support linking to a preselected channel

Readme.md

@@ -106,7 +106,7 @@ Done in Markdown this looks like:
 
 [![](https://cldup.com/WIbawiqp0Q.png)](http://slack.socket.io)
 
-Point to `https://slack.yourdomain.com`.
+Point to `https://slack.yourdomain.com`. If you're only allowing single-channel guests, you can also link to `https://slack.yourdomain.com/?channel=<name>` to have them join that particular channel.
 
 **Note:** the image for the logo of the landing page
 is retrieved from the Slack API. If your organization
@@ -136,16 +136,16 @@ By default logging is enabled.
 
 ## Developing
 
-Slackin's server side code is written in ES6. It uses babel to transpile the 
-ES6 code to a format node understands. After cloning Slackin, you should 
+Slackin's server side code is written in ES6. It uses babel to transpile the
+ES6 code to a format node understands. After cloning Slackin, you should
 install the prerequisite node libraries with npm:
 
 ```bash
 $ npm install
 ```
 
 After the libraries install, the postinstall script will run `gulp` to invoke
-babel on the source. It is important to run `gulp` manually after updating any 
+babel on the source. It is important to run `gulp` manually after updating any
 files in lib/ to update the versions in node/.
 
 ## Credits

lib/index.js

@@ -18,6 +18,20 @@ import splash from './splash';
 import iframe from './iframe';
 import log from './log';
 
+export function restrictChannels(channels, req) {
+  let channelName = req.query.channel;
+  if (
+    channels &&
+    channels.length > 0 &&
+    channelName &&
+    channels.indexOf(channelName) > -1
+  ) {
+    return [channelName];
+  } else {
+    return channels;
+  }
+}
+
 export default function slackin({
   token,
   interval = 5000, // jshint ignore:line
@@ -65,6 +79,7 @@ export default function slackin({
     let { name, logo } = slack.org;
     let { active, total } = slack.users;
     if (!name) return res.send(404);
+    let restrictedChannels = restrictChannels(channels, req);
     let page = dom('html',
       dom('head',
         dom('title',
@@ -74,7 +89,7 @@ export default function slackin({
         dom('link rel="shortcut icon" href=https://slack.global.ssl.fastly.net/272a/img/icons/favicon-32.png'),
         css && dom('link rel=stylesheet', { href: css })
       ),
-      splash({ coc, path, css, name, org, logo, channels, active, total })
+      splash({ coc, path, css, name, org, logo, active, total, channels: restrictedChannels })
     );
     res.type('html');
     res.send(page.toHTML());

test/index.js

@@ -1,8 +1,31 @@
+import assert from 'assert';
 import nock from 'nock';
 import request from 'supertest';
-import slackin from '../lib/index';
+import slackin, { restrictChannels } from '../lib/index';
 
 describe('slackin', () => {
+  describe('.restrictChannels()', () => {
+    it("returns the same list when the query is empty", () => {
+      let results = restrictChannels(['foo', 'bar'], {query: {}});
+      assert.deepEqual(results, ['foo', 'bar']);
+    });
+
+    it("returns the same list when the channel isn't in the list", () => {
+      let results = restrictChannels(['foo', 'bar'], {query: {channel: 'baz'}});
+      assert.deepEqual(results, ['foo', 'bar']);
+    });
+
+    it("returns undefined when there are no channels to restrict", () => {
+      let results = restrictChannels(undefined, {query: {channel: 'foo'}});
+      assert.equal(results, undefined);
+    });
+
+    it("returns undefined when there is an empty list of channels to restrict", () => {
+      let results = restrictChannels([], {query: {channel: 'foo'}});
+      assert.deepEqual(results, []);
+    });
+  });
+
   describe('POST /invite', () => {
     beforeEach(() => {
       nock('https://myorg.slack.com')