_in_TypeScript: { code: 8007, category: DiagnosticCategory.Error, key: "'type aliases' can only be used in TypeScript." },
-        _0_can_only_be_used_in_TypeScript: { code: 8008, category: DiagnosticCategory.Error, key: "'{0}' can only be used in TypeScript." },
-        types_can_only_be_used_in_TypeScript: { code: 8009, category: DiagnosticCategory.Error, key: "'types' can only be used in TypeScript." },
-        type_arguments_can_only_be_used_in_TypeScript: { code: 8010, category: DiagnosticCategory.Error, key: "'type arguments' can only be used in TypeScript." },
-        parameter_modifiers_can_only_be_used_in_TypeScript: { code: 8011, category: DiagnosticCategory.Error, key: "'parameter modifiers' can only be used in TypeScript." },
-        can_only_be_used_in_TypeScript: { code: 8012, category: DiagnosticCategory.Error, key: "'?' can only be used in TypeScript." },
-        property_declarations_can_only_be_used_in_TypeScript: { code: 8013, category: DiagnosticCategory.Error, key: "'property declarations' can only be used in TypeScript." },
-        enum_declarations_can_only_be_used_in_TypeScript: { code: 8014, category: DiagnosticCategory.Error, key: "'enum declarations' can only be used in TypeScript." },
-        type_assertion_expressions_can_only_be_used_in_TypeScript: { code: 8014, category: DiagnosticCategory.Error, key: "'type assertion expressions' can only be used in TypeScript." },
+        type_aliases_can_only_be_used_in_TypeScript: { code: 8008, category: DiagnosticCategory.Error, key: "'type aliases' can only be used in TypeScript." },
+        _0_can_only_be_used_in_TypeScript: { code: 8009, category: DiagnosticCategory.Error, key: "'{0}' can only be used in TypeScript." },
+        types_can_only_be_used_in_TypeScript: { code: 8010, category: DiagnosticCategory.Error, key: "'types' can only be used in TypeScript." },
+        type_arguments_can_only_be_used_in_TypeScript: { code: 8011, category: DiagnosticCategory.Error, key: "'type arguments' can only be used in TypeScript." },
+        parameter_modifiers_can_only_be_used_in_TypeScript: { code: 8012, category: DiagnosticCategory.Error, key: "'parameter modifiers' can only be used in TypeScript." },
+        can_only_be_used_in_TypeScript: { code: 8013, category: DiagnosticCategory.Error, key: "'?' can only be used in TypeScript." },
+        property_declarations_can_only_be_used_in_TypeScript: { code: 8014, category: DiagnosticCategory.Error, key: "'property declarations' can only be used in TypeScript." },
+        enum_declarations_can_only_be_used_in_TypeScript: { code: 8015, category: DiagnosticCategory.Error, key: "'enum declarations' can only be used in TypeScript." },
+        type_assertion_expressions_can_only_be_used_in_TypeScript: { code: 8016, category: DiagnosticCategory.Error, key: "'type assertion expressions' can only be used in TypeScript." },
         yield_expressions_are_not_currently_supported: { code: 9000, category: DiagnosticCategory.Error, key: "'yield' expressions are not currently supported." },
         Generators_are_not_currently_supported: { code: 9001, category: DiagnosticCategory.Error, key: "Generators are not currently supported." },
         The_arguments_object_cannot_be_referenced_in_an_arrow_function_Consider_using_a_standard_function_expression: { code: 9002, category: DiagnosticCategory.Error, key: "The 'arguments' object cannot be referenced in an arrow function. Consider using a standard function expression." },
