app is the actual Chrome extension with the manifest.json file, image assets, and the compiled javascript code.
src contains the TypeScript code which gets compiled to the JavaScript code in app.
npm run build to build with Webpack, which uses ts-loader for TypeScript.
Webpack has three entry points: background.ts, content-script.ts, and popup/popup.tsx. It compiles all dependencies into respective .js files in the app directory.
background.js is a service worker which runs on extension activation and calls chrome.declarativeContent.ShowPageAction() when specific websites are visited. ShowPageAction() displays popup.html and the corresponding .js and .css files.
Content on individual websites is read by use of a content script, which is injected into pages that match the given criteria. Content scripts run independently (inside the browser tab) from the extension and therefore can only communicate back to the extension by use of message passing.
When the popup page opens, it sends a message (get-media-title) to the content script, which responds with the detected media title if it is available.
Likewise, if a user clicks the login button, popup will send a message (do-login) which is received by the background service worker to start the login flow.
The popup page cannot be relied upon to do asynchronous work if there is a chance it will close (i.e. during login). When the popup page closes, code execution stops.
On title not found, the Anilist API responds with a 404 status code, which causes an error to be displayed in the Javascript console. There is apparently no way to disable this.