Javascript how to detect if a variable exists

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Document Subject: Javascript how to detect if a variable exists
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How to check if a javascript variable exists before using it and causing an error.

You can use http://notesninjas.com/#JSFieldExist to test to see if a field exists on a form, but sometimes you need to check to see if a variable exists.

Using

if (varname) alert("exists")

doesn't work.

You need to use typeof.

if (typeof(varname)!="undefined") alert("exists") else alert("doesnt exist')

you can also test for number string bookean object or null

From http://www.javascriptkit.com/javatutors/determinevar2.shtm