Respond to PR comments

src/compiler/core.ts

@@ -861,6 +861,13 @@ namespace ts {
         return fileExtensionIs(path, extension) ? path.substring(0, path.length - extension.length) : undefined;
     }
 
+    export function getFileExtension(path: string): string {
+        const dot = path.lastIndexOf(".");
+        if (dot !== -1) {
+            return path.substring(dot);
+        }
+    }
+
     export interface ObjectAllocator {
         getNodeConstructor(): new (kind: SyntaxKind, pos?: number, end?: number) => Node;
         getSourceFileConstructor(): new (kind: SyntaxKind, pos?: number, end?: number) => SourceFile;

src/compiler/diagnosticMessages.json

@@ -2768,6 +2768,10 @@
         "category": "Error",
         "code": 6131
     },
+    "File name '{0}' has a '{1}' extension - stripping it": {
+        "category": "Message",
+        "code": 6132
+    },
 
     "Variable '{0}' implicitly has an '{1}' type.": {
         "category": "Error",

src/compiler/program.ts

@@ -615,11 +615,26 @@ namespace ts {
     }
 
     /**
-     * @param extensions - Either supportedTypeScriptExtensions or if --allowJs, allSupportedExtensions
      * @param {boolean} onlyRecordFailures - if true then function won't try to actually load files but instead record all attempts as failures. This flag is necessary
      * in cases when we know upfront that all load attempts will fail (because containing folder does not exists) however we still need to record all failed lookup locations.
      */
     function loadModuleFromFile(candidate: string, extensions: string[], failedLookupLocation: string[], onlyRecordFailures: boolean, state: ModuleResolutionState): string {
+        // First try to keep/add an extension: importing "./foo.ts" can be matched by a file "./foo.ts", and "./foo" by "./foo.d.ts"
+        const keepOrAddExtension = loadModuleFromFileWorker(candidate, extensions, failedLookupLocation, onlyRecordFailures, state);
+        if (keepOrAddExtension) {
+            return keepOrAddExtension;
+        }
+        // Then try stripping a ".js" or ".jsx" extension and replacing it with a TypeScript one, e.g. "./foo.js" can be matched by "./foo.ts" or "./foo.d.ts"
+        if (hasJavaScriptFileExtension(candidate)) {
+            const extensionless = removeFileExtension(candidate);
+            if (state.traceEnabled) {
+                const extension = candidate.substring(extensionless.length);
+                trace(state.host, Diagnostics.File_name_0_has_a_1_extension_stripping_it, candidate, extension);
+            }
+            return loadModuleFromFileWorker(extensionless, extensions, failedLookupLocation, onlyRecordFailures, state);
+        }
+    }
+    function loadModuleFromFileWorker(candidate: string, extensions: string[], failedLookupLocation: string[], onlyRecordFailures: boolean, state: ModuleResolutionState): string {
         if (!onlyRecordFailures) {
             // check if containig folder exists - if it doesn't then just record failures for all supported extensions without disk probing
             const directory = getDirectoryPath(candidate);
@@ -628,27 +643,12 @@ namespace ts {
             }
         }
 
-        // First try to keep/add an extension: importing "./foo.ts" can be matched by a file "./foo.ts", and "./foo" by "./foo.d.ts"
-        const keepOrAddExtension = forEach(extensions, ext => {
+        return forEach(extensions, ext => {
             if (state.skipTsx && (ext === ".jsx" || ext === ".tsx")) {
                 return;
             }
             return tryLoad(fileExtensionIs(candidate, ext) ? candidate : candidate + ext);
         });
-        if (keepOrAddExtension) {
-            return keepOrAddExtension;
-        }
-
-        // Then try stripping a ".js" or ".jsx" extension and replacing it with a TypeScript one, e.g. "./foo.js" can be matched by "./foo.ts" or "./foo.d.ts"
-        return forEach(supportedJavascriptExtensions, jsExt => {
-            if (state.skipTsx && jsExt === ".jsx") {
-                return;
-            }
-            const extensionless = tryRemoveExtension(candidate, jsExt);
-            if (extensionless !== undefined) {
-                return forEach(supportedTypeScriptExtensions, tsExt => tryLoad(extensionless + tsExt));
-            }
-        });
 
