diff --git a/src/compiler/parser.ts b/src/compiler/parser.ts
index e0bf1f4c2e9..5f9ea09fd6c 100644
--- a/src/compiler/parser.ts
+++ b/src/compiler/parser.ts
@@ -407,6 +407,22 @@ namespace ts {
         return result;
     }
 
+    /* @internal */
+    export function getScriptKindFromFileName(fileName: string): ScriptKind {
+        const ext = fileName.split(".").pop();
+        switch (ext.toLowerCase()) {
+            case "js":
+                return ScriptKind.JS;
+            case "jsx":
+                return ScriptKind.JSX;
+            case "ts":
+                return ScriptKind.TS;
+            case "tsx":
+                return ScriptKind.TSX;
+            default:
+                return ScriptKind.TS;
+        }
+    }
 
     // Produces a new SourceFile for the 'newText' provided. The 'textChangeRange' parameter
     // indicates what changed between the 'text' that this SourceFile has and the 'newText'.
@@ -551,22 +567,6 @@ namespace ts {
             return result;
         }
 
-        function getScriptKindFromFileName(fileName: string): ScriptKind {
-            const ext = fileName.split(".").pop();
-            switch (ext.toLowerCase()) {
-                case "js":
-                    return ScriptKind.JS;
-                case "jsx":
-                    return ScriptKind.JSX;
-                case "ts":
-                    return ScriptKind.TS;
-                case "tsx":
-                    return ScriptKind.TSX;
-                default:
-                    return ScriptKind.TS;
-            }
-        }
-
         function getLanguageVariant(scriptKind: ScriptKind) {
             // .tsx and .jsx files are treated as jsx language variant.
             return scriptKind === ScriptKind.TSX || scriptKind === ScriptKind.JSX || scriptKind === ScriptKind.JS ? LanguageVariant.JSX : LanguageVariant.Standard;
diff --git a/src/services/services.ts b/src/services/services.ts
index 23db7b98a27..3dbd34cbedd 100644
--- a/src/services/services.ts
+++ b/src/services/services.ts
@@ -1757,7 +1757,7 @@ namespace ts {
                     hostFileName: fileName,
                     version: this.host.getScriptVersion(fileName),
                     scriptSnapshot: scriptSnapshot,
-                    scriptKind: scriptKind
+                    scriptKind: scriptKind ? scriptKind : getScriptKindFromFileName(fileName)
                 };
             }
 
@@ -2857,7 +2857,7 @@ namespace ts {
                         // We do not support the scenario where a host can modify a registered
                         // file's script kind, i.e. in one project some file is treated as ".ts"
                         // and in another as ".js"
-                        Debug.assert(hostFileInformation.scriptKind === oldSourceFile.scriptKind, "Registered script kind (" + oldSourceFile.scriptKind + ") should match new script kind (" + hostFileInformation.scriptKind +") for file: " + fileName);
+                        Debug.assert(hostFileInformation.scriptKind === oldSourceFile.scriptKind, "Registered script kind (" + oldSourceFile.scriptKind + ") should match new script kind (" + hostFileInformation.scriptKind + ") for file: " + fileName);
 
                         return documentRegistry.updateDocument(fileName, newSettings, hostFileInformation.scriptSnapshot, hostFileInformation.version, hostFileInformation.scriptKind);
                     }