fix #4041: keep void x when minify is off

Author: evanw

internal/js_parser/js_parser.go

@@ -14033,7 +14033,25 @@ func (p *parser) visitExprInOut(expr js_ast.Expr, in exprIn) (js_ast.Expr, exprO
 				}
 
 			case js_ast.UnOpVoid:
-				if p.astHelpers.ExprCanBeRemovedIfUnused(e.Value) {
+				var shouldRemove bool
+				if p.options.minifySyntax {
+					shouldRemove = p.astHelpers.ExprCanBeRemovedIfUnused(e.Value)
+				} else {
+					// This special case was added for a very obscure reason. There's a
+					// custom dialect of JavaScript called Svelte that uses JavaScript
+					// syntax with different semantics. Specifically variable accesses
+					// have side effects (!). And someone wants to use "void x" instead
+					// of just "x" to trigger the side effect for some reason.
+					//
+					// Arguably this should not be supported, because you shouldn't be
+					// running esbuild on weird kinda-JavaScript-but-not languages and
+					// expecting it to work correctly. But this one special case seems
+					// harmless enough. This is definitely not fully supported though.
+					//
+					// More info: https://github.com/evanw/esbuild/issues/4041
+					shouldRemove = isUnsightlyPrimitive(e.Value.Data)
+				}
+				if shouldRemove {
 					return js_ast.Expr{Loc: expr.Loc, Data: js_ast.EUndefinedShared}, exprOut{}
 				}
 

internal/js_parser/js_parser_test.go

@@ -230,6 +230,13 @@ func expectPrintedJSXAutomatic(t *testing.T, options JSXAutomaticTestOptions, co
 	})
 }
 
+func TestUnOp(t *testing.T) {
+	// This was important to someone for a very obscure reason. See
+	// https://github.com/evanw/esbuild/issues/4041 for more info.
+	expectPrinted(t, "let x; void 0; x", "let x;\nx;\n")
+	expectPrinted(t, "let x; void x; x", "let x;\nvoid x;\nx;\n")
+}
+
 func TestBinOp(t *testing.T) {
 	for code, entry := range js_ast.OpTable {
 		opCode := js_ast.OpCode(code)