diff --git a/CHANGES.rst b/CHANGES.rst
index 485f41f4..16f20fa5 100644
--- a/CHANGES.rst
+++ b/CHANGES.rst
@@ -5,6 +5,7 @@ mqttwarn changelog
 
 in progress
 ===========
+- [udf] Unlock JavaScript for user-defined functions. Thanks, @extremeheat.
 
 
 2023-10-15 0.35.0
diff --git a/Dockerfile b/Dockerfile
index b723fd74..f92a1e23 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -34,7 +34,7 @@ RUN --mount=type=cache,id=pip,target=/root/.cache/pip \
     true \
     && pip install --upgrade pip \
     && pip install --prefer-binary versioningit wheel \
-    && pip install --use-pep517 --prefer-binary '/src'
+    && pip install --use-pep517 --prefer-binary '/src[javascript]'
 
 # Uninstall build prerequisites again.
 RUN apt-get --yes remove --purge git && apt-get --yes autoremove
diff --git a/docs/configure/transformation.md b/docs/configure/transformation.md
index be6bb663..6a3344d0 100644
--- a/docs/configure/transformation.md
+++ b/docs/configure/transformation.md
@@ -407,6 +407,68 @@ the invocation of your function, you will create an endless loop.
 :::
 
 
+(user-defined-functions-languages)=
+### Using other languages
+
+This section is about using languages other than Python within the mqttwarn transformation
+subsystem.
+
+#### Introduction
+Imagine you are running a large cloud infrastructure for [4IR] purposes, where hardware
+appliances or industrial machines submit telemetry data to concentrators or hubs, in order
+to converge them to a message bus like [Kafka], where, because you favoured a [FaaS]
+solution, your [serverless] function handler will consume the telemetry messages, in
+order to conduct further processing, like to filter, transform, and store the data.
+Chances are that you used the [Serverless Framework], built on [Node.js], so your application
+infrastructure is effectively based on a bunch of [JavaScript] code.
+
+Now, you know how the story ends. The infrastructure costs exploded, you had to fire your
+engineering team, and now you are sitting in front of a mixture of infrastructure setup
+recipes and actual application code, and you are not able to make any sense of that at all.
+After some  digging, you are able to identify the spot where the real application logic
+evaluates a set of transformation rules, in order to mangle inbound data appropriately
+into outbound data, written in JavaScript.
+
+You quickly realize that the same thing could effectively be solved using [MQTT] and Python,
+and operated on a single server machine, because you are doing only a few hundred requests
+per second anyway. However, you don't have the time, because, well, the customer quickly
+needs a fix because they added another telemetry data field waiting to be consumed and
+handled appropriately. Well, and it's Kafka anyway, how would one get rid of _that_?
+
+After a glimpse of desperation, you are reading the fine manual again, and catch up with
+the fact that telemetry data _is actually ingested using MQTT!_ That is useful, indeed!
+Remembering conversations about a thing called _mqttwarn_ at the Späti the other day,
+and that it gained the ability to run JavaScript code recently, in order to bring cloud
+computing techniques back to personal computing, you think it would be feasible to change
+that architecture of your system in the blink of an eye.
+
+There you go: You rip out the JavaScript transformation rules into a single-file version,
+export its main entry point symbol, configure mqttwarn to use `functions = mycloud.js`,
+and adjust its settings to use your MQTT broker endpoint at the beginning of the data
+pipeline, invoke mqttwarn, and turn off Kafka. It works!
+
+:::{note}
+Rest assured we are overexaggerating a bit, and [Kafka] can only be compared to [MQTT]
+if you are also willing to compare apples with oranges, but you will get the point that
+we believe simpler systems are more often than not able to solve problems equally well,
+if not more efficient, both at runtime, and on details of maintenance and operation.
+
+Other than this, every kind of system migration should be conducted with better planning
+than outlined in our rant above.
+:::
+
+#### JavaScript
+
+For running user-defined functions code written in [JavaScript], mqttwarn uses the
+excellent [JSPyBridge] package. For adding JavaScript support to mqttwarn, install
+it using pip like `pip install --upgrade 'mqttwarn[javascript]'`, or use one of the
+available [OCI images](#using-oci-image).
