javascript - 如何使用javascript改变当前变量？

Question

我有一个小的 html 页面，如果用户单击其中一个按钮，它将更改日期。我有一个功能，它显示今天是星期几，但我希望用户能够按“>”，这会将日期从今天更改为第二天。如果按“<”，则为前一天。我不完全确定现在应该如何进行。

var date = new Date();
var days = ["Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday"];
document.getElementById("todayField").innerHTML = days[date.getDay()];

function next() {
  var date = new Date();
  var days = ["Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday"];
  document.getElementById("todayField").innerHTML = days[date.getDay()];

  if (btnNext.click) {
    days++;
    document.getElementById("todayField").innerHTML = days[date.getDay()];
  }
}

function prev() {
  var date = new Date();
  var days = ["Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday"];
  document.getElementById("todayField").innerHTML = days[date.getDay()];

  if (btnNext.click) {
    days--;
    document.getElementById("todayField").innerHTML = days[date.getDay()];
  }
}

<p>Click to change day</p>
<button type="button" name="btnPrev" onclick="next()"><</button>
<button type="button" name="btnNext" onclick="prev()">></button>
<p id="todayField"></p>

感谢所有帮助!

Answer 1

我给你准备了一把 fiddle :https://jsfiddle.net/k8g1mros/

这背后的逻辑是:

var date = new Date();
var index = date.getDay();

function prev() {
  var days = ["Sunday","Monday","Tuesday","Wednesday","Thursday","Friday","Saturday"];
  var nextIndex = index - 1 < 0 ? 6 : index-1;
  index = nextIndex
  console.log(nextIndex)
  document.getElementById("todayField").innerHTML = days[nextIndex];
}
document.getElementById("prev").addEventListener("click", function() {
  prev();
})

一些解释:

首先，您可以使用 var index=date.getDay() 获取实际日期。通过单击下一个或上一个此索引将向上/向下计数。一旦到达 0(周日)或 6(周六)，索引将重置为 0/6。

这发生在这里(对于上一个):

var nextIndex = index - 1 < 0 ? 6 : index-1;

这称为内联条件。进一步阅读:

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Conditional_Operator

How to write an inline IF statement in JavaScript?

这相当于:

var nextIndex = 0
if (index - 1 < 0 ) {
  nextIndex = 0
} else {
  nextIndex = index - 1
}

现在你明白背后的逻辑了吗？