fix #208: pass "import.meta" through for esm

CHANGELOG.md

@@ -1,5 +1,15 @@
 # Changelog
 
+## Unreleased
+
+* Pass through `import.meta` syntax ([#208](https://github.com/evanw/esbuild/issues/208))
+
+    The `import.meta` syntax is a way for code in an ES6 module to access metadata about itself. For example, `import.meta.url` in the browser is the URL of the current module.
+
+    It's a new feature that doesn't work in older browsers, so esbuild converts it to a module-local variable to avoid generating code with a syntax error. However, this is only necessary when targeting older browsers or if the output format doesn't support `import.meta`.
+
+    The `import.meta` syntax is now passed through unmodified when the target is `es2020` or newer and the output format is `esm`. This lets you use features such as `import.meta.url` in those situations.
+
 ## 0.5.16
 
 * Experimental code splitting with `--splitting` ([#16](https://github.com/evanw/esbuild/issues/16))

README.md

@@ -118,6 +118,7 @@ These syntax features are conditionally transformed for older browsers depending
 | [Optional catch binding](https://github.com/tc39/proposal-optional-catch-binding)   | `es2019`                             | `try {} catch {}`          |
 | [Optional chaining](https://github.com/tc39/proposal-optional-chaining)             | `es2020`                             | `a?.b`                     |
 | [Nullish coalescing](https://github.com/tc39/proposal-nullish-coalescing)           | `es2020`                             | `a ?? b`                   |
+| [`import.meta`](https://github.com/tc39/proposal-import-meta)                       | `es2020`                             | `import.meta`              |
 | [Class instance fields](https://github.com/tc39/proposal-class-fields)              | `esnext`                             | `class { x }`              |
 | [Static class fields](https://github.com/tc39/proposal-static-class-features)       | `esnext`                             | `class { static x }`       |
 | [Private instance methods](https://github.com/tc39/proposal-private-methods)        | `esnext`                             | `class { #x() {} }`        |

internal/bundler/bundler_test.go

@@ -4986,7 +4986,7 @@ export {default as bar} from "./bar";
 	})
 }
 
-func TestImportMeta(t *testing.T) {
+func TestImportMetaCommonJS(t *testing.T) {
 	expectBundled(t, bundled{
 		files: map[string]string{
 			"/entry.js": `
@@ -4996,6 +4996,7 @@ func TestImportMeta(t *testing.T) {
 		entryPaths: []string{"/entry.js"},
 		options: config.Options{
 			IsBundling:    true,
+			OutputFormat:  config.FormatCommonJS,
 			AbsOutputFile: "/out.js",
 		},
 		expected: map[string]string{
@@ -5007,6 +5008,27 @@ console.log(import_meta.url, import_meta.path);
 	})
 }
 
+func TestImportMetaES6(t *testing.T) {
+	expectBundled(t, bundled{
+		files: map[string]string{
+			"/entry.js": `
+				console.log(import.meta.url, import.meta.path)
+			`,
+		},
+		entryPaths: []string{"/entry.js"},
+		options: config.Options{
+			IsBundling:    true,
+			OutputFormat:  config.FormatESModule,
+			AbsOutputFile: "/out.js",
+		},
+		expected: map[string]string{
+			"/out.js": `// /entry.js
+console.log(import.meta.url, import.meta.path);
+`,
+		},
+	})
+}
+
 func TestImportMetaNoBundle(t *testing.T) {
 	expectBundled(t, bundled{
 		files: map[string]string{

internal/parser/parser.go

@@ -2816,8 +2816,14 @@ func (p *parser) parseImportExpr(loc ast.Loc) ast.Expr {
 	if p.lexer.Token == lexer.TDot {
 		p.lexer.Next()
 		if p.lexer.IsContextualKeyword("meta") {
+			r := p.lexer.Range()
 			p.lexer.Next()
 			p.hasImportMeta = true
+			if p.Target < importMetaTarget {
+				r = ast.Range{Loc: loc, Len: r.End() - loc.Start}
+				p.log.AddRangeWarning(&p.source, r, fmt.Sprintf(
+					"\"import.meta\" is from ES2020 and will be empty when targeting %s", targetTable[p.Target]))
+			}
 			return ast.Expr{Loc: loc, Data: &ast.EImportMeta{}}
 		} else {
 			p.lexer.ExpectedString("\"meta\"")
@@ -8852,7 +8858,7 @@ func (p *parser) bindingCanBeRemovedIfUnused(binding ast.Binding) bool {
 func (p *parser) exprCanBeRemovedIfUnused(expr ast.Expr) bool {
 	switch e := expr.Data.(type) {
 	case *ast.ENull, *ast.EUndefined, *ast.EBoolean, *ast.ENumber, *ast.EBigInt,
-		*ast.EString, *ast.EThis, *ast.ERegExp, *ast.EFunction, *ast.EArrow:
+		*ast.EString, *ast.EThis, *ast.ERegExp, *ast.EFunction, *ast.EArrow, *ast.EImportMeta:
 		return true
 
 	case *ast.EDot:
@@ -9169,7 +9175,8 @@ func (p *parser) prepareForVisitPass(options *config.Options) {
 		p.moduleRef = p.newSymbol(ast.SymbolHoisted, "module")
 	}
 
-	if options.IsBundling && p.hasImportMeta {
+	// Convert "import.meta" to a variable if it's not supported in the output format
+	if p.hasImportMeta && (p.Target < importMetaTarget || (options.IsBundling && !p.OutputFormat.KeepES6ImportExportSyntax())) {
 		p.importMetaRef = p.newSymbol(ast.SymbolOther, "import_meta")
 		p.moduleScope.Generated = append(p.moduleScope.Generated, p.importMetaRef)
 	} else {

internal/parser/parser_lower.go

@@ -16,6 +16,7 @@ const asyncAwaitTarget = config.ES2017
 const objectPropertyBindingTarget = config.ES2018
 const nullishCoalescingTarget = config.ES2020
 const privateNameTarget = config.ESNext
+const importMetaTarget = config.ES2020
 
 type futureSyntax uint8