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Javascript Array unshift new Date減去1天

JavaScript

慕桂英3389331 2021-06-08 13:01:39

我從以下代碼中得到了一些奇怪的結(jié)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)"相同的代碼第 1 次和第 2 次運行，但第 3 次運行給出了意想不到的結(jié)果 - 應(yīng)該是 2019 年 5 月 30 日星期四，而不是 2019 年 6 月 30 日有人能告訴我我在這里做錯了什么嗎？

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2 回答

慕慕森

TA貢獻(xiàn)1856條經(jīng)驗獲得超17個贊

最里面的new Date()總是在六月創(chuàng)建一個 Date 實例。當(dāng)您將月份日期設(shè)置為 30 時，您將日期強(qiáng)制為 6 月 30 日，而不是 5 月 30 日。

調(diào)用.setDate() 可以更改月份，但僅當(dāng)月份中的某天沒有意義時，或者更?。慊蜇?fù)）或更大（如 33）。由于 30 確實是六月中的真實日子，因此月份不會改變。

反對回復(fù) 2021-06-18

隔江千里

TA貢獻(xiàn)1906條經(jīng)驗獲得超10個贊

在這里，我將您的代碼修改為您想要的反應(yīng)：

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());

反對回復(fù) 2021-06-18