diff --git a/src/compiler/diagnosticMessages.json b/src/compiler/diagnosticMessages.json
index 552f1b04606..379a95bc405 100644
--- a/src/compiler/diagnosticMessages.json
+++ b/src/compiler/diagnosticMessages.json
@@ -1964,39 +1964,39 @@
     },
     "'type aliases' can only be used in TypeScript.": {
         "category": "Error",
-        "code": 8007
+        "code": 8008
     },
     "'{0}' can only be used in TypeScript.": {
         "category": "Error",
-        "code": 8008
+        "code": 8009
     },
     "'types' can only be used in TypeScript.": {
         "category": "Error",
-        "code": 8009
+        "code": 8010
     },
     "'type arguments' can only be used in TypeScript.": {
         "category": "Error",
-        "code": 8010
+        "code": 8011
     },
     "'parameter modifiers' can only be used in TypeScript.": {
         "category": "Error",
-        "code": 8011
+        "code": 8012
     },
     "'?' can only be used in TypeScript.": {
         "category": "Error",
-        "code": 8012
+        "code": 8013
     },
     "'property declarations' can only be used in TypeScript.": {
         "category": "Error",
-        "code": 8013
+        "code": 8014
     },
     "'enum declarations' can only be used in TypeScript.": {
         "category": "Error",
-        "code": 8014
+        "code": 8015
     },
     "'type assertion expressions' can only be used in TypeScript.": {
         "category": "Error",
-        "code": 8014
+        "code": 8016
     },
 
     "'yield' expressions are not currently supported.": {
diff --git a/src/harness/fourslash.ts b/src/harness/fourslash.ts
index 44927a7a49a..9d55fcc58b9 100644
--- a/src/harness/fourslash.ts
+++ b/src/harness/fourslash.ts
@@ -797,9 +797,6 @@ module FourSlash {
             var diagnostics = this.languageService.getSemanticDiagnostics(this.activeFile.fileName);
             var realized = ts.realizeDiagnostics(diagnostics, "\r\n");
             var actual = JSON.stringify(realized, null, "  ");
-            if (actual !== expected) {
-                ts.sys.writeFile("c:\\temp\\out.txt", actual);
-            }
             assert.equal(actual, expected);
         }
 
diff --git a/src/services/services.ts b/src/services/services.ts
index 8715a171c8b..a64358dd9fc 100644
--- a/src/services/services.ts
+++ b/src/services/services.ts
@@ -2384,7 +2384,8 @@ module ts {
             let targetSourceFile = getValidSourceFile(fileName);
 
             // For JavaScript files, we don't want to report the normal typescript semantic errors.
-            // Instead, we just report errors for using TypeScript 
+            // Instead, we just report errors for using TypeScript-only constructs from within a 
+            // JavaScript file.
             if (isJavaScript(fileName)) {
                 return getJavaScriptSemanticDiagnostics(targetSourceFile);
             }
@@ -2505,7 +2506,8 @@ module ts {
                         diagnostics.push(createDiagnosticForNode(node, Diagnostics.enum_declarations_can_only_be_used_in_TypeScript));
                         return;
                     case SyntaxKind.TypeAssertionExpression:
-                        diagnostics.push(createDiagnosticForNode(node, Diagnostics.type_assertion_expressions_can_only_be_used_in_TypeScript));
