fix #4003: minify empty try/catch blocks

CHANGELOG.md

@@ -21,6 +21,23 @@
 
     The above code will be considered valid starting with this release. This change to esbuild follows a [similar change to TypeScript](https://github.com/microsoft/TypeScript/pull/60225) which will allow this syntax starting with TypeScript 5.7.
 
+* Minify empty `try`/`catch`/`finally` blocks ([#4003](https://github.com/evanw/esbuild/issues/4003))
+
+    With this release, esbuild will now attempt to minify empty `try` blocks:
+
+    ```js
+    // Original code
+    try {} catch { foo() } finally { bar() }
+
+    // Old output (with --minify)
+    try{}catch{foo()}finally{bar()}
+
+    // New output (with --minify)
+    bar();
+    ```
+
+    This can sometimes expose additional minification opportunities.
+
 ## 0.24.0
 
 **_This release deliberately contains backwards-incompatible changes._** To avoid automatically picking up releases like this, you should either be pinning the exact version of `esbuild` in your `package.json` file (recommended) or be using a version range syntax that only accepts patch upgrades such as `^0.23.0` or `~0.23.0`. See npm's documentation about [semver](https://docs.npmjs.com/cli/v6/using-npm/semver/) for more information.

internal/bundler_tests/bundler_dce_test.go

@@ -1405,6 +1405,31 @@ func TestDeadCodeFollowingJump(t *testing.T) {
 	})
 }
 
+func TestDeadCodeInsideEmptyTry(t *testing.T) {
+	dce_suite.expectBundled(t, bundled{
+		files: map[string]string{
+			"/entry.js": `
+				try { foo() }
+				catch { require('./a') }
+				finally { require('./b') }
+
+				try {}
+				catch { require('./c') }
+				finally { require('./d') }
+			`,
+			"/a.js": ``,
+			"/b.js": ``,
+			"/c.js": `TEST FAILED`, // Dead code paths should not import code
+			"/d.js": ``,
+		},
+		entryPaths: []string{"/entry.js"},
+		options: config.Options{
+			Mode:          config.ModeBundle,
+			AbsOutputFile: "/out.js",
+		},
+	})
+}
+
 func TestRemoveTrailingReturn(t *testing.T) {
 	dce_suite.expectBundled(t, bundled{
 		files: map[string]string{

internal/bundler_tests/snapshots/snapshots_dce.txt

@@ -458,10 +458,7 @@ var A_keep = class {
   }
 }, D_keep = class {
   static {
-    try {
-    } finally {
-      foo;
-    }
+    foo;
   }
 };
 
@@ -892,6 +889,41 @@ testBreak();
 testContinue();
 testStmts();
 
+================================================================================
+TestDeadCodeInsideEmptyTry
+---------- /out.js ----------
+// a.js
+var require_a = __commonJS({
+  "a.js"() {
+  }
+});
+
+// b.js
+var require_b = __commonJS({
+  "b.js"() {
+  }
+});
+
+// d.js
+var require_d = __commonJS({
+  "d.js"() {
+  }
+});
+
+// entry.js
+try {
+  foo();
+} catch {
+  require_a();
+} finally {
+  require_b();
+}
+try {
+} catch {
+} finally {
+  require_d();
+}
+
 ================================================================================
 TestDisableTreeShaking
 ---------- /out.js ----------

internal/js_parser/js_parser.go

@@ -10774,6 +10774,13 @@ func (p *parser) visitAndAppendStmt(stmts []js_ast.Stmt, stmt js_ast.Stmt) []js_
 		p.popScope()
 
 		if s.Catch != nil {
+			old := p.isControlFlowDead
+
+			// If the try body is empty, then the catch body is dead
+			if len(s.Block.Stmts) == 0 {
+				p.isControlFlowDead = true
+			}
+
 			p.pushScopeForVisitPass(js_ast.ScopeCatchBinding, s.Catch.Loc)
 			if s.Catch.BindingOrNil.Data != nil {
 				p.visitBinding(s.Catch.BindingOrNil, bindingOpts{})
@@ -10785,6 +10792,8 @@ func (p *parser) visitAndAppendStmt(stmts []js_ast.Stmt, stmt js_ast.Stmt) []js_
 
