diff --git a/swh/loader/package/npm/loader.py b/swh/loader/package/npm/loader.py
--- a/swh/loader/package/npm/loader.py
+++ b/swh/loader/package/npm/loader.py
@@ -1,4 +1,4 @@
-# Copyright (C) 2019  The Software Heritage developers
+# Copyright (C) 2019-2020  The Software Heritage developers
 # See the AUTHORS file at the top-level directory of this distribution
 # License: GNU General Public License version 3, or any later version
 # See top-level LICENSE file for more information
@@ -8,7 +8,7 @@
 import os
 
 from codecs import BOM_UTF8
-from typing import Any, Dict, Generator, Mapping, Sequence, Tuple, Optional
+from typing import Any, Dict, Generator, List, Mapping, Optional, Sequence, Tuple, Union
 
 import attr
 import chardet
@@ -181,7 +181,27 @@
     return None
 
 
-def extract_npm_package_author(package_json) -> Person:
+def _author_str(author_data: Union[Dict, List, str]) -> str:
+    """Parse author from package.json author fields
+
+    """
+    if isinstance(author_data, dict):
+        author_str = ""
+        name = author_data.get("name")
+        if name is not None:
+            author_str += name
+        email = author_data.get("email")
+        if email is not None:
+            author_str += f" <{email}>"
+        result = author_str
+    elif isinstance(author_data, list):
+        result = _author_str(author_data[0]) if len(author_data) > 0 else ""
+    else:
+        result = author_data
+    return result
+
+
+def extract_npm_package_author(package_json: Dict[str, Any]) -> Person:
     """
     Extract package author from a ``package.json`` file content and
     return it in swh format.
@@ -194,20 +214,6 @@
         Person
 
     """
-
-    def _author_str(author_data):
-        if type(author_data) is dict:
-            author_str = ""
-            if "name" in author_data:
-                author_str += author_data["name"]
-            if "email" in author_data:
-                author_str += " <%s>" % author_data["email"]
-            return author_str
-        elif type(author_data) is list:
-            return _author_str(author_data[0]) if len(author_data) > 0 else ""
-        else:
-            return author_data
-
     for author_key in ("author", "authors"):
         if author_key in package_json:
             author_str = _author_str(package_json[author_key])
diff --git a/swh/loader/package/npm/tests/test_npm.py b/swh/loader/package/npm/tests/test_npm.py
--- a/swh/loader/package/npm/tests/test_npm.py
+++ b/swh/loader/package/npm/tests/test_npm.py
@@ -11,6 +11,7 @@
 from swh.model.model import Person, Snapshot, SnapshotBranch, TargetType
 
 from swh.loader.package.npm.loader import (
+    _author_str,
     NpmLoader,
     extract_npm_package_author,
     artifact_to_revision_id,
@@ -23,6 +24,27 @@
 )
 
 
+def test_npm_author_str():
+    for author, expected_author in [
+        ("author", "author"),
+        (
+            ["Al from quantum leap", "hal from 2001 space odyssey"],
+            "Al from quantum leap",
+        ),
+        ([], ""),
+        ({"name": "groot", "email": "groot@galaxy.org",}, "groot <groot@galaxy.org>"),
+        ({"name": "somebody",}, "somebody"),
+        ({"email": "no@one.org"}, " <no@one.org>"),  # note first elt is an extra blank
+        ({"name": "no one", "email": None,}, "no one"),
+        ({"email": None,}, ""),
+        ({"name": None}, ""),
+        ({"name": None, "email": None,}, ""),
+        ({}, ""),
+        (None, None),
+    ]:
+        assert _author_str(author) == expected_author
+
+
 def test_extract_npm_package_author(datadir):
     package_metadata_filepath = os.path.join(
         datadir, "https_replicate.npmjs.com", "org_visit1"
@@ -68,7 +90,7 @@
         },
         "homepage": "http://wcoder.github.io/highlightjs-line-numbers.js/"
     }"""
-    )  # noqa
+    )
 
     assert extract_npm_package_author(package_json) == Person(
         fullname=b"Yauheni Pakala <evgeniy.pakalo@gmail.com>",