+                        let typeAssertionExpression = <TypeAssertion>node;
+                        diagnostics.push(createDiagnosticForNode(typeAssertionExpression.type, Diagnostics.type_assertion_expressions_can_only_be_used_in_TypeScript));
                         return;
                 }
 
diff --git a/tests/cases/fourslash/getJavaScriptSemanticDiagnostics10.ts b/tests/cases/fourslash/getJavaScriptSemanticDiagnostics10.ts
new file mode 100644
index 00000000000..d5bf9e142d7
--- /dev/null
+++ b/tests/cases/fourslash/getJavaScriptSemanticDiagnostics10.ts
@@ -0,0 +1,15 @@
+/// <reference path="fourslash.ts" />
+
+// @allowNonTsExtensions: true
+// @Filename: a.tsjs
+//// function F<T>() { }
+
+verify.getSemanticDiagnostics(`[
+  {
+    "message": "'type parameter declarations' can only be used in TypeScript.",
+    "start": 11,
+    "length": 1,
+    "category": "error",
+    "code": 8004
+  }
+]`);
\ No newline at end of file
diff --git a/tests/cases/fourslash/getJavaScriptSemanticDiagnostics11.ts b/tests/cases/fourslash/getJavaScriptSemanticDiagnostics11.ts
new file mode 100644
index 00000000000..060dfa4c01a
--- /dev/null
+++ b/tests/cases/fourslash/getJavaScriptSemanticDiagnostics11.ts
@@ -0,0 +1,15 @@
+/// <reference path="fourslash.ts" />
+
+// @allowNonTsExtensions: true
+// @Filename: a.tsjs
+//// function F(): number { }
+
+verify.getSemanticDiagnostics(`[
+  {
+    "message": "'types' can only be used in TypeScript.",
+    "start": 14,
+    "length": 6,
+    "category": "error",
+    "code": 8010
+  }
+]`);
\ No newline at end of file
diff --git a/tests/cases/fourslash/getJavaScriptSemanticDiagnostics12.ts b/tests/cases/fourslash/getJavaScriptSemanticDiagnostics12.ts
new file mode 100644
index 00000000000..4a73b44725d
--- /dev/null
+++ b/tests/cases/fourslash/getJavaScriptSemanticDiagnostics12.ts
@@ -0,0 +1,15 @@
+/// <reference path="fourslash.ts" />
+
+// @allowNonTsExtensions: true
+// @Filename: a.tsjs
+//// declare var v;
+
+verify.getSemanticDiagnostics(`[
+  {
+    "message": "'declare' can only be used in TypeScript.",
+    "start": 0,
+    "length": 7,
+    "category": "error",
+    "code": 8009
+  }
+]`);
\ No newline at end of file
diff --git a/tests/cases/fourslash/getJavaScriptSemanticDiagnostics13.ts b/tests/cases/fourslash/getJavaScriptSemanticDiagnostics13.ts
new file mode 100644
index 00000000000..2717e23aba9
--- /dev/null
+++ b/tests/cases/fourslash/getJavaScriptSemanticDiagnostics13.ts
@@ -0,0 +1,15 @@
+/// <reference path="fourslash.ts" />
+
+// @allowNonTsExtensions: true
+// @Filename: a.tsjs
+//// var v: () => number;
+
+verify.getSemanticDiagnostics(`[
+  {
+    "message": "'types' can only be used in TypeScript.",
+    "start": 7,
+    "length": 12,
+    "category": "error",
+    "code": 8010
+  }
+]`);
\ No newline at end of file
diff --git a/tests/cases/fourslash/getJavaScriptSemanticDiagnostics14.ts b/tests/cases/fourslash/getJavaScriptSemanticDiagnostics14.ts
new file mode 100644
index 00000000000..c67659379fc
--- /dev/null
+++ b/tests/cases/fourslash/getJavaScriptSemanticDiagnostics14.ts
@@ -0,0 +1,15 @@
+/// <reference path="fourslash.ts" />
+
+// @allowNonTsExtensions: true
+// @Filename: a.tsjs
+//// Foo<number>();
+
+verify.getSemanticDiagnostics(`[
+  {
+    "message": "'type arguments' can only be used in TypeScript.",
+    "start": 4,
+    "length": 6,
+    "category": "error",
+    "code": 8011
+  }
+]`);
