FlexSearch is a superb, browser-based search engine. It supports a non-blocking asynchronous processing model using Web workers and works great with Svelte/Vue/React and static site builders like Eleventy and Gatsby. It provides the search engine for the blog you're reading.
The notion of an effective browser-based search engine is a little eye-brow raising. Your truly have to see FlexSearch in action to believe what it is—it is highly performant. But don't take my word for it, get this Git repo and run the example on your box. On my PC, FlexSearch indexed 5000 documents in about 700ms. For many sites, FlexSearch is a worthy alternative to Algolia, ElasticSearch, and TypeSense.
This set of articles shows how to get FlexSearch up and running with Eleventy. However, most of what's presented here works with any Node/JS/TS-based site.