Document payload normalization

doc/content/integrations/payload-formatters/_index.md

@@ -9,29 +9,29 @@ This section explains how to set up payload formatters for a specific end device
 
 <!--more-->
 
-In addition to the written instructions linked below, a video with instructions for creating Javascript and device repository payload formatters is available on [The Things Network youtube channel](https://youtu.be/4tii7MiD-yM).
+In addition to the written instructions linked below, a video with instructions for creating JavaScript and device repository payload formatters is available on [The Things Network YouTube channel](https://youtu.be/4tii7MiD-yM).
 
 <details><summary>Show video</summary>
 {{< youtube "4tii7MiD-yM" >}}
 </details>
 
 ## Types of Payload Formatters
 
-{{% tts %}} supports three types of payload formatters: Javascript, CayenneLPP, and Repository payload formatters.
+{{% tts %}} supports three types of payload formatters: JavaScript, CayenneLPP, and Repository payload formatters.
 
-### Javascript
+### JavaScript
 
-{{% tts %}} allows you to write your own custom payload formatters in Javascript. To find out how to write a custom Javascript payload formatter, see the [Javascript Payload Formatters]({{< ref "/integrations/payload-formatters/javascript" >}}) section.
+{{% tts %}} allows you to write your own custom payload formatters in JavaScript. To find out how to write a custom JavaScript payload formatter, see the [JavaScript Payload Formatters]({{< ref "/integrations/payload-formatters/javascript" >}}) section.
 
 ### CayeneLPP
 
 {{% tts %}} can automatically decode CayenneLPP formatted payloads, no custom code neccessary. To find out how to use CayenneLPP payload formatters, see the [CayenneLPP]({{< relref "cayenne" >}}) section.
 
 ### Repository
 
-Device manufacturers may submit payload formatters designed to work with their devices. These are publicly available in {{% tts %}} [Device Repository](https://github.com/TheThingsNetwork/lorawan-devices/tree/master).
+Device manufacturers may submit payload formatters designed to work with their devices. These are publicly available in {{% tts %}} [Device Repository](https://github.com/TheThingsNetwork/lorawan-devices).
 
-Device repository payload formatters work right out of the box - no customization needed!
+Device repository payload formatters work right out of the box – no customization needed!
 
 ## Application and Device Specific Payload Formatters
 

doc/content/integrations/payload-formatters/device-repo.md

@@ -3,6 +3,6 @@ title: "Device Repository"
 description: ""
 ---
 
-Device manufacturers may submit payload formatters designed to work with their devices. The source code for these is publicly available in the [lorawan-devices Github Repository](https://github.com/TheThingsNetwork/lorawan-devices/tree/master) and the devices are on display in The Things Industries [Device Repository](https://www.thethingsnetwork.org/device-repository/).
+Device manufacturers may submit payload formatters designed to work with their devices. The data and code are publicly available in the [Github Repository](https://github.com/TheThingsNetwork/lorawan-devices). You can find all devices in the Device Repository on [this page](https://www.thethingsnetwork.org/device-repository/).
 
-If you're a device manufacturer and you'd like to add your device to the open source device repo, find out how [here](https://github.com/TheThingsNetwork/lorawan-devices/blob/master/README.md). Adding your device makes it easy for users to use it with {{% tts %}}, and is a good way to publicly document information and code related to your device.
+If you're a device manufacturer and you would like to add your device to the repository, find out how [here](https://github.com/TheThingsNetwork/lorawan-devices/blob/master/README.md). Adding your device makes it easy for users to use it with {{% tts %}}, and is a good way to publicly document information and code related to your device.

doc/content/integrations/payload-formatters/javascript/_index.md

@@ -1,28 +1,28 @@
 ---
-title: "Javascript"
+title: "JavaScript"
 description: ""
 alias: "/integrations/payload-formatters/javascript"
 ---
 
-Javascript payload formatters allow you to write your own functions to encode or decode messages. Javascript functions are executed using an [JavaScript ECMAScript 5.1](https://www.ecma-international.org/ecma-262/5.1/) engine.
+JavaScript payload formatters allow you to write functions to encode or decode messages. JavaScript functions are executed using an [JavaScript ECMAScript 5.1](https://www.ecma-international.org/ecma-262/5.1/) runtime.
 
 <!--more-->
 
 Tips:
+
 - The payload formatters should be simple and lightweight. 
 - Use arithmetic operations and bit shifts to convert binary data to fields. 
 - Avoid using non-trivial logic or polyfills. 
 
-{{< note >}} Payload formatters use ECMAScript 5 (2009), which has some distinct differences compared to newer, commonly used ECMAScript revisions. See [here](https://www.javatpoint.com/es5-vs-es6) for a quick comparison. Notably, `let`, `const`, and arrow functions are not supported by ES5. {{</ note >}}
-
-{{< note >}} For security, the runtime does not support modules, `require` syntax, or any input/output other than defined below. {{</ note >}}
+{{< note >}}
+Payload formatters use ECMAScript 5.1 (2009), which has some distinct differences compared to newer, commonly used ECMAScript revisions. See [here](https://www.javatpoint.com/es5-vs-es6) for a quick comparison.
 
