From c4c6df968ed9008e51975d2e0773d36f70abe341 Mon Sep 17 00:00:00 2001
From: Ron Buckton <ron.buckton@microsoft.com>
Date: Mon, 19 Aug 2019 15:40:15 -0700
Subject: [PATCH] Fix default behavior for transpileModule when fileName not
 provided

---
 src/services/transpile.ts                         |  2 +-
 src/testRunner/unittests/services/transpile.ts    | 15 ++++++++++++---
 .../transpile/Infer correct file extension.js     |  2 ++
 .../Infer correct file extension.oldTranspile.js  |  2 ++
 4 files changed, 17 insertions(+), 4 deletions(-)
 create mode 100644 tests/baselines/reference/transpile/Infer correct file extension.js
 create mode 100644 tests/baselines/reference/transpile/Infer correct file extension.oldTranspile.js

diff --git a/src/services/transpile.ts b/src/services/transpile.ts
index bdc27ac5e22cf..51d1582c564c9 100644
--- a/src/services/transpile.ts
+++ b/src/services/transpile.ts
@@ -66,7 +66,7 @@ namespace ts {
         options.noResolve = true;
 
         // if jsx is specified then treat file as .tsx
-        const inputFileName = transpileOptions.fileName || (options.jsx ? "module.tsx" : "module.ts");
+        const inputFileName = transpileOptions.fileName || (transpileOptions.compilerOptions && transpileOptions.compilerOptions.jsx ? "module.tsx" : "module.ts");
         const sourceFile = createSourceFile(inputFileName, input, options.target!); // TODO: GH#18217
         if (transpileOptions.moduleName) {
             sourceFile.moduleName = transpileOptions.moduleName;
diff --git a/src/testRunner/unittests/services/transpile.ts b/src/testRunner/unittests/services/transpile.ts
index 6e8f3bcac9c6d..a86f4bdfc9e60 100644
--- a/src/testRunner/unittests/services/transpile.ts
+++ b/src/testRunner/unittests/services/transpile.ts
@@ -3,6 +3,7 @@ namespace ts {
 
         interface TranspileTestSettings {
             options?: TranspileOptions;
+            noSetFileName?: boolean;
         }
 
         function transpilesCorrectly(name: string, input: string, testSettings: TranspileTestSettings) {
@@ -30,15 +31,19 @@ namespace ts {
 
                 transpileOptions.compilerOptions.sourceMap = true;
 
-                if (!transpileOptions.fileName) {
-                    transpileOptions.fileName = transpileOptions.compilerOptions.jsx ? "file.tsx" : "file.ts";
+                let unitName = transpileOptions.fileName;
+                if (!unitName) {
+                    unitName = transpileOptions.compilerOptions.jsx ? "file.tsx" : "file.ts";
+                    if (!testSettings.noSetFileName) {
+                        transpileOptions.fileName = unitName;
+                    }
                 }
 
                 transpileOptions.reportDiagnostics = true;
 
                 justName = "transpile/" + name.replace(/[^a-z0-9\-. ]/ig, "") + (transpileOptions.compilerOptions.jsx ? Extension.Tsx : Extension.Ts);
                 toBeCompiled = [{
-                    unitName: transpileOptions.fileName,
+                    unitName,
                     content: input
                 }];
 
@@ -456,5 +461,9 @@ var x = 0;`, {
         transpilesCorrectly("Supports 'as const' arrays", `([] as const).forEach(k => console.log(k));`, {
             options: { compilerOptions: { module: ModuleKind.CommonJS } }
         });
+
+        transpilesCorrectly("Infer correct file extension", `const fn = <T>(a: T) => a`, {
+            noSetFileName: true
+        });
     });
 }
diff --git a/tests/baselines/reference/transpile/Infer correct file extension.js b/tests/baselines/reference/transpile/Infer correct file extension.js
new file mode 100644
index 0000000000000..76701b98d64fc
--- /dev/null
+++ b/tests/baselines/reference/transpile/Infer correct file extension.js	
@@ -0,0 +1,2 @@
+var fn = function (a) { return a; };
+//# sourceMappingURL=module.js.map
\ No newline at end of file
diff --git a/tests/baselines/reference/transpile/Infer correct file extension.oldTranspile.js b/tests/baselines/reference/transpile/Infer correct file extension.oldTranspile.js
new file mode 100644
index 0000000000000..76701b98d64fc
--- /dev/null
+++ b/tests/baselines/reference/transpile/Infer correct file extension.oldTranspile.js	
@@ -0,0 +1,2 @@
+var fn = function (a) { return a; };
+//# sourceMappingURL=module.js.map
\ No newline at end of file