 			p.lowerObjectRestInCatchBinding(s.Catch)
 			p.popScope()
+
+			p.isControlFlowDead = old
 		}
 
 		if s.Finally != nil {
@@ -10793,6 +10802,44 @@ func (p *parser) visitAndAppendStmt(stmts []js_ast.Stmt, stmt js_ast.Stmt) []js_
 			p.popScope()
 		}
 
+		// Drop the whole thing if the try body is empty
+		if p.options.minifySyntax && len(s.Block.Stmts) == 0 {
+			keepCatch := false
+
+			// Certain "catch" blocks need to be preserved:
+			//
+			//   try {} catch { let foo } // Can be removed
+			//   try {} catch { var foo } // Must be kept
+			//
+			if s.Catch != nil {
+				for _, stmt2 := range s.Catch.Block.Stmts {
+					if shouldKeepStmtInDeadControlFlow(stmt2) {
+						keepCatch = true
+						break
+					}
+				}
+			}
+
+			// Make sure to preserve the "finally" block if present
+			if !keepCatch {
+				if s.Finally == nil {
+					return stmts
+				}
+				finallyNeedsBlock := false
+				for _, stmt2 := range s.Finally.Block.Stmts {
+					if statementCaresAboutScope(stmt2) {
+						finallyNeedsBlock = true
+						break
+					}
+				}
+				if !finallyNeedsBlock {
+					return append(stmts, s.Finally.Block.Stmts...)
+				}
+				block := s.Finally.Block
+				stmt = js_ast.Stmt{Loc: s.Finally.Loc, Data: &block}
+			}
+		}
+
 	case *js_ast.SSwitch:
 		s.Test = p.visitExpr(s.Test)
 		p.pushScopeForVisitPass(js_ast.ScopeBlock, s.BodyLoc)

internal/js_parser/js_parser_test.go

@@ -6548,6 +6548,23 @@ func TestMangleCatch(t *testing.T) {
 	expectPrintedMangle(t, "if (y) try { throw 1 } catch (x) {} else eval('x')", "if (y) try {\n  throw 1;\n} catch {\n}\nelse eval(\"x\");\n")
 }
 
+func TestMangleEmptyTry(t *testing.T) {
+	expectPrintedMangle(t, "try { throw 0 } catch (e) { foo() }", "try {\n  throw 0;\n} catch {\n  foo();\n}\n")
+	expectPrintedMangle(t, "try {} catch (e) { var foo }", "try {\n} catch {\n  var foo;\n}\n")
+
+	expectPrintedMangle(t, "try {} catch (e) { foo() }", "")
+	expectPrintedMangle(t, "try {} catch (e) { foo() } finally {}", "")
+
+	expectPrintedMangle(t, "try {} finally { foo() }", "foo();\n")
+	expectPrintedMangle(t, "try {} catch (e) { foo() } finally { bar() }", "bar();\n")
+
+	expectPrintedMangle(t, "try {} finally { var x = foo() }", "var x = foo();\n")
+	expectPrintedMangle(t, "try {} catch (e) { foo() } finally { var x = bar() }", "var x = bar();\n")
+
+	expectPrintedMangle(t, "try {} finally { let x = foo() }", "{\n  let x = foo();\n}\n")
+	expectPrintedMangle(t, "try {} catch (e) { foo() } finally { let x = bar() }", "{\n  let x = bar();\n}\n")
+}
+
 func TestAutoPureForObjectCreate(t *testing.T) {
 	expectPrinted(t, "Object.create(null)", "/* @__PURE__ */ Object.create(null);\n")
 	expectPrinted(t, "Object.create({})", "/* @__PURE__ */ Object.create({});\n")