diff --git a/.prettierignore b/.prettierignore
index 524ec8d7e91..9f5a1e6cb82 100644
--- a/.prettierignore
+++ b/.prettierignore
@@ -7,4 +7,4 @@ temp
 rollup
 **/.git
 **/node_modules
-
+scripts
diff --git a/crates/rolldown/tests/esbuild/import_star/import_namespace_undefined_property_side_effect_free_file/artifacts.snap b/crates/rolldown/tests/esbuild/import_star/import_namespace_undefined_property_side_effect_free_file/artifacts.snap
index 57f11410e0f..20103a1b318 100644
--- a/crates/rolldown/tests/esbuild/import_star/import_namespace_undefined_property_side_effect_free_file/artifacts.snap
+++ b/crates/rolldown/tests/esbuild/import_star/import_namespace_undefined_property_side_effect_free_file/artifacts.snap
@@ -7,50 +7,50 @@ input_file: crates/rolldown/tests/esbuild/import_star/import_namespace_undefined
 
 ```js
 // foo/no-side-effects.cjs
-var require_no-side-effects = __commonJS({
+var require_no_side_effects = __commonJS({
 'foo/no-side-effects.cjs'(exports, module) {
 console.log('cjs')
 }
 });
 // foo/no-side-effects.mjs
-var no-side-effects_ns$1 = {
+var no_side_effects_ns$1 = {
 
 };
 console.log('mjs')
 // foo/no-side-effects.js
-var no-side-effects_ns = {
+var no_side_effects_ns = {
 
 };
 console.log('js')
-export { no-side-effects_ns as no-side-effects_ns$2, no-side-effects_ns$1 as no-side-effects_ns, no-side-effects_ns$2 as no-side-effects_ns$1 };
+export { no_side_effects_ns as no_side_effects_ns$2, no_side_effects_ns$1 as no_side_effects_ns, no_side_effects_ns$2 as no_side_effects_ns$1 };
 ```
 # entry-default_js.js
 
 ```js
-import { no-side-effects_ns as no-side-effects_ns$2, no-side-effects_ns$1 as no-side-effects_ns, no-side-effects_ns$2 as no-side-effects_ns$1 } from "./2.js";
+import { no_side_effects_ns as no_side_effects_ns$2, no_side_effects_ns$1 as no_side_effects_ns, no_side_effects_ns$2 as no_side_effects_ns$1 } from "./2.js";
 // entry-default.js
-var no-side-effects_ns$2 = __toESM(require_no-side-effects());
+var no_side_effects_ns$2 = __toESM(require_no_side_effects());
 
 
 
 console.log(
-	no-side-effects_ns.default,
-	no-side-effects_ns$1.default,
-	no-side-effects_ns$2.default,
+	no_side_effects_ns.default,
+	no_side_effects_ns$1.default,
+	no_side_effects_ns$2.default,
 )
 ```
 # entry-nope_js.js
 
 ```js
-import { no-side-effects_ns as no-side-effects_ns$2, no-side-effects_ns$1 as no-side-effects_ns, no-side-effects_ns$2 as no-side-effects_ns$1 } from "./2.js";
+import { no_side_effects_ns as no_side_effects_ns$2, no_side_effects_ns$1 as no_side_effects_ns, no_side_effects_ns$2 as no_side_effects_ns$1 } from "./2.js";
 // entry-nope.js
-var no-side-effects_ns$2 = __toESM(require_no-side-effects());
+var no_side_effects_ns$2 = __toESM(require_no_side_effects());
 
 
 
 console.log(
-	no-side-effects_ns.nope,
-	no-side-effects_ns$1.nope,
-	no-side-effects_ns$2.nope,
+	no_side_effects_ns.nope,
+	no_side_effects_ns$1.nope,
+	no_side_effects_ns$2.nope,
 )
 ```
diff --git a/crates/rolldown_common/src/module_path.rs b/crates/rolldown_common/src/module_path.rs
index 331ab458c3d..d33714c854e 100644
--- a/crates/rolldown_common/src/module_path.rs
+++ b/crates/rolldown_common/src/module_path.rs
@@ -62,8 +62,7 @@ impl ResourceId {
         return [unique_name_of_parent_dir, "_index"].concat();
       }
     }
-    // TODO: ensure valid identifier
-    return unique_name.to_string();
+    return ensure_valid_identifier(unique_name);
   }
 }
 
@@ -72,3 +71,23 @@ impl AsRef<str> for ResourceId {
     self.0.path.as_str()
   }
 }
+
+fn ensure_valid_identifier(s: &str) -> String {
+  let mut ident = String::new();
+  let mut need_gap = false;
+  for i in s.chars() {
+    if i.is_ascii_alphabetic() || (i.is_ascii_digit() && !ident.is_empty()) {
+      if need_gap {
+        ident.push('_');
+        need_gap = false;
+      }
+      ident.push(i);
+    } else if !ident.is_empty() {
+      need_gap = true;
+    }
+  }
+  if ident.is_empty() {
+    ident.push('_');
+  }
+  ident
+}