javascript 检查 === 未定义时未捕获的 ReferenceError（未定义）
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Uncaught ReferenceError (not defined) when checking === undefined
提问by WBT
I have the following code:
我有以下代码：
simpleExample.html:
simpleExample.html：
<!DOCTYPE html>
<html lang="en">
<head>
<title>Simple example</title>
</head>
<body>
Open the Console.
<script src="js/simpleExampleJS.js"></script>
</body>
</html>
js/simpleExampleJS.js:
js/simpleExampleJS.js：
MyObject = {
COMPUTER_GREETING: "Hello World!",
hello: function() {
console.log(MyObject.COMPUTER_GREETING);
}
};
checkSomeGlobal = function() {
if(someGlobal === undefined) {
console.log("someGlobal is undefined & handled without an error.");
} else {
console.log("someGlobal is defined.");
}
};
MyObject.hello();
checkSomeGlobal();
When I run this, I get:
当我运行这个时，我得到：
Hello World!
Uncaught ReferenceError: someGlobal is not defined
at checkSomeGlobal (simpleExampleJS.js:9)
at simpleExampleJS.js:17
(The first line of output generally indicates that the code is loading and running).
（第一行输出一般表示代码正在加载和运行）。
MDN indicatesthat a potentially undefined variable can be used as the left-hand-size of a strict equal/non-equal comparison. Yet when checkingif(someGlobal === undefined)that line of code produces an errorbecause the variable is undefined, instead of making the comparison evaluate to true. How can I check for and handle this undefined variable case without an error?
MDN表明一个潜在的未定义变量可以用作严格相等/不等比较的左侧大小。然而，在检查if(someGlobal === undefined)该行代码时会产生错误，因为变量未定义，而不是使比较评估为true。如何检查和处理这个未定义的变量情况而不会出错？
回答by SLaks
That error is saying that there is no such variable (it was never declared), not that its value is undefined.
该错误是说没有这样的变量（它从未被声明过），而不是它的值是undefined.
To check whether a variable exists, you can write typeof someGlobal
要检查变量是否存在，您可以编写 typeof someGlobal
