fix mangle syntax edge case with "==" and "!="

internal/bundler/bundler_dce_test.go

@@ -1687,3 +1687,39 @@ func TestDCETypeOf(t *testing.T) {
 		},
 	})
 }
+
+func TestDCETypeOfEqualsString(t *testing.T) {
+	dce_suite.expectBundled(t, bundled{
+		files: map[string]string{
+			"/entry.js": `
+				var hasBar = typeof bar !== 'undefined'
+				if (false) console.log(hasBar)
+			`,
+		},
+		entryPaths: []string{"/entry.js"},
+		options: config.Options{
+			Mode:          config.ModeBundle,
+			OutputFormat:  config.FormatIIFE,
+			AbsOutputFile: "/out.js",
+		},
+	})
+}
+
+func TestDCETypeOfEqualsStringMangle(t *testing.T) {
+	dce_suite.expectBundled(t, bundled{
+		files: map[string]string{
+			"/entry.js": `
+				// Everything here should be removed as dead code due to tree shaking
+				var hasBar = typeof bar !== 'undefined'
+				if (false) console.log(hasBar)
+			`,
+		},
+		entryPaths: []string{"/entry.js"},
+		options: config.Options{
+			Mode:          config.ModeBundle,
+			OutputFormat:  config.FormatIIFE,
+			MangleSyntax:  true,
+			AbsOutputFile: "/out.js",
+		},
+	})
+}

internal/bundler/snapshots/snapshots_dce.txt

@@ -7,6 +7,21 @@ console.log("unused import");
 TestDCETypeOf
 ---------- /out.js ----------
 
+================================================================================
+TestDCETypeOfEqualsString
+---------- /out.js ----------
+(() => {
+  // entry.js
+  if (false)
+    console.log(hasBar);
+})();
+
+================================================================================
+TestDCETypeOfEqualsStringMangle
+---------- /out.js ----------
+(() => {
+})();
+
 ================================================================================
 TestDataURLLoaderRemoveUnused
 ---------- /out.js ----------

internal/bundler/snapshots/snapshots_default.txt

@@ -3371,15 +3371,6 @@ var import_return_asi = __toModule(require_return_asi());
 var import_return_asi2 = __toModule(require_return_asi2());
 var import_return_asi3 = __toModule(require_return_asi3());
 
-// bad-typeof.js
-typeof x == "null";
-
-// node_modules/bad-typeof.js
-typeof x == "null";
-
-// plugin-dir/node_modules/bad-typeof.js
-typeof x == "null";
-
 // equals-neg-zero.js
 x === -0;
 

internal/js_parser/js_parser.go

@@ -13417,6 +13417,15 @@ func (p *parser) exprCanBeRemovedIfUnused(expr js_ast.Expr) bool {
 		case js_ast.BinOpStrictEq, js_ast.BinOpStrictNe, js_ast.BinOpComma,
 			js_ast.BinOpLogicalOr, js_ast.BinOpLogicalAnd, js_ast.BinOpNullishCoalescing:
 			return p.exprCanBeRemovedIfUnused(e.Left) && p.exprCanBeRemovedIfUnused(e.Right)
+
+		// For "==" and "!=", pretend the operator was actually "===" or "!==". If
+		// we know that we can convert it to "==" or "!=", then we can consider the
+		// operator itself to have no side effects. This matters because our mangle
+		// logic will convert "typeof x === 'object'" into "typeof x == 'object'"
+		// and since "typeof x === 'object'" is considered to be side-effect free,
+		// we must also consider "typeof x == 'object'" to be side-effect free.
+		case js_ast.BinOpLooseEq, js_ast.BinOpLooseNe:
+			return canChangeStrictToLoose(e.Left, e.Right) && p.exprCanBeRemovedIfUnused(e.Left) && p.exprCanBeRemovedIfUnused(e.Right)
 		}
 	}