Javascript Array unshift new Date减去1天

Question

我从以下代码中得到了一些奇怪的结果：a = [];a[0] = new Date();console.log("1 Element Added: "+a.length + " - " + a.toString());//"1 Element Added: 1 - Sun Jun 02 2019 12:13:35 GMT-0400 (Eastern Daylight Time)"a.unshift(new Date(new Date(new Date().setDate(a[0].getDate() - 1))));console.log("First Unshift: "+a.length + " - " + a.toString());//"First Unshift: 2 - Sat Jun 01 2019 12:13:35 GMT-0400 (Eastern Daylight Time),Sun Jun 02 2019 12:13:35 GMT-0400 (Eastern Daylight Time)"a.unshift(new Date(new Date(new Date().setDate(a[0].getDate() - 1))));console.log("Second Unshift: "+a.length + " - " + a.toString());//"Second Unshift: 3 - Fri May 31 2019 12:13:35 GMT-0400 (Eastern Daylight Time),Sat Jun 01 2019 12:13:35 GMT-0400 (Eastern Daylight Time),Sun Jun 02 2019 12:13:35 GMT-0400 (Eastern Daylight Time)"a.unshift(new Date(new Date(new Date().setDate(a[0].getDate() - 1))));console.log("Third Unshift: "+a.length + " - " + a.toString());//"Third Unshift: 4 - Sun Jun 30 2019 12:13:35 GMT-0400 (Eastern Daylight Time),Fri May 31 2019 12:13:35 GMT-0400 (Eastern Daylight Time),Sat Jun 01 2019 12:13:35 GMT-0400 (Eastern Daylight Time),Sun Jun 02 2019 12:13:35 GMT-0400 (Eastern Daylight Time)"相同的代码第 1 次和第 2 次运行，但第 3 次运行给出了意想不到的结果 - 应该是 2019 年 5 月 30 日星期四，而不是 2019 年 6 月 30 日有人能告诉我我在这里做错了什么吗？

Answer 1

最里面的new Date()总是在六月创建一个 Date 实例。当您将月份日期设置为 30 时，您将日期强制为 6 月 30 日，而不是 5 月 30 日。

调用.setDate() 可以更改月份，但仅当月份中的某天没有意义时，或者更小（零或负）或更大（如 33）。由于 30 确实是六月中的真实日子，因此月份不会改变。

Answer 2

在这里，我将您的代码修改为您想要的反应：

a = [];

a[0] = new Date();

console.log("1 Element Added: "+a.length + " - " + a.toString());

//"1 Element Added: 1 - Sun Jun 02 2019 12:13:35 GMT-0400 (Eastern Daylight Time)"

a.unshift(new Date(a[0]));

a[0].setDate(a[0].getDate()-1);

console.log("First Unshift: "+a.length + " - " + a.toString());

//"First Unshift: 2 - Sat Jun 01 2019 12:13:35 GMT-0400 (Eastern Daylight Time),Sun Jun 02 2019 12:13:35 GMT-0400 (Eastern Daylight Time)"

a.unshift(new Date(a[0]));

a[0].setDate(a[0].getDate()-1);

console.log("Second Unshift: "+a.length + " - " + a.toString());

//"Second Unshift: 3 - Fri May 31 2019 12:13:35 GMT-0400 (Eastern Daylight Time),Sat Jun 01 2019 12:13:35 GMT-0400 (Eastern Daylight Time),Sun Jun 02 2019 12:13:35 GMT-0400 (Eastern Daylight Time)"

a.unshift(new Date(a[0]));

a[0].setDate(a[0].getDate()-1);

console.log("Third Unshift: "+a.length + " - " + a.toString());