chore: import upstream snapshot with attribution

docs/md/how_to/javascript/worker.md

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+# Accessing the Perspective engine via a `Client` instance
+
+An instance of a `Client` is needed to talk to a Perspective `Server`, of which
+there are a few varieties available in JavaScript.
+
+## Web Worker (Browser)
+
+Perspective's Web Worker client is actually a `Client` and `Server` rolled into
+one. Instantiating this `Client` will also create a _dedicated_ Perspective
+`Server` in a Web Worker process.
+
+To use it, you'll need to instantiate a Web Worker `perspective` engine via the
+`worker()` method. This will create a new Web Worker (browser) and load the
+WebAssembly binary. All calculation and data accumulation will occur in this
+separate process.
+
+```javascript
+const client = await perspective.worker();
+```
+
+The `worker` symbol will expose the full `perspective` API for one managed Web
+Worker process. You are free to create as many as your browser supports, but be
+sure to keep track of the `worker` instances themselves, as you'll need them to
+interact with your data in each instance.
+
+## Websocket (Browser)
+
+Alternatively, with a Perspective server running in Node.js, Python or Rust, you
+can create a _virtual_ `Client` via the `websocket()` method.
+
+```javascript
+const client = perspective.websocket("http://localhost:8080/");
+```
+
+## Node.js
+
+The Node.js runtime for the `@perspective-dev/client` module runs in-process by
+default and does not implement a `child_process` interface, so no need to call
+the `.worker()` factory function. Instead, the `perspective` library exports the
+functions directly and run synchronously in the main process.
+
+```javascript
+const client = require("@perspective-dev/client");
+```