+
+You can find an example implementation for a `filter` function written in JavaScript
+at the [OwnTracks-to-ntfy example tutorial](#owntracks-ntfy-variants-udf).
+
+
+
 ## User-defined function examples
 
 In this section, you can explore a few example scenarios where user-defined
@@ -507,6 +569,12 @@ was received on, here `owntracks/jane/phone`. The name of the section will be
 the `section` argument, here `owntracks/#/phone`.
 :::
 
+:::{note}
+The recipe about how to use [](#owntracks-ntfy-variants-udf) demonstrates
+corresponding examples for writing a `filter` function.
+:::
+
+
 (decode-topic)=
 ### `alldata`: Decoding topic names
 
@@ -631,10 +699,19 @@ weather,topic=tasmota/temp/ds/1 temperature=19.7 1517525319000
 ```
 
 
+[4IR]: https://en.wikipedia.org/wiki/Fourth_Industrial_Revolution
+[FaaS]: https://en.wikipedia.org/wiki/Cloud_computing#Serverless_computing_or_Function-as-a-Service_(FaaS)
 [geo-fence]: https://en.wikipedia.org/wiki/Geo-fence
 [InfluxDB line format]: https://docs.influxdata.com/influxdb/v1.8/write_protocols/line_protocol_tutorial/
+[JavaScript]: https://en.wikipedia.org/wiki/JavaScript
 [Jinja2 templates]: https://jinja.palletsprojects.com/templates/
+[JSPyBridge]: https://pypi.org/project/javascript/
+[Kafka]: https://en.wikipedia.org/wiki/Apache_Kafka
+[MQTT]: https://en.wikipedia.org/wiki/MQTT
+[Node.js]: https://en.wikipedia.org/wiki/Node.js
+[OwnTracks]: https://owntracks.org
 [Tasmota]: https://github.com/arendst/Tasmota
 [Tasmota JSON status response for a DS18B20 sensor]: https://tasmota.github.io/docs/JSON-Status-Responses/#ds18b20
-[OwnTracks]: https://owntracks.org
+[serverless]: https://en.wikipedia.org/wiki/Serverless_computing
+[Serverless Framework]: https://github.com/serverless/serverless
 [waypoint]: https://en.wikipedia.org/wiki/Waypoint
diff --git a/docs/usage/pip.md b/docs/usage/pip.md
index 465f83b7..31770753 100644
--- a/docs/usage/pip.md
+++ b/docs/usage/pip.md
@@ -11,6 +11,11 @@ that.
 pip install --upgrade mqttwarn
 ```
 
+Add JavaScript support for user-defined functions.
+```bash
+pip install --upgrade 'mqttwarn[javascript]'
+```
+
 You can also add support for a specific service plugin.
 
 ```bash
diff --git a/examples/owntracks-ntfy/mqttwarn-owntracks.js b/examples/owntracks-ntfy/mqttwarn-owntracks.js
new file mode 100644
index 00000000..a9af3303
--- /dev/null
+++ b/examples/owntracks-ntfy/mqttwarn-owntracks.js
@@ -0,0 +1,35 @@
+/**
+ *
+ * Forward OwnTracks low-battery warnings to ntfy.
+ * https://mqttwarn.readthedocs.io/en/latest/examples/owntracks-battery/readme.html
+ *
+ */
+
+// mqttwarn filter function, returning true if the message should be ignored.
+// In this case, ignore all battery level telemetry values above a certain threshold.
+function owntracks_batteryfilter(topic, message) {
+    let ignore = true;
+    let data;
+
+    // Decode inbound message.
+    try {
+        data = JSON.parse(message);
+    } catch {
+        data = null;
+    }
+
+    // Evaluate filtering rule.
+    if (data && "batt" in data && data.batt !== null) {
+        ignore = Number.parseFloat(data.batt) > 20;
+    }
+
+    return ignore;
+}
+
+// Status message.
+console.log("Loaded JavaScript module.");
+
+// Export symbols.
