diff --git a/tests/unit/extensions/test_expressjs.py b/tests/unit/extensions/test_expressjs.py
new file mode 100644
index 000000000..ba05dc0b7
--- /dev/null
+++ b/tests/unit/extensions/test_expressjs.py
@@ -0,0 +1,153 @@
+# -*- Mode:Python; indent-tabs-mode:nil; tab-width:4 -*-
+#
+# Copyright 2024 Canonical Ltd.
+#
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License version 3 as
+# published by the Free Software Foundation.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+import pytest
+
+from rockcraft import extensions
+from rockcraft.errors import ExtensionError
+
+
+@pytest.fixture(name="expressjs_input_yaml")
+def expressjs_input_yaml_fixture():
+    return {
+        "name": "foo-bar",
+        "base": "ubuntu@24.04",
+        "platforms": {"amd64": {}},
+        "extensions": ["expressjs-framework"],
+    }
+
+
+@pytest.fixture
+def expressjs_extension(mock_extensions, monkeypatch):
+    monkeypatch.setenv("ROCKCRAFT_ENABLE_EXPERIMENTAL_EXTENSIONS", "1")
+    extensions.register("expressjs-framework", extensions.ExpressJSFramework)
+
+
+@pytest.fixture
+def expressjs_project_name():
+    return "test-expressjs-project"
+
+
+@pytest.fixture
+def package_json_file(tmp_path, expressjs_project_name):
+    (tmp_path / "package.json").write_text(
+        f"""{{
+    "name": "{expressjs_project_name}",
+    "scripts": {{
+        "start": "node ./bin/www"
+    }}
+}}"""
+    )
+
+
+@pytest.mark.usefixtures("expressjs_extension", "package_json_file")
+def test_expressjs_extension_default(
+    tmp_path, expressjs_project_name, expressjs_input_yaml
+):
+    applied = extensions.apply_extensions(tmp_path, expressjs_input_yaml)
+
+    assert applied == {
+        "base": "ubuntu@24.04",
+        "name": "foo-bar",
+        "platforms": {
+            "amd64": {},
+        },
+        "run-user": "_daemon_",
+        "parts": {
+            "expressjs-framework/install-app": {
+                "plugin": "npm",
+                "npm-include-node": False,
+                "source": "app/",
+                "organise": {
+                    f"lib/node_modules/{expressjs_project_name}/package.json": "app/package.json",
+                },
+                "override-prime": f"rm -rf lib/node_modules/{expressjs_project_name}",
+            },
+            "expressjs-framework/runtime-dependencies": {
+                "plugin": "nil",
+                "stage-packages": [
+                    "ca-certificates_data",
+                    "libpq5",
+                    "node",
+                ],
+            },
+        },
+        "services": {
+            "app": {
+                "command": "npm start",
+                "on-failure": "shutdown",
+                "on-success": "shutdown",
+                "override": "replace",
+                "startup": "enabled",
+                "working-dir": "/app",
+            },
+        },
+    }
+
+
+@pytest.mark.usefixtures("expressjs_extension")
+def test_expressjs_no_package_json_error(tmp_path, expressjs_input_yaml):
+    with pytest.raises(ExtensionError) as exc:
+        extensions.apply_extensions(tmp_path, expressjs_input_yaml)
+    assert str(exc.value) == "missing package.json file"
+    assert str(exc.value.doc_slug) == "/reference/extensions/expressjs-framework"
+
+
+@pytest.mark.parametrize(
+    "package_json_contents, error_message",
+    [
+        ("{}", "missing start script"),
+        ('{"scripts":{}}', "missing start script"),
+        ('{"scripts":{"start":"node ./bin/www"}}', "missing application name"),
+    ],
+)
+@pytest.mark.usefixtures("expressjs_extension")
+def test_expressjs_invalid_package_json_scripts_error(
+    tmp_path, expressjs_input_yaml, package_json_contents, error_message
+):
+    (tmp_path / "package.json").write_text(package_json_contents)
+    with pytest.raises(ExtensionError) as exc:
+        extensions.apply_extensions(tmp_path, expressjs_input_yaml)
+    assert str(exc.value) == error_message
+    assert str(exc.value.doc_slug) == "/reference/extensions/expressjs-framework"
+
+
+@pytest.mark.parametrize(
+    "existing_files, missing_files, expected_organise",
+    [
+        pytest.param(
+            ["lib/node_modules/test-expressjs-project/app.js"],
+            [],
+            {"lib/node_modules/test-expressjs-project/app.js": "app/app.js"},
+            id="single file defined",
+        ),
+    ],
+)
+@pytest.mark.usefixtures("expressjs_extension", "package_json_file")
+def test_expressjs_install_app_prime_to_organise_map(
+    tmp_path, expressjs_input_yaml, existing_files, missing_files, expected_organise
+):
+    for file in existing_files:
+        (tmp_path / file).parent.mkdir(parents=True)
+        (tmp_path / file).touch()
+    prime_files = [*existing_files, *missing_files]
+    expressjs_input_yaml["parts"] = {
+        "expressjs-framework/install-app": {"prime": prime_files}
+    }
+    applied = extensions.apply_extensions(tmp_path, expressjs_input_yaml)
+    assert (
+        applied["parts"]["expressjs-framework/install-app"]["organise"]
+        == expected_organise
+    )