Add docs and migration guide for JS tracker v4 (#1047)

docs/collecting-data/collecting-from-own-applications/javascript-trackers/node-js-tracker/migration-guides/index.md

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+---
+title: "Migration guides"
+date: "2021-04-12"
+sidebar_position: 4800
+---
+
+```mdx-code-block
+import DocCardList from '@theme/DocCardList';
+
+<DocCardList />
+```

docs/collecting-data/collecting-from-own-applications/javascript-trackers/node-js-tracker/migration-guides/v3-to-v4-migration-guide/index.md

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+---
+title: "Migrating from v3 to v4"
+sidebar_position: 1100
+---
+
+```mdx-code-block
+import Tabs from '@theme/Tabs';
+import TabItem from '@theme/TabItem';
+```
+
+Version 4 is a major release with several breaking changes to defaults and behavior.
+
+## Behavior changes
+
+### Removed the use of the got library in favor of fetch
+
+The tracker no longer needs the got library for making HTTP requests to the collector.
+This has been replaced with the [fetch API](https://nodejs.org/dist/latest-v18.x/docs/api/globals.html).
+The change has an effect on the public API of the tracker which had to previously accommodate the got library – the following change results from this.
+
+### Removed `tracker` and `emitter` functions in favor or `newTracker`
+
+The initialization of a tracker instance has changed – instead of initializing an emitter (using `gotEmitter`) and tracker (using `tracker`) separately, the SDK handles initialization of both using the `newTracker` call.
+The `newTracker` call now accepts configuration objects for the tracker and emitter as arguments.
+
+[See the new initialization options here.](/docs/collecting-data/collecting-from-own-applications/javascript-trackers/node-js-tracker/node-js-tracker-v4/initialization/)
+
+### Dropped support for older Node.JS
+
+The support for the following Node.JS versions has been dropped:
+
+* Drop Node.js <18 support

docs/collecting-data/collecting-from-own-applications/javascript-trackers/node-js-tracker/node-js-tracker-v4/index.md

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+---
+title: "Node.js Tracker v4"
+date: "2021-03-24"
+sidebar_position: 200
+---
+
+```mdx-code-block
+import Badges from '@site/src/components/Badges';
+
+<Badges badgeType="Actively Maintained"></Badges>
+```
+
+The Snowplow Node.js Tracker allows you to track Snowplow events from your Node.js applications.
+
+The tracker should be straightforward to use if you are comfortable with JavaScript development; any prior experience with other Snowplow trackers is helpful but not necessary.
+
+```mdx-code-block
+import DocCardList from '@theme/DocCardList';
+
+<DocCardList />
+```

docs/collecting-data/collecting-from-own-applications/javascript-trackers/node-js-tracker/node-js-tracker-v4/initialization/index.md