+module.exports = {
+    "owntracks_batteryfilter": owntracks_batteryfilter,
+};
diff --git a/examples/owntracks-ntfy/readme-variants.md b/examples/owntracks-ntfy/readme-variants.md
new file mode 100644
index 00000000..8f91d485
--- /dev/null
+++ b/examples/owntracks-ntfy/readme-variants.md
@@ -0,0 +1,71 @@
+---
+orphan: true
+---
+
+(owntracks-ntfy-variants)=
+
+# OwnTracks-to-ntfy setup variants
+
+
+## About
+
+This section informs you about additional configuration and operation variants of the
+[](#owntracks-ntfy-recipe) recipe. For example, you may want to use Docker or Podman
+to run both mqttwarn and ntfy, or you may want to use another language than Python to
+implement your filtering function.
+
+
+## Docker and Podman
+
+### Running mqttwarn as container
+This command will run mqttwarn in a container, using the `docker` command to launch it.
+Alternatively, `podman` can be used. It expects an MQTT broker to be running on `localhost`,
+so it uses the `--network=host` option. The command will mount the configuration file and
+the user-defined functions file correctly, and will invoke mqttwarn with the corresponding
+`--config-file` option.
+```shell
+docker run --rm -it --network=host --volume=$PWD:/etc/mqttwarn \
+  ghcr.io/jpmens/mqttwarn-standard \
+  mqttwarn --config-file=mqttwarn-owntracks.ini
+```
+
+### Running ntfy as container
+While this tutorial uses the ntfy service at ntfy.sh, it is possible to run your own
+instance. For example, use Docker or Podman.
+```shell
+docker run --name=ntfy --rm -it --publish=5555:80 \
+  binwiederhier/ntfy serve --base-url="http://localhost:5555"
+```
+In this case, please adjust the ntfy configuration section `[config:ntfy]` to use
+a different URL, and make sure to restart mqttwarn afterwards.
+```ini
+[config:ntfy]
+targets   = {'testdrive': 'http://localhost:5555/testdrive'}
+```
+
+
+(owntracks-ntfy-variants-udf)=
+
+## Alternative languages for user-defined functions
+
+### JavaScript
+
+In order to try that on the OwnTracks-to-ntfy example, use the alternative
+`mqttwarn-owntracks.js` implementation by adjusting the `functions` setting within the
+`[defaults]` section of your configuration file, and restart mqttwarn.
+```ini
+[defaults]
+functions = mqttwarn-owntracks.js
+```
+
+The JavaScript function `owntracks_batteryfilter()` implements the same rule as the
+previous one, which was written in Python.
+
+:::{literalinclude} mqttwarn-owntracks.js
+:language: javascript
+:::
+
+:::{attention}
+The feature to run JavaScript code is currently considered to be experimental.
+Please use it responsibly.
+:::
diff --git a/examples/owntracks-ntfy/readme.md b/examples/owntracks-ntfy/readme.md
index 6d636f0b..5c557390 100644
--- a/examples/owntracks-ntfy/readme.md
+++ b/examples/owntracks-ntfy/readme.md
@@ -96,34 +96,13 @@ echo 'foobar' | mosquitto_pub -t 'owntracks/testdrive' -l
 
 ## Appendix
 
-This section demonstrates a few alternative methods for solving different aspects of this
-recipe, and also includes administrative information.
-
-### Running mqttwarn as container
-This command will run mqttwarn in a container, using the `docker` command to launch it.
-Alternatively, `podman` can be used. It expects an MQTT broker to be running on `localhost`,
-so it uses the `--network=host` option. The command will mount the configuration file and
-the user-defined functions file correctly, and will invoke mqttwarn with the corresponding
-`--config-file` option. 
-```shell
-docker run --rm -it --network=host --volume=$PWD:/etc/mqttwarn \
-  ghcr.io/jpmens/mqttwarn-standard \
-  mqttwarn --config-file=mqttwarn-owntracks.ini
-```
+### Configuration and operation variants
 
-### Running ntfy as container
-While this tutorial uses the ntfy service at ntfy.sh, it is possible to run your own
-instance. For example, use Docker or Podman.
