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Asked: 2026-05-31T19:21:05+00:00

Here is my code: <!DOCTYPE html PUBLIC -//W3C//DTD XHTML 1.0 Transitional//EN http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd> <html xmlns=http://www.w3.org/1999/xhtml>

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Here is my code:

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

<html xmlns="http://www.w3.org/1999/xhtml">
<head>
    <title>
        JS Test
    </title>
        <script type="text/javascript" src="jquery-1.4.2.min.js"></script>
        <script type="text/javascript">
        if(someObj != null || typeof someObj != 'undefined')
        {
            alert("Success.");
        }
        else
        {
            alert("Failed.");
        }
    </script>
</head>
<body>

Why it always return an error message-“someObj is not defined” to me?
I want to put the codes to foot of page to check whether the someObj has been declared or not.

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1 Answer

  1. Editorial Team

    Do it the other way around:

        if(typeof someObj != 'undefined' && someObj != null)
    {
        alert("Success.");
    }
    else
    {
        alert("Failed.");
    }

    That is, don’t try to actually use someObj until after you are sure it has been defined as confirmed with typeof.

    If in fact the variable isn’t defined, when you start your if condition with someObj != null you get the error “someObj is not defined” because – well, it isn’t defined.

    If you first test it with typeof then it won’t do the null comparison unless it actually is defined.

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