forbid typescript's "export =" in esm

CHANGELOG.md

@@ -71,6 +71,10 @@
 
     Getting this change out should hopefully make the linker rewrite go more smoothly.
 
+* You can no longer mix ESM exports and CommonJS exports in the same file
+
+    Previously doing this was supported and caused the file to become a CommonJS module with the ESM exports becoming CommonJS exports. However, now that ESM code is no longer allowed inside a CommonJS wrapper, doing this is no longer supported. When you try to do this the ESM exports will win and references to the CommonJS `exports` and `module` variables will be passed through unmodified just like any other global variable reference. In addition, the TypeScript-specific `export =` syntax is not allowed in an ESM file. This is an alias for `module.exports =` and is already forbidden by the TypeScript compiler in an ESM file.
+
 ## Unreleased
 
 * Fix overlapping chunk names when code splitting is active ([#928](https://github.com/evanw/esbuild/issues/928))

internal/js_parser/js_parser.go

@@ -8110,7 +8110,24 @@ func (p *parser) visitAndAppendStmt(stmts []js_ast.Stmt, stmt js_ast.Stmt) []js_
 		}
 
 	case *js_ast.SExportEquals:
-		// "module.exports = value"
+		if p.hasESModuleSyntax {
+			var msgData logger.MsgData
+			exportRange := js_lexer.RangeOfIdentifier(p.source, stmt.Loc)
+			text := "Export assignment cannot be used in an ECMAScript module"
+			if equalsRange := p.source.RangeOfOperatorAfter(logger.Loc{Start: exportRange.End()}, "="); equalsRange.Len > 0 {
+				msgData = logger.RangeData(&p.source, equalsRange, text+" (consider using \"export default\" instead)")
+				msgData.Location.Suggestion = "default"
+			} else {
+				msgData = logger.RangeData(&p.source, exportRange, text)
+			}
+			p.log.AddMsg(logger.Msg{
+				Kind:  logger.Error,
+				Data:  msgData,
+				Notes: p.whyESModuleNotes(),
+			})
+		}
+
+		// "export = value" => "module.exports = value"
 		stmts = append(stmts, js_ast.AssignStmt(
 			js_ast.Expr{Loc: stmt.Loc, Data: &js_ast.EDot{
 				Target:  js_ast.Expr{Loc: stmt.Loc, Data: &js_ast.EIdentifier{Ref: p.moduleRef}},

internal/js_parser/js_parser_lower.go

@@ -191,6 +191,25 @@ func (p *parser) markStrictModeFeature(feature strictModeFeature, r logger.Range
 	}
 }
 
+func (p *parser) whyESModuleNotes() []logger.MsgData {
+	var why string
+	var keywordRange logger.Range
+	if p.es6ImportKeyword.Len > 0 {
+		why = "This file is considered an ECMAScript module because of the \"import\" keyword"
+		keywordRange = p.es6ImportKeyword
+	} else if p.es6ExportKeyword.Len > 0 {
+		why = "This file is considered an ECMAScript module because of the \"export\" keyword"
+		keywordRange = p.es6ExportKeyword
+	} else if p.topLevelAwaitKeyword.Len > 0 {
+		why = "This file is considered an ECMAScript module because of the top-level \"await\" keyword"
+		keywordRange = p.topLevelAwaitKeyword
+	}
+	if why != "" {
+		return []logger.MsgData{logger.RangeData(&p.source, keywordRange, why)}
+	}
+	return nil
+}
+
 // Mark the feature if "loweredFeature" is unsupported. This is used when one
 // feature is implemented in terms of another feature.
 func (p *parser) markLoweredSyntaxFeature(feature compat.JSFeature, r logger.Range, loweredFeature compat.JSFeature) {

internal/js_parser/ts_parser_test.go

@@ -1321,10 +1321,27 @@ func TestTSImport(t *testing.T) {
 
 // This is TypeScript-specific export syntax
 func TestTSExportEquals(t *testing.T) {
+	// Check that "export =" is relocated to the bottom of the file
+	expectPrintedTS(t, "export = x; let x = 1", "let x = 1;\nmodule.exports = x;\n")
+
 	// This use of the "export" keyword should not trigger strict mode because
 	// this syntax works in CommonJS modules, not in ECMAScript modules
 	expectPrintedTS(t, "export = []", "module.exports = [];\n")
 	expectPrintedTS(t, "export = []; with ({}) ;", "with ({})\n  ;\nmodule.exports = [];\n")
+
+	// Disallow "export =" with ECMAScript modules
+	expectParseErrorTS(t, "export = []; import 'x'",
+		`<stdin>: error: Export assignment cannot be used in an ECMAScript module (consider using "export default" instead)
+<stdin>: note: This file is considered an ECMAScript module because of the "import" keyword
+`)
+	expectParseErrorTS(t, "export = []; export {}",
+		`<stdin>: error: Export assignment cannot be used in an ECMAScript module (consider using "export default" instead)
+<stdin>: note: This file is considered an ECMAScript module because of the "export" keyword
+`)
+	expectParseErrorTS(t, "export = []; await 0",
+		`<stdin>: error: Export assignment cannot be used in an ECMAScript module (consider using "export default" instead)
+<stdin>: note: This file is considered an ECMAScript module because of the top-level "await" keyword
+`)
 }
 
 // This is TypeScript-specific import syntax