-```shell
-docker run --name=ntfy --rm -it --publish=5555:80 \
-  binwiederhier/ntfy serve --base-url="http://localhost:5555"
-```
-In this case, please adjust the ntfy configuration section `[config:ntfy]` to use
-a different URL, and make sure to restart mqttwarn afterwards.
-```ini
-[config:ntfy]
-targets   = {'testdrive': 'http://localhost:5555/testdrive'}
-```
+There are different variants to configure and operate this setup. For example,
+you may want to use Docker or Podman to run both mqttwarn and ntfy, or you may
+want to use another language than Python to implement your filtering function.
+We summarized the available options on the [](#owntracks-ntfy-variants) page,
+together with corresponding guidelines how to use them.
 
 ### Backlog
 :::{todo}
diff --git a/mqttwarn/util.py b/mqttwarn/util.py
index 3739b8d2..c8b59b8d 100644
--- a/mqttwarn/util.py
+++ b/mqttwarn/util.py
@@ -13,6 +13,7 @@
 import os
 import re
 import string
+import threading
 import types
 import typing as t
 from pathlib import Path
@@ -147,8 +148,10 @@ def load_module_from_file(path: t.Union[str, Path]) -> types.ModuleType:
         loader = importlib.machinery.SourceFileLoader(fullname=name, path=str(path))
     elif path.suffix == ".pyc":
         loader = importlib.machinery.SourcelessFileLoader(fullname=name, path=str(path))
+    elif path.suffix in [".js", ".javascript"]:
+        return load_source_js(name, str(path))
     else:
-        raise ImportError(f"Loading file failed (only .py and .pyc): {path}")
+        raise ImportError(f"Loading file type failed (only .py, .pyc, .js, .javascript): {path}")
     spec = importlib.util.spec_from_loader(loader.name, loader)
     if spec is None:
         raise ModuleNotFoundError(f"Failed loading module from file: {path}")
@@ -216,16 +219,17 @@ def import_symbol(name: str, parent: t.Optional[types.ModuleType] = None) -> typ
     return import_symbol(remaining_names, parent=module)
 
 
-def load_functions(filepath: t.Optional[str] = None) -> t.Optional[types.ModuleType]:
+def load_functions(filepath: t.Optional[t.Union[str, Path]] = None) -> t.Optional[types.ModuleType]:
 
     if not filepath:
         return None
 
+    filepath = str(filepath)
+
     if not os.path.isfile(filepath):
         raise IOError("'{}' not found".format(filepath))
 
-    py_mod = load_module_from_file(filepath)
-    return py_mod
+    return load_module_from_file(filepath)
 
 
 def load_function(name: str, py_mod: t.Optional[types.ModuleType]) -> t.Callable:
@@ -277,3 +281,39 @@ def load_file(path: t.Union[str, Path], retry_tries=None, retry_interval=0.075,
         except:  # pragma: nocover
             pass
     return reader
+
+
+def imp_new_module(name):
+    """
+    Create a new module.
+
+    The module is not entered into sys.modules.
+
+    """
+    return types.ModuleType(name)
+
+
+def module_factory(name, variables):
+    """
+    Create a synthetic Python module object.
+
+    Derived from:
+    https://www.oreilly.com/library/view/python-cookbook/0596001673/ch15s03.html
+    """
+    module = imp_new_module(name)
+    module.__dict__.update(variables)
+    module.__file__ = "<synthesized>"
+    return module
+
+
+def load_source_js(mod_name, filepath):
+    """
+    Load a JavaScript module, and import its exported symbols into a synthetic Python module.
