Default to language_version: system if node and npm are installed

tests/languages/node_test.py

@@ -0,0 +1,47 @@
+import sys
+from unittest import mock
+
+import pytest
+
+import pre_commit.constants as C
+from pre_commit import parse_shebang
+from pre_commit.languages.node import get_default_version
+
+
+ACTUAL_GET_DEFAULT_VERSION = get_default_version.__wrapped__
+
+
+@pytest.fixture
+def is_linux():
+    with mock.patch.object(sys, 'platform', 'linux'):
+        yield
+
+
+@pytest.fixture
+def is_win32():
+    with mock.patch.object(sys, 'platform', 'win32'):
+        yield
+
+
+@pytest.fixture
+def find_exe_mck():
+    with mock.patch.object(parse_shebang, 'find_executable') as mck:
+        yield mck
+
+
+@pytest.mark.usefixtures('is_linux')
+def test_sets_system_when_node_and_npm_are_available(find_exe_mck):
+    find_exe_mck.return_value = '/path/to/exe'
+    assert ACTUAL_GET_DEFAULT_VERSION() == 'system'
+
+
+@pytest.mark.usefixtures('is_linux')
+def test_uses_default_when_node_and_npm_are_not_available(find_exe_mck):
+    find_exe_mck.return_value = None
+    assert ACTUAL_GET_DEFAULT_VERSION() == C.DEFAULT
+
+
+@pytest.mark.usefixtures('is_win32')
+def test_sets_default_on_windows(find_exe_mck):
+    find_exe_mck.return_value = '/path/to/exe'
+    assert ACTUAL_GET_DEFAULT_VERSION() == C.DEFAULT