         function tryLoad(fileName: string): string {
             if (!onlyRecordFailures && state.host.fileExists(fileName)) {

tests/baselines/reference/moduleResolution.trace.json

@@ -0,0 +1,32 @@
+[
+    "======== Resolving module './a' from 'C:/Users/anhans/TypeScript/tests/cases/compiler/b.ts'. ========",
+    "Module resolution kind is not specified, using 'NodeJs'.",
+    "Loading module as file / folder, candidate module location 'C:/Users/anhans/TypeScript/tests/cases/compiler/a'.",
+    "File 'C:/Users/anhans/TypeScript/tests/cases/compiler/a.ts' exist - use it as a name resolution result.",
+    "======== Module name './a' was successfully resolved to 'C:/Users/anhans/TypeScript/tests/cases/compiler/a.ts'. ========",
+    "======== Resolving module './a.ts' from 'C:/Users/anhans/TypeScript/tests/cases/compiler/c.ts'. ========",
+    "Module resolution kind is not specified, using 'NodeJs'.",
+    "Loading module as file / folder, candidate module location 'C:/Users/anhans/TypeScript/tests/cases/compiler/a.ts'.",
+    "File 'C:/Users/anhans/TypeScript/tests/cases/compiler/a.ts' exist - use it as a name resolution result.",
+    "======== Module name './a.ts' was successfully resolved to 'C:/Users/anhans/TypeScript/tests/cases/compiler/a.ts'. ========",
+    "======== Resolving module './a.js' from 'C:/Users/anhans/TypeScript/tests/cases/compiler/d.ts'. ========",
+    "Module resolution kind is not specified, using 'NodeJs'.",
+    "Loading module as file / folder, candidate module location 'C:/Users/anhans/TypeScript/tests/cases/compiler/a.js'.",
+    "File 'C:/Users/anhans/TypeScript/tests/cases/compiler/a.js.ts' does not exist.",
+    "File 'C:/Users/anhans/TypeScript/tests/cases/compiler/a.js.tsx' does not exist.",
+    "File 'C:/Users/anhans/TypeScript/tests/cases/compiler/a.js.d.ts' does not exist.",
+    "File name 'C:/Users/anhans/TypeScript/tests/cases/compiler/a.js' has a '.js' extension - stripping it",
+    "File 'C:/Users/anhans/TypeScript/tests/cases/compiler/a.ts' exist - use it as a name resolution result.",
+    "======== Module name './a.js' was successfully resolved to 'C:/Users/anhans/TypeScript/tests/cases/compiler/a.ts'. ========",
+    "======== Resolving module './jquery.js' from 'C:/Users/anhans/TypeScript/tests/cases/compiler/jquery_user_1.ts'. ========",
+    "Module resolution kind is not specified, using 'NodeJs'.",
+    "Loading module as file / folder, candidate module location 'C:/Users/anhans/TypeScript/tests/cases/compiler/jquery.js'.",
+    "File 'C:/Users/anhans/TypeScript/tests/cases/compiler/jquery.js.ts' does not exist.",
+    "File 'C:/Users/anhans/TypeScript/tests/cases/compiler/jquery.js.tsx' does not exist.",
+    "File 'C:/Users/anhans/TypeScript/tests/cases/compiler/jquery.js.d.ts' does not exist.",
+    "File name 'C:/Users/anhans/TypeScript/tests/cases/compiler/jquery.js' has a '.js' extension - stripping it",
+    "File 'C:/Users/anhans/TypeScript/tests/cases/compiler/jquery.ts' does not exist.",
+    "File 'C:/Users/anhans/TypeScript/tests/cases/compiler/jquery.tsx' does not exist.",
+    "File 'C:/Users/anhans/TypeScript/tests/cases/compiler/jquery.d.ts' exist - use it as a name resolution result.",
+    "======== Module name './jquery.js' was successfully resolved to 'C:/Users/anhans/TypeScript/tests/cases/compiler/jquery.d.ts'. ========"
+]

tests/cases/compiler/moduleResolution.ts

@@ -1,3 +1,4 @@
+// @traceResolution: true
 // @Filename: a.ts
 export default 0;