docs(manuals): add ts version of react component

Some time ago Gatsby started to support Typescript natively. Also the frontend of remark42 is written in Typescript. So I thought it's a good idea to add a Typescript version of the component.

site/src/docs/manuals/integration-with-gatsby/index.md

@@ -1,5 +1,5 @@
 # Using Remark42 in Gatsby projects
-## Sample Comments.js React component:
+## React component written in Javascript
 
 ```js
 
@@ -96,3 +96,109 @@ const Comments = ({ location }) => {
 export default Comments
 
 ```
+
+## React component written in Typescript
+
+```ts
+import { Fragment, useEffect } from 'react';
+
+declare global {
+  interface Window {
+    REMARK42: any;
+    remark_config: any;
+  }
+}
+
+// This function will insert the usual <script> tag of
+// Remark42 into the specified DOM location (parentElement)
+const insertScript = (id: string, parentElement: HTMLElement) => {
+  const script = window.document.createElement('script');
+  script.type = 'text/javascript';
+  script.async = true;
+  script.id = id;
+  /* For Gatsby it's important to manually provide the URL
+  and make sure it does not contain a trailing slash ("/").
+  Because otherwise the comments for paths with/without 
+  the trailing slash are stored separately in the BoltDB database.
+  When following a Gatsby Link a page is loaded without the trailing slash,
+  but when refreshing the page (F5) it is loaded with the trailing slash.
+  So essentially every URL can become duplicated in the DB and you may not see
+  your comments from the inverse URL at your present URL.
+  Making sure url is provided without the trailing slash
+  in the remark42 config solves this. */
+  let url = window.location.origin + window.location.pathname;
+  if (url.endsWith('/')) {
+    url = url.slice(0, -1);
+  }
+  // Now the actual config and script-fetching function:
+  script.innerHTML = `
+    var remark_config = {
+      host: "https://remark42.example.com",
+      site_id: "example-name",
+      url: "${url}",
+      theme: "dark",
+      components: ["embed"],
+    };
+    !function(e,n){for(var o=0;o<e.length;o++){var r=n.createElement("script"),c=".js",d=n.head||n.body;"noModule"in r?(r.type="module",c=".mjs"):r.async=!0,r.defer=!0,r.src=remark_config.host+"/web/"+e[o]+c,d.appendChild(r)}}(remark_config.components||["embed"],document);`;
+  parentElement.appendChild(script);
+};
+
+/* This function removes the previously added script from the DOM.
+Might be necessary when page transitions happen, to make sure a new instance 
+is created, pointing to the current URL. Although this might actually 
+not be necessary, please do your own testing. */
+const removeScript = (id: string, parentElement: HTMLElement) => {
+  const script = window.document.getElementById(id);
+  if (script) {
+    parentElement.removeChild(script);
+  }
+};
+
+// This function will be provided to useEffect and will insert the script
+// when the component is mounted and remove it when it unmounts
+const manageScript = () => {
+  if (!window) {
+    return () => {};
+  }
+  const { document } = window;
+  if (document.getElementById('remark42')) {
+    insertScript('comments-script', document.body);
+  }
+  return () => removeScript('comments-script', document.body);
+};
+
+/* Another function for another useEffect - this is the most crucial part for
+Gatsby compatibility. It will ensure that each page loads its own appropriate
+comments, and that comments will be properly loaded */
+const recreateRemark42Instance = () => {
+  if (!window) {
+    return;
+  }
+  const remark42 = window.REMARK42;
+  if (remark42) {
+    remark42.destroy();
+    remark42.createInstance(window.remark_config);
+  }
+};
+
+type CommentsProps = {
+  location: string;
+};
+
+// The location prop is {props.location.pathname} from the parent component.
+// I.e. invoke the component like this in the parent: <Comments location={props.location.pathname} />
+export function Comments({ location }: CommentsProps) {
+  // Insert the two useEffect hooks. Maybe you can combine them into one? Feel free if you want to.
+  useEffect(manageScript, [location]);
+  useEffect(recreateRemark42Instance, [location]);
+
+  return (
+    <Fragment>
+      <h2>Comments</h2>
+      {/* This div is the target for actual comments insertion */}
+      <div id="remark42" />
+    </Fragment>
+  );
+}
+
+```