为什么在此代码中计数已减少？

Question

Why in this code count has been decremented first before deleting the last element of storage object? In this way ,will it not delete the second last element instead of last?

var stack = function () {
  this.count = 0;
  this.storage = {};
  this.push = function (value) {
    this.storage[this.count] = value;
    this.count++;
  }
  this.pop = function () {
    if (this.count === 0) {
      return undefined;
    }
    else {
      this.count--;
      var result = this.storage[this.count];
      delete this.storage[this.count];
      return result;
    }
  }
}

Answer 1

In (most) programming languages, arrays are zero-based.

So, for ['foo'] , count will be 1 , but 'foo' is at index 0 .

So, the last element in an array will always be at index array.length - 1 .

---

That said, if you make this.storage an array, the whole else block can be replaced.

Since this.storage acts as an array any way, make it an array:

this.storage = [];

Then you can use:

else {
  this.count--;
  return this.storage.pop();
}

Array.prototype.pop removes the last element from the array, and returns said element.

Answer 2

因为数组的索引为0，所以第一个元素存储在第0个索引处，第2个元素存储在第1个索引处，依此类推

Answer 3

Count等于数据结构中第一个空闲位置的索引，然后在当前count处进行加法，此后递增，并通过对称性递减，以指向已释放的最后一个元素，因此count指向最后释放的位置。