Allow an import of "foo.js" to be matched by a file "foo.ts"

src/compiler/core.ts

@@ -849,13 +849,18 @@ namespace ts {
     const extensionsToRemove = [".d.ts", ".ts", ".js", ".tsx", ".jsx"];
     export function removeFileExtension(path: string): string {
         for (const ext of extensionsToRemove) {
-            if (fileExtensionIs(path, ext)) {
-                return path.substr(0, path.length - ext.length);
+            const extensionless = tryRemoveExtension(path, ext);
+            if (extensionless !== undefined) {
+                return extensionless;
             }
         }
         return path;
     }
 
+    export function tryRemoveExtension(path: string, extension: string): string {
+        return fileExtensionIs(path, extension) ? path.substring(0, path.length - extension.length) : undefined;
+    }
+
     export interface ObjectAllocator {
         getNodeConstructor(): new (kind: SyntaxKind, pos?: number, end?: number) => Node;
         getSourceFileConstructor(): new (kind: SyntaxKind, pos?: number, end?: number) => SourceFile;

src/compiler/program.ts

@@ -615,6 +615,7 @@ 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.
      */
@@ -626,13 +627,29 @@ namespace ts {
                 onlyRecordFailures = !directoryProbablyExists(directory, state.host);
             }
         }
-        return forEach(extensions, tryLoad);
 
-        function tryLoad(ext: string): string {
-            if (ext === ".tsx" && state.skipTsx) {
-                return undefined;
+        if (state.skipTsx)
+            extensions = filter(extensions, ext => ext !== "tsx");
+
+        // 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 =>
+           tryLoad(fileExtensionIs(candidate, ext) ? candidate : candidate + ext));
+        if (keepOrAddExtension) {
+            return keepOrAddExtension;
+        }
+
+        // Then try stripping a ".js" or ".jsx" extension and replacing it with a different one, e.g. "./foo.js" can be matched by "./foo.ts" or "./foo.d.ts"
+        return forEach(supportedJavascriptExtensions, jsExt => {
+            const extensionless = tryRemoveExtension(candidate, jsExt);
+            if (extensionless !== undefined) {
+                return forEach(supportedTypeScriptExtensions, ext => {
+                    if (ext !== jsExt)
+                        return tryLoad(extensionless + ext);
+                });
             }
-            const fileName = fileExtensionIs(candidate, ext) ? candidate : candidate + ext;
+        });
+
+        function tryLoad(fileName: string): string {
             if (!onlyRecordFailures && state.host.fileExists(fileName)) {
                 if (state.traceEnabled) {
                     trace(state.host, Diagnostics.File_0_exist_use_it_as_a_name_resolution_result, fileName);

src/harness/harness.ts

@@ -1,7 +1,7 @@
 
 //
 // Copyright (c) Microsoft Corporation.  All rights reserved.
-// 
+//
 // Licensed under the Apache License, Version 2.0 (the "License");
 // you may not use this file except in compliance with the License.
 // You may obtain a copy of the License at
@@ -350,7 +350,7 @@ namespace Utils {
         assert.equal(node1.end, node2.end, "node1.end !== node2.end");
         assert.equal(node1.kind, node2.kind, "node1.kind !== node2.kind");
 
-        // call this on both nodes to ensure all propagated flags have been set (and thus can be 
+        // call this on both nodes to ensure all propagated flags have been set (and thus can be
         // compared).
         assert.equal(ts.containsParseError(node1), ts.containsParseError(node2));
         assert.equal(node1.flags, node2.flags, "node1.flags !== node2.flags");
@@ -751,7 +751,7 @@ namespace Harness {
         (emittedFile: string, emittedLine: number, emittedColumn: number, sourceFile: string, sourceLine: number, sourceColumn: number, sourceName: string): void;
     }
 
-    // Settings 
+    // Settings
     export let userSpecifiedRoot = "";
     export let lightMode = false;
 
@@ -790,7 +790,7 @@ namespace Harness {
             fileName: string,
             sourceText: string,
             languageVersion: ts.ScriptTarget) {
-            // We'll only assert invariants outside of light mode. 
+            // We'll only assert invariants outside of light mode.
             const shouldAssertInvariants = !Harness.lightMode;
 
             // Only set the parent nodes if we're asserting invariants.  We don't need them otherwise.
@@ -935,7 +935,7 @@ namespace Harness {
             libFiles?: string;
         }
 
-        // Additional options not already in ts.optionDeclarations 
+        // Additional options not already in ts.optionDeclarations
         const harnessOptionDeclarations: ts.CommandLineOption[] = [
             { name: "allowNonTsExtensions", type: "boolean" },
             { name: "useCaseSensitiveFileNames", type: "boolean" },
@@ -1187,7 +1187,7 @@ namespace Harness {
                 errLines.forEach(e => outputLines.push(e));
 
                 // do not count errors from lib.d.ts here, they are computed separately as numLibraryDiagnostics
-                // if lib.d.ts is explicitly included in input files and there are some errors in it (i.e. because of duplicate identifiers) 
+                // if lib.d.ts is explicitly included in input files and there are some errors in it (i.e. because of duplicate identifiers)
                 // then they will be added twice thus triggering 'total errors' assertion with condition
                 // 'totalErrorsReportedInNonLibraryFiles + numLibraryDiagnostics + numTest262HarnessDiagnostics, diagnostics.length
 
@@ -1497,7 +1497,7 @@ namespace Harness {
             };
             testUnitData.push(newTestFile2);
 
-            // unit tests always list files explicitly 
+            // unit tests always list files explicitly
             const parseConfigHost: ts.ParseConfigHost = {
                 readDirectory: (name) => []
             };