How to find out if a variable has been defined
It’s two articles now, it must be a series! Here’s how to find the out if a variable has been defined in my three favorite languages:
PHP
/* v has not been declared */ if (isset($v)) echo "v has been defined"; else echo "v has _not_ been defined";
JavaScript
Please note that in javascript a runtime error would be generated when trying to access a variable that has not been defined. However, any variable that you create will automatically become a property of the window object. Therefor, checking if the variable has been added to the scope of window will result in a condition that can be checked. Also note that this will only work in a browser, which of course is how most people experience javascript anyway.
As an example: “window.doesnotexist==null” results in true. Be careful not to overwrite stuff that already does exist in the window.
/* v has not been declared */
if (window.v)
alert("v has been defined");
else
alert("v has _not_ been defined");
(Visual) Basic
In Visual Basic you will only be able to use this code if “option explicit” is off, meaning in VBScript, VBA and VB6.0 there should not be a line on the top which says “Option Explicit”. In the .NET environment there is a setting for this in the options dialog (project properties). In the case Option Explicit is on, variables will always be assigned a default value, so you can never check e.g. if an integer variable has been assigned a value of 0.
You can either check using the typename function or using the isEmpty function.
As an example: Debug.Print TypeName(doesnotexist)=”Empty” shows True
if isempty(v) then debug.print "v has been defined" else debug.print "v has _not_ been defined" end if
3 comments so far
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Your JavaScript isn’t quite right. It’s perfectly legitimate to assign null to a variable:
v = null
This makes it defined and then makes it okay to use, but your «if(window.v)» test will still print “v has _not_ been defined”. Ditto if we assign false to v (or 0, or the empty string). Better is to use «if(window.v === undefined)» (note the triple equals). But even that doesn’t catch the case where we assign undefined to a variable. We can’t distinguish between an undefined variable and a variable whose value is undefined.
We can use a exception handler though:
try { v; alert(‘defined’) } catch(_) { alert(‘undefined’) }
which seems kinda sneaky to me.
drj11, you’re very right. I’ll change the code. I tried to make the code as simple as possible, and instead made it a bit too simple. Thanks.
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