as you maybe know, that if you use a complex structure in JS it's passed as a reference.

var ref = {something : 0};
console.log(ref);
function mutable(ref) {
 ref['something'] = 1;
}

mutable(ref);
console.log(ref);

function breakReference(ref) {
 var localRef = JSON.parse(JSON.stringify(ref));
 localRef['something'] = 2;
}

console.log(ref);

this is one classic side-effect that happens in a lot of apps because developers don't understand that references are mutable.

this is just why you should care about it or at least you should know what it does.

Amn: Immutability in this case means .... you pass any parameter and it stays the same on the outside despite changes on the inside and the other way around.