Javascript中的字符串切换案例无法按预期工作 [英] Switch-Case for strings in Javascript not working as expected

问题描述

所以我对字符串和switch-case有这个问题，我会尽量保持它的简单。

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有值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");
}

这样可行，但是：

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

那不是。但是这样可行：

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，而不是在switch-case中匹配？

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

推荐答案

keyCode 是一个整数，而不是一个字符串。当您使用 == 时，转换是隐式完成的。但是，交换机使用等价的 === ，这不允许隐式转换。您可以使用以下方法轻松测试：

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.

ECMAScript ，第5版第12.11节（转换声明）。如果 input 等于 clauseSelector ，则由===运算符定义，解释器将输入case语句。在上面的例子中，输入是65（整数）和clauseSelector是'65'（字符串），它不是 === 。

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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