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webapp/platform/client/README.md

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+# Mattermost Client
+
+[![npm version](https://img.shields.io/npm/v/@mattermost/client?style=flat)](https://www.npmjs.com/package/@mattermost/client)
+
+This package contains the JavaScript/TypeScript client for [Mattermost](https://github.com/mattermost/mattermost). It's used by [the Mattermost web app](https://github.com/mattermost/mattermost/tree/master/webapp/channels) and related projects.
+
+## Installation
+
+### JavaScript
+
+```sh
+$ npm install @mattermost/client
+```
+
+### TypeScript
+
+```sh
+$ npm install @mattermost/client @mattermost/types
+```
+
+## Usage
+
+### Rest Client
+
+To use this client, create an instance of `Client4`, set the server URL, and log in, and then you can start making requests.
+
+```js
+import {Client4} from '@mattermost/client';
+
+const client = new Client4();
+client.setUrl('https://mymattermostserver.example.com');
+
+client.login('username', 'password').then((user) => {
+    // ...
+});
+```
+
+If you already have a session token or a user access token, you can call `Client4.setToken` instead of logging in.
+
+```js
+import {Client4} from '@mattermost/client';
+
+const client = new Client4();
+client.setUrl('https://mymattermostserver.example.com');
+
+client.setToken('accesstoken');
+```
+
+If needed, methods exist to set other headers such as the User-Agent (`Client4.setUserAgent`), the CSRF token (`Client4.setCSRF`), or any extra headers you wish to include (`Client4.setHeader`).
+
+Methods of `Client4` which make requests to the server return a `Promise` which does the following:
+
+- On success, the promise resolves to a `ClientResponse<T>` object which contains the the [Response](https://developer.mozilla.org/en-US/docs/Web/API/Response) (`response`), a [Map](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Map) of headers (`headers`), and the data sent from the server (`data`).
+- On an error, the promise rejects with a `ClientError` which contains the error message and the URL being requested. If the error happened on the server, the status code and an error ID (`server_error_id`) are included.
+
+```js
+let user;
+try {
+    user = (await client.getUser('userid')).data;
+} catch (e) {
+    console.error(`An error occurred when making a request to ${e.url}: ${e.message}`);
+}
+```
+
+### WebSocket Client
+
+To use the WebSocket client, create an instance of `WebSocketClient` and then call its `initialize` method with the connection URL and an optional session token or user access token. After that, you can call the client's `addMessageListener` method to register a listener which will be called whenever a WebSocket message is received from the server.
+
+```js
+import {WebSocketClient} from '@mattermost/client';
+
+// If you already have an instance of Client4, you can call its getWebSocketUrl method to get this URL
+const connectionUrl = 'https://mymattermostserver.example.com/api/v4/websocket';
+
+// In a browser, the token may be passed automatically from a cookie
+const authToken = process.env.TOKEN;
+
+const wsClient = new WebSocketClient();
+wsClient.initialize(connectionUrl, authToken);
+
+wsClient.addMessageListener((msg) => {
+    if (msg.event === 'posted') {
+        console.log('New post received', JSON.parse(msg.data.post));
+    }
+});
+```
+
+#### Node.js
+
+Note that `WebSocketClient` expects `globalThis.WebSocket` to be defined as it was originally written for use in the Mattermost web app. If you're using it in a Node.js environment, you should set `globalThis.WebSocket` before instantiating the `WebSocketClient`.
+
+```js
+import WebSocket from 'ws';
+
+if (!globalThis.WebSocket) {
+    globalThis.WebSocket = WebSocket;
+}
+
+const wsClient = new WebSocketClient();
+```
+
+This can also be done using dynamic imports if you're using them.
+
+```js
+if (!globalThis.WebSocket) {
+    const {WebSocket} = await import('ws');
+    globalThis.WebSocket = WebSocket;
+}
+
+const wsClient = new WebSocketClient();
+```
+
+## Compilation and Packaging
+
+As a member of Mattermost with write access to our NPM organization, you can build and publish this package by running the following commands:
+
+```bash
+npm run build --workspace=platform/client
+npm publish --workspace=platform/client
+```
+
+Make sure to increment the version number in `package.json` first! You can add `-0`, `-1`, etc for pre-release versions.