Javascript 中字符串的 Switch-Case 未按预期工作
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Switch-Case for strings in Javascript not working as expected
提问by Coltin
So I have this problem with strings and switch-case, and I'll try to keep it as simple as possible.
所以我对字符串和 switch-case 有这个问题，我会尽量保持简单。
Here event.keyCode has the value "65", and is the result of a keydown event of 'a' (using JQuery).
这里 event.keyCode 的值为“65”，是 'a' 的 keydown 事件的结果（使用 JQuery）。
if (event.keyCode == "65") {
alert("hmmmm");
}
That works, but:
这有效，但是：
switch (event.keyCode) {
case '65':
alert("Yay!");
break;
}
That doesn't. However this will work:
那没有。但是，这将起作用：
switch ('65') {
case '65':
alert("Yay!");
break;
}
And if I do this:
如果我这样做：
var t = '65';
switch (t) {
case '65':
alert("Yay!");
break;
}
It works. And then I tried this:
有用。然后我尝试了这个：
var t = event.keyCode;
switch (t) {
case '65':
alert("Yay!");
break;
}
But it fails!
但它失败了！
So why does it match in the if-block at the beginning, but not for the switch-case?
那么为什么它在开头的 if 块中匹配，而在 switch-case 中不匹配？
回答by Matthew Flaschen
keyCodeis 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:
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，开关使用等效的===，它不允许隐式转换。您可以使用以下方法轻松测试：
switch (65) {
case '65':
alert("Yay!");
break;
}
As expected, it does not alert.
正如预期的那样，它不会发出警报。
This is stated in ECMAScript, 5th edition section 12.11 (switch statement). The interpreter will enter a case statement if "inputis equal to clauseSelectoras defined by the === operator". inputis 65 (integer) and clauseSelector is '65' (string) in my above example, which are not ===.
这在ECMAScript，第 5 版第 12.11 节（switch 语句）中有说明。如果“input等于clauseSelector由 === 运算符定义”，解释器将输入 case 语句。 input在我上面的例子中是 65（整数），而 clauseSelector 是 '65'（字符串），它们不是===.
