Lesson 11 — Config merge

Layers of settings

Real apps combine settings from several layers: built-in defaults, then a config file, then environment variables. Later layers win. The spread syntax { ...a, ...b } copies a and then b into a new object, so keys in b overwrite keys in a.

{ ...{ port: 3000 }, ...{ port: 8080 } }   // { port: 8080 }

Why it matters: missing keys fall back safely, and you can always override one setting without copying the rest.