A JavaScript engine is the browser component that parses and executes webpage scripts. It turns code from the page into actions the browser can perform, such as changing content, making requests, or running interactive features. Many engines also use just-in-time (JIT) compilation to speed up execution by converting hot script paths into machine code while the page is running.
This component matters because it sits on a high-value attack surface: bugs in script parsing, JIT optimization, or memory handling can let a malicious webpage trigger crashes, data leaks, or even code execution in the browser process. Defenders treat engine updates as critical, because privacy-focused browsers often inherit the same engine code and vulnerabilities from upstream projects. In practice, reducing JavaScript exposure and keeping the browser updated are common ways to limit risk from engine flaws.