\ No newline at end of file
diff --git a/tests/cases/fourslash/getJavaScriptSemanticDiagnostics15.ts b/tests/cases/fourslash/getJavaScriptSemanticDiagnostics15.ts
new file mode 100644
index 00000000000..756e17d9a34
--- /dev/null
+++ b/tests/cases/fourslash/getJavaScriptSemanticDiagnostics15.ts
@@ -0,0 +1,15 @@
+/// <reference path="fourslash.ts" />
+
+// @allowNonTsExtensions: true
+// @Filename: a.tsjs
+//// function F(public p) { }
+
+verify.getSemanticDiagnostics(`[
+  {
+    "message": "'parameter modifiers' can only be used in TypeScript.",
+    "start": 11,
+    "length": 6,
+    "category": "error",
+    "code": 8012
+  }
+]`);
\ No newline at end of file
diff --git a/tests/cases/fourslash/getJavaScriptSemanticDiagnostics16.ts b/tests/cases/fourslash/getJavaScriptSemanticDiagnostics16.ts
new file mode 100644
index 00000000000..1ce8f3a767d
--- /dev/null
+++ b/tests/cases/fourslash/getJavaScriptSemanticDiagnostics16.ts
@@ -0,0 +1,15 @@
+/// <reference path="fourslash.ts" />
+
+// @allowNonTsExtensions: true
+// @Filename: a.tsjs
+//// function F(p?) { }
+
+verify.getSemanticDiagnostics(`[
+  {
+    "message": "'?' can only be used in TypeScript.",
+    "start": 12,
+    "length": 1,
+    "category": "error",
+    "code": 8013
+  }
+]`);
\ No newline at end of file
diff --git a/tests/cases/fourslash/getJavaScriptSemanticDiagnostics17.ts b/tests/cases/fourslash/getJavaScriptSemanticDiagnostics17.ts
new file mode 100644
index 00000000000..fad657f9b50
--- /dev/null
+++ b/tests/cases/fourslash/getJavaScriptSemanticDiagnostics17.ts
@@ -0,0 +1,15 @@
+/// <reference path="fourslash.ts" />
+
+// @allowNonTsExtensions: true
+// @Filename: a.tsjs
+//// function F(a: number) { }
+
+verify.getSemanticDiagnostics(`[
+  {
+    "message": "'types' can only be used in TypeScript.",
+    "start": 14,
+    "length": 6,
+    "category": "error",
+    "code": 8010
+  }
+]`);
\ No newline at end of file
diff --git a/tests/cases/fourslash/getJavaScriptSemanticDiagnostics18.ts b/tests/cases/fourslash/getJavaScriptSemanticDiagnostics18.ts
new file mode 100644
index 00000000000..6c2578d6e85
--- /dev/null
+++ b/tests/cases/fourslash/getJavaScriptSemanticDiagnostics18.ts
@@ -0,0 +1,15 @@
+/// <reference path="fourslash.ts" />
+
+// @allowNonTsExtensions: true
+// @Filename: a.tsjs
+//// class C { v }
+
+verify.getSemanticDiagnostics(`[
+  {
+    "message": "'property declarations' can only be used in TypeScript.",
+    "start": 10,
+    "length": 1,
+    "category": "error",
+    "code": 8014
+  }
+]`);
\ No newline at end of file
diff --git a/tests/cases/fourslash/getJavaScriptSemanticDiagnostics19.ts b/tests/cases/fourslash/getJavaScriptSemanticDiagnostics19.ts
new file mode 100644
index 00000000000..6dc83c38327
--- /dev/null
+++ b/tests/cases/fourslash/getJavaScriptSemanticDiagnostics19.ts
@@ -0,0 +1,15 @@
+/// <reference path="fourslash.ts" />
+
+// @allowNonTsExtensions: true
+// @Filename: a.tsjs
+//// enum E { }
+
+verify.getSemanticDiagnostics(`[
+  {
+    "message": "'enum declarations' can only be used in TypeScript.",
+    "start": 5,
+    "length": 1,
+    "category": "error",
+    "code": 8015
+  }
+]`);
\ No newline at end of file
diff --git a/tests/cases/fourslash/getJavaScriptSemanticDiagnostics20.ts b/tests/cases/fourslash/getJavaScriptSemanticDiagnostics20.ts
new file mode 100644
index 00000000000..cf5f16b827a
--- /dev/null
+++ b/tests/cases/fourslash/getJavaScriptSemanticDiagnostics20.ts
