(obj) => { obj.x = 5; return obj.x; }

[1,2,3]

(arr) => { arr[1] = {}; return arr[2]; }

{"arr":[1,2,3]}

(obj) => { obj.arr.push(4); return 42; }

{"x":2,"y":2}

(obj) => { return Object.keys(obj); }