fix #910: esm "export {}" must still use cjs "exports"

CHANGELOG.md

@@ -1,5 +1,11 @@
 # Changelog
 
+## Unreleased
+
+* Fix bug when ESM file has empty exports and is converted to CommonJS ([#910](https://github.com/evanw/esbuild/issues/910))
+
+    A file containing the contents `export {}` is still considered to be an ESM file even though it has no exports. However, if a file containing this edge case is converted to CommonJS internally during bundling (e.g. when it is the target of `require()`), esbuild failed to mark the `exports` symbol from the CommonJS wrapping closure as used even though it is actually needed. This resulted in an output file that crashed when run. The `exports` symbol is now considered used in this case, so the bug has been fixed.
+
 ## 0.8.54
 
 * Fix ordering issue with private class methods ([#901](https://github.com/evanw/esbuild/issues/901))

internal/bundler/bundler_default_test.go

@@ -2752,6 +2752,26 @@ func TestMinifiedExportsAndModuleFormatCommonJS(t *testing.T) {
 	})
 }
 
+func TestEmptyExportClauseBundleAsCommonJSIssue910(t *testing.T) {
+	default_suite.expectBundled(t, bundled{
+		files: map[string]string{
+			"/entry.js": `
+				console.log(require('./types.mjs'))
+			`,
+			"/types.mjs": `
+				export {}
+			`,
+		},
+		entryPaths: []string{"/entry.js"},
+		options: config.Options{
+			Mode:          config.ModeBundle,
+			OutputFormat:  config.FormatCommonJS,
+			AbsOutputFile: "/out.js",
+			Platform:      config.PlatformNode,
+		},
+	})
+}
+
 // The minifier should not remove "use strict" or join it with other expressions
 func TestUseStrictDirectiveMinifyNoBundle(t *testing.T) {
 	default_suite.expectBundled(t, bundled{

internal/bundler/linker.go

@@ -1672,8 +1672,10 @@ func (c *linkerContext) createExportsForFile(sourceIndex uint32) {
 			nsExportNonLocalDependencies = append(nsExportNonLocalDependencies, dep)
 		}
 
-		// Pull in the "__markAsModule" symbol later
+		// Pull in the "__markAsModule" symbol later. Also make sure the "exports"
+		// variable is marked as used because we used it above.
 		repr.meta.needsMarkAsModuleSymbolFromRuntime = true
+		repr.ast.UsesExportsRef = true
 	}
 
 	// "__export(exports, { foo: () => foo })"

internal/bundler/snapshots/snapshots_default.txt

@@ -451,6 +451,17 @@ var import_foo = __toModule(require_foo());
 var import_bar = __toModule(require_bar());
 console.log(import_foo.foo(), import_bar.bar());
 
+================================================================================
+TestEmptyExportClauseBundleAsCommonJSIssue910
+---------- /out.js ----------
+// types.mjs
+var require_types = __commonJS((exports2) => {
+  __markAsModule(exports2);
+});
+
+// entry.js
+console.log(require_types());
+
 ================================================================================
 TestExportChain
 ---------- /out.js ----------