扩展内建类

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extends Array

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class PowerArray extends Array {
  isEmpty() {
    return this.length === 0;
  }

  // filter/map/... 等函数不再传递PowerArray
  static get [Symbol.species] () {
    return Array;
  }
}

let arr = new PowerArray(1, 2, 3);
alert(arr.isEmpty()); // false

let filter_array = arr.filter(i => i > 1);
alert(filter_array.isEmpty()); //Error