-{{< warning >}} The maximum size of a user-defined Javascript payload formatter is 40KB (40960 characters), unless the source of the payload formatter is [Device Repository]({{< ref "/integrations/payload-formatters/device-repo" >}}). {{</ warning>}}
+For security, the runtime does not support modules (`require` syntax) or any network or file system access.
+{{</ note >}}
 
-There are three different types of {{% tts %}} JavaScript payload formatters:
+{{< warning >}} The maximum size of a JavaScript payload formatter is 40KB (40,960 bytes). Payload formatters loaded from the [Device Repository]({{< ref "/integrations/payload-formatters/device-repo" >}}) can be larger. {{</ warning>}}
 
-- [Uplink Decoder]({{< ref "/integrations/payload-formatters/javascript/uplink-decoder" >}})
-- [Downlink Encoder]({{< ref "/integrations/payload-formatters/javascript/downlink-encoder" >}})
-- [Downlink Decoder]({{< ref "/integrations/payload-formatters/javascript/downlink-decoder" >}})
+Learn more about payload formatters and examples:
 
-Read the documentation below to further learn about these formatters and find associated examples.
+- [Uplink]({{< relref "uplink" >}})
+- [Downlink]({{< relref "downlink" >}})

doc/content/integrations/payload-formatters/javascript/downlink/_index.md

@@ -0,0 +1,125 @@
+---
+title: "Downlink"
+description: ""
+weight: 2
+aliases:
+  - "/integrations/payload-formatters/javascript/downlink-encoder"
+  - "/integrations/payload-formatters/javascript/downlink-decoder"
+---
+
+The `encodeDownlink()` function is called when a downlink message with decoded payload is scheduled to be sent to the end device. The `encodeDownlink()` function encodes the JSON object of the downlink message to binary payload for transmission to the end device.
+
+The `decodeDownlink()` function does the opposite of `encodeDownlink()`: it decodes the binary payload back to decoded payload.
+
+<!--more-->
+
+The functions take an input object and return an output object:
+
+```js
+function encodeDownlink(input) {
+  // input has the following structure:
+  // {
+  //   field: "value"
+  // }
+  return {
+    bytes: [1, 2, 3], // FRMPayload (byte array)
+    fPort: 1,
+    warnings: ["warning 1", "warning 2"], // optional
+    errors: ["error 1", "error 2"] // optional (if set, the encoding failed)
+  }
+}
+
+function decodeDownlink(input) {
+  // input has the following structure:
+  // {
+  //   bytes: [1, 2, 3], // FRMPayload (byte array)
+  //   fPort: 1
+  // }
+  return {
+    data: {
+      field: "value"
+    },
+    warnings: ["warning 1", "warning 2"], // optional
+    errors: ["error 1", "error 2"] // optional (if set, the decoding failed)
+  }
+}
+```
+
+Encoded and decoded downlink payload are incorporated in the [`as.down.data.receive`]({{< ref "/reference/api/events#event:as.down.data.receive" >}}) event, where the `data` object contains:
+
+```json
+{
+  "f_port": 1,
+  "frm_payload": "AQID",
+  "decoded_payload": {
+    "field": "value"
+  }
+}
+```
+
+The encoded payload is in `frm_payload` (Base64 encoded), while the decoded payload is in `decoded_payload`.
+
+If an error is present in `errors`, the payload is invalid and the message will be dropped. Any warnings in `warnings` are informative.
+
+{{< note >}} You can test your downlink encoder and decoder as well as schedule downlinks from {{% tts %}} Console. {{</ note >}}
+
+## Downlink Example: The Things Node
+
+Here is an example of an encoder function that uses the `color` field to send a byte on a specific FPort:
+
+```js
+var colors = ["red", "green", "blue"];
+
+function encodeDownlink(input) {
+  return {
+    bytes: [colors.indexOf(input.data.color)],
+    fPort: 4,
+  };
+}
+
+function decodeDownlink(input) {
+  switch (input.fPort) {
+  case 4:
+    return {
+      data: {
+        color: colors[input.bytes[0]]
+      }
+    }
+  default:
+    return {
+      errors: ["unknown FPort"]
+    }
+  }
+}
+```
+
+If the downlink with the following JSON payload is sent by the application:
+
+```json
+{
+  "color": "green"
+}
+```
+
+The output of the `encodeDownlink()` function will have the following structure:
+
+```json
+{
+  "bytes": [1],
+  "fPort": 4
+}
+```
+
+{{< figure src="downlink-encoder.png" alt="Testing a downlink encoder" >}}
+
+The `data` object of the `as.down.data.receive` event will therefore contain:
+
+```json
+{
+  "f_port": 4,
+  "frm_payload": "AQ==",
+  "decoded_payload": {
+    "color": "green"
+  }
+}
+```