i am not a web dev, so please correct me if i am wrong, but a runtime is essentially a specific version of browser in some capacity, more comparable to blink in some sense (minus the html+css parts). so there is a js engine (v8 or firefox’s spider if i am not wrong) and that essentially just executes the js, but it needs stuff around it (think for example io or cli interface). none of the run times implement independent js engines, as they are really hard, but surrounding stuff is relatively easier, and likes of deno/node/bun implement that.

And as other commenter said - it has other stuff too, for example a package manager, transpiler (most common would be converting typescript to js i think), bundler (just makes a bundle kinda like java jar), etc.

A bundler, a transpiler, a runtime (designed to be a drop-in replacement for Node.js), test runner, and a package manager - all in one.

Bun’s single-file executables turned out to be perfect for distributing CLI tools. You can compile any JavaScript project into a self-contained binary—runs anywhere, even if the user doesn’t have Bun or Node installed. Works with native addons. Fast startup. Easy to distribute.

It’s more of a node.js competitor, but it doesn’t use v8, it uses safaris js engine. https://bun.com/blog/how-bun-supports-v8-apis-without-using-v8-part-1