Fix bug: Converting 'module.exports = {...}' to ES6 doesn't introduce a default export (#24141)

src/harness/fourslash.ts

@@ -2293,9 +2293,13 @@ Actual: ${stringify(fullActual)}`);
         }
 
         public verifyCurrentFileContent(text: string) {
-            const actual = this.getFileContent(this.activeFile.fileName);
+            this.verifyFileContent(this.activeFile.fileName, text);
+        }
+
+        private verifyFileContent(fileName: string, text: string) {
+            const actual = this.getFileContent(fileName);
             if (actual !== text) {
-                throw new Error(`verifyCurrentFileContent failed:\n${showTextDiff(text, actual)}`);
+                throw new Error(`verifyFileContent failed:\n${showTextDiff(text, actual)}`);
             }
         }
 
@@ -2483,7 +2487,7 @@ Actual: ${stringify(fullActual)}`);
 
             this.applyCodeActions(details.codeActions);
 
-            this.verifyNewContent(options);
+            this.verifyNewContent(options, ts.flatMap(details.codeActions, a => a.changes.map(c => c.fileName)));
         }
 
         public verifyRangeIs(expectedText: string, includeWhiteSpace?: boolean) {
@@ -2570,13 +2574,25 @@ Actual: ${stringify(fullActual)}`);
                 this.applyEdits(change.fileName, change.textChanges, /*isFormattingEdit*/ false);
             }
 
-            this.verifyNewContent(options);
+            this.verifyNewContent(options, action.changes.map(c => c.fileName));
         }
 
-        private verifyNewContent(options: FourSlashInterface.NewContentOptions) {
+        private verifyNewContent(options: FourSlashInterface.NewContentOptions, changedFiles: ReadonlyArray<string>) {
+            const assertedChangedFiles = !options.newFileContent || typeof options.newFileContent === "string"
+                ? [this.activeFile.fileName]
+                : ts.getOwnKeys(options.newFileContent);
+            assert.deepEqual(assertedChangedFiles, changedFiles);
+
             if (options.newFileContent !== undefined) {
                 assert(!options.newRangeContent);
-                this.verifyCurrentFileContent(options.newFileContent);
+                if (typeof options.newFileContent === "string") {
+                    this.verifyCurrentFileContent(options.newFileContent);
+                }
+                else {
+                    for (const fileName in options.newFileContent) {
+                        this.verifyFileContent(fileName, options.newFileContent[fileName]);
+                    }
+                }
             }
             else {
                 this.verifyRangeIs(options.newRangeContent, /*includeWhitespace*/ true);
@@ -4781,7 +4797,7 @@ namespace FourSlashInterface {
 
     export interface NewContentOptions {
         // Exactly one of these should be defined.
-        newFileContent?: string;
+        newFileContent?: string | { readonly [filename: string]: string };
         newRangeContent?: string;
     }
 

src/services/codefixes/convertToEs6Module.ts

@@ -233,7 +233,7 @@ namespace ts.codefix {
                     Debug.assertNever(prop);
             }
         });
-        return statements && [statements, true];
+        return statements && [statements, false];
     }
 
     function convertNamedExport(

tests/cases/fourslash/fourslash.ts

@@ -179,7 +179,7 @@ declare namespace FourSlashInterface {
         isInCommentAtPosition(onlyMultiLineDiverges?: boolean): void;
         codeFix(options: {
             description: string,
-            newFileContent?: string,
+            newFileContent?: string | { readonly [fileName: string]: string },
             newRangeContent?: string,
             errorCode?: number,
             index?: number,

tests/cases/fourslash/refactorConvertToEs6Module_export_moduleDotExports_changesImports.ts

@@ -14,11 +14,9 @@
 
 verify.codeFix({
     description: "Convert to ES6 module",
-    newFileContent: "export default 0;",
+    newFileContent: {
+        "/a.js": "export default 0;",
+        "/b.ts": 'import a from "./a";',
+        "/c.js": 'const a = require("./a").default;',
+    }
 });
-
-goTo.file("/b.ts");
-verify.currentFileContentIs('import a from "./a";');
-
-goTo.file("/c.js");
-verify.currentFileContentIs('const a = require("./a").default;');

tests/cases/fourslash/refactorConvertToEs6Module_export_object.ts

@@ -12,6 +12,9 @@
 ////    C: class {},
 ////};
 
+// @Filename: /b.js
+////const a = require("./a.js");
+
 verify.codeFix({
     description: "Convert to ES6 module",
     newFileContent: