feat(minifier): constant fold 'x'.toString() and true.toString()

crates/oxc_ecmascript/src/constant_evaluation/value_type.rs

@@ -44,6 +44,10 @@ impl ValueType {
     pub fn is_object(self) -> bool {
         self == Self::Object
     }
+
+    pub fn is_undetermined(self) -> bool {
+        self == Self::Undetermined
+    }
 }
 
 /// `get_known_value_type`

crates/oxc_minifier/src/ast_passes/peephole_fold_constants.rs

@@ -2,6 +2,7 @@ use oxc_ast::ast::*;
 use oxc_ecmascript::{
     constant_evaluation::{ConstantEvaluation, ConstantValue, ValueType},
     side_effects::MayHaveSideEffects,
+    ToJsString,
 };
 use oxc_span::{GetSpan, SPAN};
 use oxc_syntax::{
@@ -35,7 +36,8 @@ impl<'a> Traverse<'a> for PeepholeFoldConstants {
             Expression::StaticMemberExpression(e) => Self::try_fold_static_member_expr(e, ctx),
             Expression::LogicalExpression(e) => Self::try_fold_logical_expr(e, ctx),
             Expression::ChainExpression(e) => Self::try_fold_optional_chain(e, ctx),
-            Expression::CallExpression(e) => Self::try_fold_number_constructor(e, ctx),
+            Expression::CallExpression(e) => Self::try_fold_number_constructor(e, ctx)
+                .or_else(|| Self::try_fold_to_string(e, ctx)),
             _ => None,
         } {
             *expr = folded_expr;
@@ -601,6 +603,26 @@ impl<'a, 'b> PeepholeFoldConstants {
             }),
         ))
     }
+
+    fn try_fold_to_string(e: &CallExpression<'a>, ctx: Ctx<'a, 'b>) -> Option<Expression<'a>> {
+        let Expression::StaticMemberExpression(member_expr) = &e.callee else { return None };
+        if member_expr.property.name != "toString" {
+            return None;
+        }
+        if !e.arguments.is_empty() {
+            return None;
+        }
+        let object = &member_expr.object;
+        if !matches!(
+            ValueType::from(object),
+            ValueType::String | ValueType::Boolean | ValueType::Number
+        ) {
+            return None;
+        }
+        object
+            .to_js_string()
+            .map(|value| ctx.ast.expression_string_literal(object.span(), value, None))
+    }
 }
 
 /// <https://github.com/google/closure-compiler/blob/v20240609/test/com/google/javascript/jscomp/PeepholeFoldConstantsTest.java>
@@ -1733,4 +1755,12 @@ mod test {
         test("Number('1')", "1");
         test_same("var Number; Number(1)");
     }
+
+    #[test]
+    fn test_fold_to_string() {
+        test("'x'.toString()", "'x'");
+        test("1 .toString()", "'1'");
+        test("true.toString()", "'true'");
+        test("false.toString()", "'false'");
+    }
 }