+    """
+    import javascript
+
+    js_code = load_file(filepath, retry_tries=0).read().decode("utf-8")
+    module = {}
+    javascript.eval_js(js_code)
+    threading.Event().wait(0.01)
+    return module_factory(mod_name, module["exports"])
diff --git a/pyproject.toml b/pyproject.toml
index 29498aea..4e2c3aff 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -56,6 +56,7 @@ ignore = [
 extend-exclude = [
   # Always.
   ".venv*",
+  "example_*",
   "tests/etc/functions_bad.py",
   # Temporary.
   "examples",
diff --git a/setup.py b/setup.py
index 1075769c..072bc408 100644
--- a/setup.py
+++ b/setup.py
@@ -53,6 +53,9 @@
         "gspread>=2.1.1",
         "oauth2client>=4.1.2",
     ],
+    "javascript": [
+        "javascript==1!1.0.1; python_version>='3.7'",
+    ],
     "mysql": [
         "mysql",
     ],
@@ -200,6 +203,8 @@
         "Operating System :: POSIX",
         "Operating System :: Unix",
         "Operating System :: MacOS",
+        "Operating System :: Microsoft :: Windows",
+        "Programming Language :: JavaScript",
         "Programming Language :: Python",
         "Programming Language :: Python :: 3.6",
         "Programming Language :: Python :: 3.7",
@@ -229,7 +234,7 @@
     author="Jan-Piet Mens, Ben Jones, Andreas Motl",
     author_email="jpmens@gmail.com",
     url="https://github.com/mqtt-tools/mqttwarn",
-    keywords="mqtt notification plugins data acquisition push transformation engine mosquitto",
+    keywords="mqtt notification plugins data acquisition push transformation engine mosquitto python javascript",
     packages=find_packages(),
     include_package_data=True,
     package_data={
diff --git a/tests/test_util.py b/tests/test_util.py
index 716133f9..d8c8fb5b 100644
--- a/tests/test_util.py
+++ b/tests/test_util.py
@@ -4,6 +4,7 @@
 
 import py_compile
 import re
+import sys
 import time
 import types
 from builtins import str
@@ -138,7 +139,9 @@ def test_load_functions():
     # Load functions file that is not a python file
     with pytest.raises(ImportError) as excinfo:
         load_functions(filepath=configfile_full)
-    assert re.match(r"Loading file failed \(only .py and .pyc\): .+full.ini", str(excinfo.value))
+    assert re.match(
+        re.escape("Loading file type failed (only .py, .pyc, " ".js, .javascript)") + ": .+full.ini", str(excinfo.value)
+    )
 
     # Load bad functions file
     with pytest.raises(Exception):
@@ -159,6 +162,46 @@ def test_load_functions_pyc(tmp_path):
     assert py_mod is not None
 
 
+@pytest.mark.skipif(sys.version_info < (3, 7), reason="JavaScript support only works on Python >= 3.7")
+def test_load_functions_javascript_success(tmp_path):
+    """
+    Verify that JavaScript module loading, including symbol exporting and invocation, works well.
+    """
+    jsfile = tmp_path / "test.js"
+    jsfile.write_text("module.exports = { forty_two: function() { return 42; } };")
+    jsmod = load_functions(jsfile)
+    assert jsmod.forty_two() == 42
+
+
+@pytest.mark.skipif(sys.version_info < (3, 7), reason="JavaScript support only works on Python >= 3.7")
+def test_load_functions_javascript_compile_failure(tmp_path):
+    """
+    Verify that JavaScript module loading, including symbol exporting and invocation, works well.
+    """
+    from javascript.errors import JavaScriptError
+
+    jsfile = tmp_path / "test.js"
+    jsfile.write_text("Hotzenplotz")
+    with pytest.raises(JavaScriptError) as ex:
+        load_functions(jsfile)
+    assert ex.match("ReferenceError: Hotzenplotz is not defined")
+
+
+@pytest.mark.skipif(sys.version_info < (3, 7), reason="JavaScript support only works on Python >= 3.7")
+def test_load_functions_javascript_runtime_failure(tmp_path):
+    """
+    Verify that JavaScript module loading, including symbol exporting and invocation, works well.
+    """
+    from javascript.errors import JavaScriptError
+
+    jsfile = tmp_path / "test.js"
+    jsfile.write_text("module.exports = { foo: function() { bar(); } };")
+    jsmod = load_functions(jsfile)
+    with pytest.raises(JavaScriptError) as ex:
+        jsmod.foo()
+    assert ex.match("ReferenceError: bar is not defined")
+
+
 def test_load_function():
 
     # Load valid functions file