11-array-proxy.js

/* UTILS */

const getAverageScore = (score) =>
  score.length ? score.reduce((acc, point) => acc + point) / score.length : 0;

/* PROXY */

const handleSet = (obj, prop, value) => {
  const isNumber = !isNaN(parseInt(prop));

  if (isNumber) {
    const index = parseInt(prop);
    console.log(index);

    if (index >= 10) {
      throw new RangeError("Array limit reached");
    }

    Reflect.set(obj, prop, value);
  }

  Reflect.set(obj, prop, value);
  return true;
};

const handleGet = (obj, prop) => {
  if (prop === "average") {
    return getAverageScore(obj);
  }

  if (prop in obj) {
    return Reflect.get(obj, prop);
  }
};

const createArrayProxy = (arr) => {
  const proxy = new Proxy(arr, {
    set: handleSet,
    get: handleGet
  });

  return proxy;
};

/* INIT */

let arr = createArrayProxy([]);