Javascript
Javascript is the only first class citizen language of the browser. It has also in recent years become a prime player in backend servers, thanks to NodeJS.
NodeJS
NodeJS is the most common server runtime for javascript to run javascript outside the browser. This has made Javascript into a powerful community in the realm of backend server programming for the web since now programming web app frontends & backends can be done in the same language.
Jest
Jest is perhaps the most common testing library for the Javascript & Typescript ecosystems. More detailed information can be found in the jest page.