feature instead of app

"killer feature" makes more sense in this context IMHO.

src/documentation/0029-node-event-loop/index.md

@@ -10,7 +10,7 @@ category: learn
 
 The **Event Loop** is one of the most important aspects to understand about Node.js.
 
-Why is this so important? Because it explains how Node.js can be asynchronous and have non-blocking I/O, and so it explains basically the "killer app" of Node.js, the thing that made it this successful.
+Why is this so important? Because it explains how Node.js can be asynchronous and have non-blocking I/O, and so it explains basically the "killer feature" of Node.js, the thing that made it this successful.
 
 The Node.js JavaScript code runs on a single thread. There is just one thing happening at a time.