Checking whether a variable is set in javascript

Question

I want a JS function that says whether a variable is set. I've defined isset() below.

It behaves strangely as described in the comments. Any ideas?

<html>
    <head></head>
    <body>
        <script type="text/javascript">

          function isset(variableName) {
            return typeof variableName != 'undefined';
          }

          a = 5;
          console.log(isset(a)); 
          console.log(isset(b)); // returns an error.
        </script>
    </body>
</html>

variableName is not the same as varname. Why try to access an undeclared variable? — user1106925
– user1106925, Commented Apr 5, 2013 at 21:45
Since you changed the code, change your code comment too since it should now return true. — user1106925
– user1106925, Commented Apr 5, 2013 at 21:47

ThiefMaster · Accepted Answer · 2013-04-05 22:52:45Z

5

You need to perform the typeof check directly, not inside a function.

The reason is exactly the same one like in PHP (where your function would also break unless you used pass-by-reference):

isset(b) results in an exception being thrown because b is an undefined variable and passing it as an argument tries to read it which you cannot do. So the typef check is never executed.

edited Apr 5, 2013 at 22:52

answered Apr 5, 2013 at 21:44

ThiefMaster

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1 Comment

Don P Over a year ago

But inside of my isset() function, isn't there a local variable "a" with a value of 5?

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