Runtime JavaScript execution is code that runs inside an application while the app is already running, instead of being fixed only at build time. In Electron apps, this can include preload scripts, renderer logic, or dynamically loaded modules that change behavior after installation.
This matters in cyber security because runtime code can be altered, injected, or abused without changing the visible app shell. A utility may still convert files or open documents normally while hidden JavaScript performs extra actions in the background, such as reaching sensitive APIs, handling stolen data, or enabling screen capture. Defenders look for this risk by reviewing preload scripts, enforcing context isolation, validating IPC messages, and limiting access to privileged features. The key lesson is that a working interface does not guarantee safe code paths underneath it.