var arr = [1, 2, 3, 'a', 'b', 'c', 'd'];
Array.prototype.copySlice =function() {
  var newArr = [];
  var len = this.length;
  var argLen = arguments.length;
  if(arguments.length == 1) {//一個參數(shù)
    var startNum = arguments[0] > 0 ? arguments[0] : (len + arguments[0]);
    for(var i = startNum; i < len; i++) {
      newArr.push(arr[i]);
    }
  }
  else if(arguments.length == 2) {//兩個參數(shù)
    var startNum = arguments[0] > 0 ? arguments[0] : (len + arguments[0]);
    var endNum = arguments[1] > 0 ? arguments[1] : (len + arguments[1]);
    if(startNum >= endNum) {//起始索引大于終止索引，返回[]
      return newArr;
    }
    else {
      for(var i = startNum; i < endNum; i++) {
        newArr.push(arr[i]);
      }
    }
  }
  return newArr;
};
console.log(arr.length); // 7
// 一個參數(shù)
console.log(arr.copySlice(2)); // [3, "a", "b", "c", "d"]
// 兩個參數(shù)
console.log(arr.copySlice(3, 6));  //["a", "b", "c"]
console.log(arr);  //[1, 2, 3, "a", "b", "c", "d"]
// 接收負數(shù)
console.log(arr.copySlice(-2)); // ["c", "d"]
console.log(arr.copySlice(-5, 6)); //[3, "a", "b", "c"]
//結束位置小于起始位置，返回空數(shù)組。
console.log(arr.copySlice(-5, -6)); //[]
console.log(arr.copySlice(5, 5)); //[]
console.log(arr.copySlice(5, )); // ["c", "d"]