Handle the json extension when ResolvedModule result is json file

Fixes microsoft#24932

Author: sheetalkamat

src/parser/utilities.ts

@@ -7995,7 +7995,7 @@ namespace ts {
     }
 
     export function tryGetExtensionFromPath(path: string): Extension | undefined {
-        return find<Extension>(supportedTypescriptExtensionsForExtractExtension, e => fileExtensionIs(path, e)) || find(supportedJavascriptExtensions, e => fileExtensionIs(path, e));
+        return find<Extension>(extensionsToRemove, e => fileExtensionIs(path, e));
     }
 
     function getAnyExtensionFromPathWorker(path: string, extensions: string | ReadonlyArray<string>, stringEqualityComparer: (a: string, b: string) => boolean) {

src/testRunner/unittests/tscWatchMode.ts

@@ -2532,4 +2532,43 @@ declare module "fs" {
             checkWatchedDirectoriesDetailed(host, [mainPackageRoot, linkedPackageRoot, `${mainPackageRoot}/node_modules/@types`, `${projectRoot}/node_modules/@types`], 1, /*recursive*/ true);
         });
     });
+
+    describe("tsc-watch with custom module resolution", () => {
+        const projectRoot = "/user/username/projects/project";
+        const configFileJson: any = {
+            compilerOptions: { module: "commonjs", resolveJsonModule: true },
+            files: ["index.ts"]
+        };
+        const mainFile: File = {
+            path: `${projectRoot}/index.ts`,
+            content: "import settings from './settings.json';"
+        };
+        const config: File = {
+            path: `${projectRoot}/tsconfig.json`,
+            content: JSON.stringify(configFileJson)
+        };
+        const settingsJson: File = {
+            path: `${projectRoot}/settings.json`,
+            content: JSON.stringify({ content: "Print this" })
+        };
+
+        it("verify that module resolution with json extension works when returned without extension", () => {
+            const files = [libFile, mainFile, config, settingsJson];
+            const host = createWatchedSystem(files, { currentDirectory: projectRoot });
+            const compilerHost = createWatchCompilerHostOfConfigFile(config.path, {}, host);
+            const parsedCommandResult = parseJsonConfigFileContent(configFileJson, host, config.path);
+            compilerHost.resolveModuleNames = (moduleNames, containingFile) => moduleNames.map(m => {
+                const result = resolveModuleName(m, containingFile, parsedCommandResult.options, compilerHost);
+                const resolvedModule = result.resolvedModule!;
+                return {
+                    resolvedFileName: resolvedModule.resolvedFileName,
+                    isExternalLibraryImport: resolvedModule.isExternalLibraryImport,
+                    originalFileName: resolvedModule.originalPath,
+                };
+            });
+            const watch = createWatchProgram(compilerHost);
+            const program = watch.getCurrentProgram().getProgram();
+            checkProgramActualFiles(program, [mainFile.path, libFile.path, settingsJson.path]);
+        });
+    });
 }