fix: make Promise.withResolvers polyfill file a module (#493)

Author: dsherret

rs-lib/src/polyfills/mod.rs

@@ -179,3 +179,23 @@ impl PolyfillTester {
     })
   }
 }
+
+#[cfg(test)]
+mod test {
+  use super::*;
+
+  #[test]
+  fn polyfill_scripts_are_modules() {
+    // a polyfill may end up being the only one in the output file, so each
+    // must be a module on its own for `declare global` to be valid
+    for polyfill in all_polyfills() {
+      let text = polyfill.get_file_text();
+      assert!(
+        text.lines().any(
+          |line| line.starts_with("export ") || line.starts_with("import ")
+        ),
+        "polyfill script was not a module: {text}"
+      );
+    }
+  }
+}

rs-lib/src/polyfills/scripts/es2021.promise-withResolvers.ts

@@ -20,3 +20,5 @@ if (Promise.withResolvers === undefined) {
     return out;
   };
 }
+
+export {};

tests/integration.test.ts

@@ -927,6 +927,25 @@ Deno.test("should build and test polyfill project", async () => {
   });
 });
 
+Deno.test("should build and test the promise with resolvers polyfill project", async () => {
+  // this polyfill ends up alone in the polyfill file, so ensure the generated
+  // file is still a module and thus type checks (see #440)
+  await runTest("polyfill_promise_with_resolvers_project", {
+    entryPoints: ["mod.ts"],
+    outDir: "./npm",
+    shims: {
+      ...getAllShimOptions(false),
+      deno: "dev",
+    },
+    package: {
+      name: "polyfill-package",
+      version: "1.0.0",
+    },
+  }, (output) => {
+    output.assertExists("esm/_dnt.polyfills.js");
+  });
+});
+
 Deno.test("should build and test the array find last polyfill project", async () => {
   await runTest("polyfill_array_find_last_project", {
     entryPoints: ["mod.ts"],
@@ -1316,6 +1335,7 @@ async function runTest(
     | "polyfill_project"
     | "polyfill_array_from_async_project"
     | "polyfill_array_find_last_project"
+    | "polyfill_promise_with_resolvers_project"
     | "polyfill_import_meta_project"
     | "module_mappings_project"
     | "node_types_project"

tests/polyfill_promise_with_resolvers_project/mod.ts

@@ -0,0 +1,5 @@
+// this project must only use the `Promise.withResolvers` polyfill so that it
+// ends up alone in the generated polyfill file (see #440)
+export function withResolvers<T>() {
+  return Promise.withResolvers<T>();
+}

tests/polyfill_promise_with_resolvers_project/mod_test.ts

@@ -0,0 +1,24 @@
+import { withResolvers } from "./mod.ts";
+
+function assertEquals(a: unknown, b: unknown) {
+  if (a !== b) {
+    throw new Error(`${a} did not equal ${b}`);
+  }
+}
+
+Deno.test("should resolve", async () => {
+  const { promise, resolve } = withResolvers<number>();
+  setTimeout(() => resolve(5), 10);
+  assertEquals(await promise, 5);
+});
+
+Deno.test("should reject", async () => {
+  const { promise, reject } = withResolvers<number>();
+  setTimeout(() => reject(new Error("test")), 10);
+  try {
+    await promise;
+    throw new Error("Did not throw.");
+  } catch (err) {
+    assertEquals((err as Error).message, "test");
+  }
+});