Prototype and for...in

wat.js

// using ES5, but still relevant

var map = {
    a: 1,
    b: 2
};
Object.prototype.thisIsReallyReallyPrivate = 'foo'

for (var key in map) {
    console.log(key);
    // a
    // b
    // thisIsReallyReallyPrivate
}

// instead use:
function newMap() {return Object.create(null)}
map = newMap();
map.a = 1;
map.b = 2;

for (var key in map) {
    console.log(key);
    // a
    // b
}