StackOverflow2013

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plurals

PO
primarykey
Id
17265764
data
AcceptedAnswerId
0
AnswerCount
0
ClosedDate
CommentCount
0
CommunityOwnedDate
CreationDate
2013-06-23T22:03:04.207
FavoriteCount
0
LastActivityDate
2013-06-23T22:16:37.707
LastEditDate
2013-06-23T22:16:37.707
LastEditorUserId
1202830
OwnerUserId
1202830
ParentId
17265706
PostTypeId
2
Score
7
ViewCount
0
LastEditorDisplayName
text
Body
There is a difference between an existing variable that happens to contain the <code>undefined</code> value, and a variable that does not exist at all. If a variable name doesn't exist, then it's an error to try to reference it. The <code>typeof</code> operator is a special case: it does accept a name even if there is no such variable of that name. But only when the name is actually used with the <code>typeof</code> operator. In your function example, you use a name that does not exist, and there's no <code>typeof</code> operator at the point where the name is used. That's where the error occurs. It doesn't matter that you are passing the name into a function that will use <code>typeof</code>; that function never gets called because the error has already happened. Your function would work as expected if you give it an existing variable name. Then it would tell you if that variable has the <code>undefined</code> value or not: <pre><code>var boo; // now boo exists but has the undefined value console.log( isUndefined(boo) ); // will log 'true' </code></pre> If you were checking a global variable, it would work if you said <code>window.boo</code> instead of just <code>boo</code>. That's because it is not an error to reference a nonexistent property of an object; it just gives you the <code>undefined</code> value when you do this: <pre><code>// logs 'true' if there is no global 'boo' console.log( isUndefined(window.boo) ); </code></pre> That wouldn't work if you wanted to check whether a local variable exists, though, because it won't be a property of the <code>window</code> object.
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1. This table or related slice is empty.
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1. POUnderstanding functions and undefined
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1. PTAnswer
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1. USMichael Geary
UserOwnerUserId
1. USMichael Geary
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