Add test for .prettierrc with string reference to ESM config package (#3868)

* Initial plan

* Add test fixture for string ESM config issue

Co-authored-by: ntotten <[email protected]>

* Update test comment to address code review

Co-authored-by: ntotten <[email protected]>

---------

Co-authored-by: copilot-swe-agent[bot] <[email protected]>
Co-authored-by: ntotten <[email protected]>

Author: Copilot

src/test/suite/string-esm-config.test.ts

@@ -0,0 +1,18 @@
+import * as assert from "assert";
+import { format, getText } from "./formatTestUtils.js";
+import { ensureExtensionActivated } from "./testUtils.js";
+
+describe("String ESM Config Support", () => {
+  before(async () => {
+    await ensureExtensionActivated();
+  });
+
+  // This test validates that .prettierrc with a string reference to an ESM config package works
+  // Reproduces issue where extension fails with "Invalid prettier configuration file detected"
+  // when .prettierrc contains a string pointing to an ESM prettier config package
+  it("it formats with .prettierrc containing string reference to ESM config package", async () => {
+    const { actual } = await format("string-esm-config", "test.js");
+    const expected = await getText("string-esm-config", "test.result.js");
+    assert.equal(actual, expected);
+  });
+});

test-fixtures/string-esm-config/.prettierrc

@@ -0,0 +1 @@
+"test-prettier-config-esm"

test-fixtures/string-esm-config/package.json

@@ -0,0 +1,16 @@
+{
+  "name": "string-esm-config",
+  "version": "1.0.0",
+  "description": "Test folder for .prettierrc with string reference to ESM config package",
+  "private": true,
+  "type": "module",
+  "scripts": {
+    "test": "echo \"Error: no test specified\" && exit 1"
+  },
+  "author": "Prettier",
+  "license": "MIT",
+  "devDependencies": {
+    "prettier": "^3.4.2",
+    "test-prettier-config-esm": "file:./test-prettier-config-esm"
+  }
+}

test-fixtures/string-esm-config/test-prettier-config-esm/index.js

@@ -0,0 +1,9 @@
+// ESM Prettier config package
+const config = {
+  semi: true,
+  singleQuote: false,
+  tabWidth: 2,
+  trailingComma: "es5",
+};
+
+export default config;

test-fixtures/string-esm-config/test-prettier-config-esm/package.json

@@ -0,0 +1,12 @@
+{
+  "name": "test-prettier-config-esm",
+  "version": "1.0.0",
+  "description": "ESM Prettier config package for testing",
+  "type": "module",
+  "main": "index.js",
+  "exports": {
+    ".": "./index.js"
+  },
+  "author": "Prettier",
+  "license": "MIT"
+}

test-fixtures/string-esm-config/test.js

@@ -0,0 +1 @@
+const foo={bar:1,baz:2};

test-fixtures/string-esm-config/test.result.js

@@ -0,0 +1 @@
+const foo = { bar: 1, baz: 2 };

test-fixtures/test.code-workspace

@@ -57,6 +57,9 @@
     {
       "path": "esm-plugin-import"
     },
+    {
+      "path": "string-esm-config"
+    },
     {
       "path": "monorepo-subfolder/subfolder"
     },