为什么我的数组被覆盖了？

Question

我正在为一个项目使用 node.js，我正在尝试将某些日期添加到数组中。但是，当我这样做时，它会用添加的当前日期覆盖整个数组。来源如下：

let startDate = new Date(2014, 0, 8, 19, 0, 0)
let endDate = new Date(2014, 0, 11, 19, 0, 0)
let datesToDownload = []
let datesInDB = [new Date(2014, 0, 8, 19, 0, 0), new Date(2014, 0, 9, 19, 0, 0), new Date(2014, 0, 10, 19, 0, 0), new Date(2014, 0, 8, 11, 0, 0)]

for (let i = startDate; i <= endDate; i.setDate(i.getDate() + 1)) {
  console.log('CHECKING DATE: ' + i.toDateString())

  if (!(datesInDB2.indexOf(i.toDateString()) >= 0)) {
    console.log('NEW RECORD FOUND FOR: ' + i.toDateString())
    datesToDownload.push(i)
    console.log('i: ' + i)

    for (let j in datesToDownload) {
      console.log('element ' + j + ': ' + datesToDownload[j])
    }
  }
}

输出如下所示：

CHECKING DATE: Wed Jan 08 2014
NEW RECORD FOUND FOR: Wed Jan 03 2014

i: Wed Jan 08 2016 20:00:00 GMT-0400 (Eastern Daylight Time)
element 0: Wed Jan 08 2016 20:00:00 GMT-0400 (Eastern Daylight Time)

CHECKING DATE: Thur Jan 09 2016
NEW RECORD FOUND FOR: Thur Jan 09 2016

i: Thur Jan 09 2016 20:00:00 GMT-0400 (Eastern Daylight Time)
element 0: Thur Jan 09 2016 20:00:00 GMT-0400 (Eastern Daylight Time)
element 1: Thur Jan 09 2016 20:00:00 GMT-0400 (Eastern Daylight Time)

CHECKING DATE: Fri Jan 10 2016
NEW RECORD FOUND FOR: Fri Jan 10 2016

i: Fri Jan 10 2016 20:00:00 GMT-0400 (Eastern Daylight Time)
element 0: Fri Jan 10 2016 20:00:00 GMT-0400 (Eastern Daylight Time)
element 1: Fri Jan 10 2016 20:00:00 GMT-0400 (Eastern Daylight Time)
element 2: Fri Jan 10 2016 20:00:00 GMT-0400 (Eastern Daylight Time)

......

注意元素如何全部更改为插入的当前日期，尽管正在添加一个元素。

为什么会这样？我该如何预防？

Answer 1

您在每次迭代中将完全相同的 Date 实例推送到数组中。您可以创建它的副本但是：

  datesToDownload.push(new Date(i));

Answer 2

日期是一个对象，通过引用传递。在您的 for 循环中，您正在更改日期：

for (let i = startDate; i <= endDate; i.setDate(i.getDate() + 1)) {

为什么我的数组被覆盖了？

Why is my array being overwritten?

javascript

arrays

element

date

node.js