Explain what a single page app is and how to make one SEO-friendly

A single page application (SPA) is a web application that loads a single HTML document and updates its content in the browser via JavaScript, rather than requesting a new page from the server on each navigation. This model provides application-like UX but presents challenges for search engine indexing because the initial HTML does not contain the rendered content.

In current practice, SPAs are made SEO-friendly by producing HTML on the server rather than relying solely on client-side rendering. The available strategies are server-side rendering (SSR), static site generation (SSG), incremental static regeneration (ISR), and streaming with React Server Components. Each strategy offers a different tradeoff between freshness, server cost, and time to first paint, and modern frameworks such as Next.js, Nuxt, Remix, SvelteKit, and Astro support selecting the strategy per route.