direct-proxy.js

let NegativeIndices = (array) => {
  return new Proxy(array, {
    get: (receiver, name) => {
      let index;
      console.log('Proxy#get', array, name);
      index = parseInt(name);
      if (!isNaN(index) && index < 0) {
        return array[array.length + index];
      } else {
        return array[name];
      }
    }
  });
};

/*
* Negative array indices:
* array = NegativeIndices [4, 420, 42]
* array[-1] is 42
*/