Object-Array-Proxy.js

var flatten = arr =>
  arr.reduce(
    (acc, val) => acc.concat(Array.isArray(val) ? flatten(val) : val),
    [],
  );

var o = [];

var p = new Proxy(o, {
  set(obj, id, value) {
    obj[id] = flatten(value);
    return obj;
  },
});

p.arr = arr;