@@ -0,0 +1,15 @@
+/// <reference path="fourslash.ts" />
+
+// @allowNonTsExtensions: true
+// @Filename: a.tsjs
+//// var v = <string>undefined;
+
+verify.getSemanticDiagnostics(`[
+  {
+    "message": "'type assertion expressions' can only be used in TypeScript.",
+    "start": 9,
+    "length": 6,
+    "category": "error",
+    "code": 8016
+  }
+]`);
\ No newline at end of file
diff --git a/tests/cases/fourslash/getJavaScriptSemanticDiagnostics4.ts b/tests/cases/fourslash/getJavaScriptSemanticDiagnostics4.ts
index 27635cb34b7..8505798e230 100644
--- a/tests/cases/fourslash/getJavaScriptSemanticDiagnostics4.ts
+++ b/tests/cases/fourslash/getJavaScriptSemanticDiagnostics4.ts
@@ -10,6 +10,6 @@ verify.getSemanticDiagnostics(`[
     "start": 0,
     "length": 6,
     "category": "error",
-    "code": 8008
+    "code": 8009
   }
 ]`);
\ No newline at end of file
diff --git a/tests/cases/fourslash/getJavaScriptSemanticDiagnostics6.ts b/tests/cases/fourslash/getJavaScriptSemanticDiagnostics6.ts
new file mode 100644
index 00000000000..09cd23ffbf6
--- /dev/null
+++ b/tests/cases/fourslash/getJavaScriptSemanticDiagnostics6.ts
@@ -0,0 +1,15 @@
+/// <reference path="fourslash.ts" />
+
+// @allowNonTsExtensions: true
+// @Filename: a.tsjs
+//// interface I { }
+
+verify.getSemanticDiagnostics(`[
+  {
+    "message": "'interface declarations' can only be used in TypeScript.",
+    "start": 10,
+    "length": 1,
+    "category": "error",
+    "code": 8006
+  }
+]`);
\ No newline at end of file
diff --git a/tests/cases/fourslash/getJavaScriptSemanticDiagnostics7.ts b/tests/cases/fourslash/getJavaScriptSemanticDiagnostics7.ts
new file mode 100644
index 00000000000..77c0b1f4e0d
--- /dev/null
+++ b/tests/cases/fourslash/getJavaScriptSemanticDiagnostics7.ts
@@ -0,0 +1,15 @@
+/// <reference path="fourslash.ts" />
+
+// @allowNonTsExtensions: true
+// @Filename: a.tsjs
+//// module M { }
+
+verify.getSemanticDiagnostics(`[
+  {
+    "message": "'module declarations' can only be used in TypeScript.",
+    "start": 7,
+    "length": 1,
+    "category": "error",
+    "code": 8007
+  }
+]`);
\ No newline at end of file
diff --git a/tests/cases/fourslash/getJavaScriptSemanticDiagnostics8.ts b/tests/cases/fourslash/getJavaScriptSemanticDiagnostics8.ts
new file mode 100644
index 00000000000..c0a27621539
--- /dev/null
+++ b/tests/cases/fourslash/getJavaScriptSemanticDiagnostics8.ts
@@ -0,0 +1,15 @@
+/// <reference path="fourslash.ts" />
+
+// @allowNonTsExtensions: true
+// @Filename: a.tsjs
+//// type a = b;
+
+verify.getSemanticDiagnostics(`[
+  {
+    "message": "'type aliases' can only be used in TypeScript.",
+    "start": 0,
+    "length": 11,
+    "category": "error",
+    "code": 8008
+  }
+]`);
\ No newline at end of file
diff --git a/tests/cases/fourslash/getJavaScriptSemanticDiagnostics9.ts b/tests/cases/fourslash/getJavaScriptSemanticDiagnostics9.ts
new file mode 100644
index 00000000000..6b7eed188fe
--- /dev/null
+++ b/tests/cases/fourslash/getJavaScriptSemanticDiagnostics9.ts
@@ -0,0 +1,15 @@
+/// <reference path="fourslash.ts" />
+
+// @allowNonTsExtensions: true
+// @Filename: a.tsjs
+//// public function F() { }
+
+verify.getSemanticDiagnostics(`[
+  {
+    "message": "'public' can only be used in TypeScript.",
+    "start": 0,
+    "length": 6,
+    "category": "error",
+    "code": 8009
+  }
+]`);
\ No newline at end of file