Conveniently, all three of the major JS engines can be extracted from the browsers they are developed for, and used in other projects. Node and Bun famously use

Conveniently, all three of the major JS engines can be extracted from the browsers they are developed for, and used in other projects. Node and Bun famously use V8 and the WebKit one, and Servo I believe embeds SpiderMonkey. If you want to start a new browser project, and you're not interested in writing a JS engine from scratch, there are three off-the-shelf options there to choose from.