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+---
+title: "Initialization"
+date: "2021-04-07"
+sidebar_position: 2000
+---
+
+Assuming you have completed the [Node.js Tracker Setup](/docs/collecting-data/collecting-from-own-applications/javascript-trackers/node-js-tracker/node-js-tracker-v4/setup/index.md) for your project, you are now ready to initialize the Tracker.
+
+Require the Node.js Tracker module into your code like so:
+
+```javascript
+const snowplow = require('@snowplow/node-tracker');
+const tracker = snowplow.newTracker( /* ... */ );
+```
+
+or, if using ES Modules, you can import the module like so:
+
+```javascript
+import { newTracker } from '@snowplow/node-tracker';
+```
+
+The `newTracker` call takes 2 arguments – the tracker and emitter configuration. The tracker configuration specifies the tracker namespace and app name. The emitter configuration specifies how events are queued locally and sent to the Snowplow collector.
+
+```javascript
+const tracker = newTracker({
+  namespace: "my-tracker", // Namespace to identify the tracker instance. It will be attached to all events which the tracker fires.
+  appId: "my-app", // Application identifier
+  encodeBase64: false, // Whether to use base64 encoding for the self-describing JSON. Defaults to true.
+}, {
+  endpoint: "https://collector.mydomain.net", // Collector endpoint
+  eventMethod: "post", // Method - defaults to POST
+  bufferSize: 1, // Only send events once n are buffered. Defaults to 1 for GET requests and 10 for POST requests.
+});
+```
+
+There are a number of additional parameters that may optionally be configured.
+Please refer to the following API docs for the full list:
+
+1. [Tracker configuration options](https://snowplow.github.io/snowplow-javascript-tracker/docs/node-tracker/markdown/node-tracker.trackerconfiguration).
+2. [Emitter configuration options](https://snowplow.github.io/snowplow-javascript-tracker/docs/node-tracker/markdown/node-tracker.emitterconfigurationbase).
+
+### Using multiple Emitters
+
+In case you want to send the events to multiple Snowplow collectors, you can provide a list of emitter configurations in the `newTracker` call – one for each collector.
+This may look as follows.
+
+```javascript
+const tracker = newTracker({
+  namespace: "my-tracker", // Namespace to identify the tracker instance. It will be attached to all events which the tracker fires.
+  appId: "my-app", // Application identifier
+  encodeBase64: false, // Whether to use base64 encoding for the self-describing JSON. Defaults to true.
+}, [
+  {
+    endpoint: "https://collector1.mydomain.net", // First collector endpoint
+  },
+  {
+    endpoint: "https://collector2.mydomain.net", // Second collector endpoint
+  },
+]);
+```
+
+### Create your own Emitter
+
+Emitter is an object responsible for queuing tracked events and making requests to the Snowplow collector.
+
+By default, the Node tracker makes use of the [fetch API](https://developer.mozilla.org/en-US/docs/Web/API/Fetch_API) to make the HTTP requests.
+If you want to use a different library or logic for queuing and sending events, you can provide a custom `Emitter` implementation.
+
+Emitters must conform to an [`Emitter` interface](https://github.com/snowplow/snowplow-javascript-tracker/blob/master/trackers/node-tracker/docs/markdown/node-tracker.emitter.md), which looks like:
+
+```typescript
+/**
+ * Emitter is responsible for sending events to the collector.
+ * It manages the event queue and sends events in batches depending on configuration.
+ */
+interface Emitter {
+    /**
+     * Forces the emitter to send all events in the event store to the collector.
+     * @returns A Promise that resolves when all events have been sent to the collector.
+     */
+    flush: () => Promise<void>;
+    /**
+     * Adds a payload to the event store or sends it to the collector.
+     * @param payload - A payload to be sent to the collector
+     * @returns Promise that resolves when the payload has been added to the event store or sent to the collector
+     */
+    input: (payload: Payload) => Promise<void>;
+    /**
+     * Updates the collector URL to which events will be sent.
+     * @param url - New collector URL
+     */
+    setCollectorUrl: (url: string) => void;
+    /**
+     * Sets the server anonymization flag.
+     */
+    setAnonymousTracking: (anonymous: boolean) => void;
+    /**
+     * Updates the buffer size of the emitter.
+     */
+    setBufferSize: (bufferSize: number) => void;
+}
+```
+
+Once you implement your own `Emitter`, you can pass it to the `newTracker` call as follows.
+
+```ts
+export const expressTracker = newTracker(
+  {
+    namespace: "my-tracker",
+    appId: "my-app",
+  },
+  {
+    customEmitter: () => ({
+      input: (payload) => {
+        return Promise.resolve();
+      },
+      flush: () => {
+        return Promise.resolve();
+      },
+      setCollectorUrl: (url) => {},
+      setAnonymousTracking: (anonymous) => {},
+      setBufferSize: (bufferSize) => {},
+    }),
+  }
+);
+
+```

docs/collecting-data/collecting-from-own-applications/javascript-trackers/node-js-tracker/node-js-tracker-v4/setup/index.md

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+---
+title: "Setup"
+date: "2021-03-31"
+sidebar_position: 1000
+---
+
+## Compatibility
+
+The Snowplow Node.js Tracker is tested and compatible with Node.js versions from 18 to 20. Installation requires [npm](https://www.npmjs.org/), [pnpm](https://pnpm.js.org/) or [yarn](https://yarnpkg.com/).
+
+## Installation
+
+Setting up the tracker should be straightforward if you are familiar with npm:
+
+```bash
+npm install @snowplow/node-tracker
+```
+
+alternative you can use pnpm or yarn:
+
+```bash
+pnpm add @snowplow/node-tracker
+```
+
+```bash
+yarn add @snowplow/node-tracker
+```
+
+The Snowplow Node.js Tracker is also **bundled with TypeScript types and interfaces** so will integrate easily with TypeScript applications.

docs/collecting-data/collecting-from-own-applications/javascript-trackers/node-js-tracker/node-js-tracker/node-js-tracker-v3/api-reference.md

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+---
+type: link
+title: API reference
+sidebar_position: 5000
+href: https://snowplow.github.io/snowplow-javascript-tracker/docs/node-tracker/node-tracker.api 
+---