Javascript 中字符串的 Switch-Case 未按预期工作

2022-01-19 00:00:00 string switch-statement jquery javascript

所以我在字符串和 switch-case 方面遇到了这个问题，我会尽量保持简单.

So I have this problem with strings and switch-case, and I'll try to keep it as simple as possible.

这里 event.keyCode 的值为65"，是a"的 keydown 事件的结果(使用 JQuery).

Here event.keyCode has the value "65", and is the result of a keydown event of 'a' (using JQuery). 

if (event.keyCode == "65") {
   alert("hmmmm");
}

这行得通，但是:

switch (event.keyCode) {
   case '65':
      alert("Yay!");
      break;
}

那不是.但是，这将起作用:

That doesn't. However this will work:

switch ('65') {
   case '65':
      alert("Yay!");
      break;
}

如果我这样做:

var t = '65';
switch (t) {
   case '65':
      alert("Yay!");
      break;
}

它有效.然后我尝试了这个:

It works. And then I tried this:

var t = event.keyCode;
switch (t) {
   case '65':
      alert("Yay!");
      break;
}

但它失败了！

那么为什么它在开头的 if-block 中匹配，但在 switch-case 中却不匹配?

So why does it match in the if-block at the beginning, but not for the switch-case?

推荐答案

keyCode 是整数，而不是字符串.当您使用 == 时，转换是隐式完成的.但是，该开关使用 === 的等效项，它不允许隐式转换.您可以通过以下方式轻松测试:

keyCode is an integer, not a string. When you use ==, the conversion is done implicitly. However, the switch uses the equivalent of ===, which doesn't allow implicit conversions. You can test this easily with:

switch (65) {
   case '65':
      alert("Yay!");
      break;
}

正如预期的那样，它不会发出警报.

As expected, it does not alert.

这在 ECMAScript 中有说明，第 5 版第 12.11 节(switch 语句).如果input 等于 === 运算符定义的 clauseSelector"，解释器将输入 case 语句.在我上面的示例中，input 是 65(整数)，而 ClauseSelector 是 '65'(字符串)，它们不是 ===.

This is stated in ECMAScript, 5th edition section 12.11 (switch statement). The interpreter will enter a case statement if "input is equal to clauseSelector as defined by the === operator". input is 65 (integer) and clauseSelector is '65' (string) in my above example, which are not ===.

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