fix: avoid npm reinstall for empty devDependencies

news/7566.bugfix.md

@@ -0,0 +1 @@
+Avoid rerunning npm installs during source-only reloads when npm omits an empty `devDependencies` field from `package.json`.

reflex/utils/frontend_skeleton.py

@@ -19,6 +19,8 @@
 
 logger = logging.getLogger(__name__)
 
+_DEV_DEPENDENCIES_KEY = "devDependencies"
+
 
 def initialize_gitignore(
     gitignore_file: Path = constants.GitIgnore.FILE,
@@ -368,7 +370,12 @@ def sync_root_package_json_to_web() -> bool:
 
     output_path = get_web_lockfile_path(constants.PackageJson.PATH)
     rendered = _compile_package_json()
-    if _read_package_json_object(output_path) == json.loads(rendered):
+    existing_package_json = _read_package_json_object(output_path)
+    rendered_package_json = json.loads(rendered)
+    for package_json in (existing_package_json, rendered_package_json):
+        if package_json.get(_DEV_DEPENDENCIES_KEY) == {}:
+            package_json.pop(_DEV_DEPENDENCIES_KEY)
+    if existing_package_json == rendered_package_json:
         return False
 
     changed = output_path.exists()

tests/units/utils/test_frontend_skeleton.py

@@ -67,6 +67,25 @@ def test_sync_root_package_json_ignores_formatting(
         assert package_json_path.stat().st_mtime_ns == original_mtime
 
 
+def test_sync_root_package_json_ignores_empty_dev_dependencies(
+    package_json_path: Path,
+):
+    """An absent and empty devDependencies field are equivalent for npm.
+
+    Args:
+        package_json_path: The rendered package.json path.
+    """
+    package_json = json.loads(package_json_path.read_text())
+    package_json.pop("devDependencies", None)
+    package_json_path.write_text(json.dumps(package_json, indent=2))
+    original_content = package_json_path.read_text()
+    original_mtime = package_json_path.stat().st_mtime_ns
+
+    assert frontend_skeleton.sync_root_package_json_to_web() is False
+    assert package_json_path.read_text() == original_content
+    assert package_json_path.stat().st_mtime_ns == original_mtime
+
+
 @pytest.mark.parametrize(
     ("field", "value"),
     [

tests/units/utils/test_js_runtimes.py

@@ -1,11 +1,13 @@
 """Tests for reflex.utils.js_runtimes install gating."""
 
+import json
 from pathlib import Path
 from unittest import mock
 
 import pytest
+from reflex_base import constants
 
-from reflex.utils import js_runtimes
+from reflex.utils import frontend_skeleton, js_runtimes
 
 BUN_PATH = Path("/usr/bin/bun")
 NPM_PATH = Path("/usr/bin/npm")
@@ -89,6 +91,44 @@ def _patch_unsupported_node(monkeypatch) -> None:
     monkeypatch.setattr(js_runtimes, "get_node_version", lambda: None)
 
 
+def test_npm_empty_dev_dependencies_preserves_frontend_install_cache(
+    tmp_path: Path, monkeypatch: pytest.MonkeyPatch
+):
+    """An npm manifest omitting empty devDependencies must not invalidate installs.
+
+    Args:
+        tmp_path: The temporary app directory.
+        monkeypatch: The pytest monkeypatch fixture.
+    """
+    monkeypatch.chdir(tmp_path)
+    web_dir = tmp_path / constants.Dirs.WEB
+    web_dir.mkdir()
+    monkeypatch.setattr(frontend_skeleton, "get_web_dir", lambda: web_dir)
+    monkeypatch.setattr(js_runtimes, "get_web_dir", lambda: web_dir)
+
+    root_package_json = frontend_skeleton.get_root_lockfile_path(
+        constants.PackageJson.PATH
+    )
+    root_package_json.parent.mkdir()
+    root_package_json.write_text(json.dumps({"dependencies": {"test-pkg": "1.0.0"}}))
+
+    npm_package_json = json.loads(frontend_skeleton._compile_package_json())
+    npm_package_json.pop("devDependencies", None)
+    web_package_json = frontend_skeleton.get_web_lockfile_path(
+        constants.PackageJson.PATH
+    )
+    web_package_json.write_text(json.dumps(npm_package_json, indent=2))
+
+    cache_file = js_runtimes._frontend_packages_cache_path()
+    cache_file.touch()
+    original_content = web_package_json.read_text()
+
+    js_runtimes._sync_root_lockfiles_for_frontend_install()
+
+    assert cache_file.exists()
+    assert web_package_json.read_text() == original_content
+
+
 def test_install_never_starts_with_unsupported_node(monkeypatch, install_mocks, caplog):
     """A failed node-version preflight must leave no lockfile side effects.