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Re: Undefined array element becomes defined and null

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On 18/12/2011 21:37, Tim Streater wrote:
> I have a JavaScript array, myArray. I happen to know that let's say
> element 27 is undefined - it's never been created. It appears that I can
> detect this with:
>
> if (myArray[27]==null)
> {
> alert (myArray[27]);
> }
>
>
> The alert puts up 'undefined'. Is it valid to be able to detect the
> undefined stater of element 27 in this way?
>
> Initially I thought perhaps that the act of accessing the element
> created it and set it to null, but apparently not if the alert is
> anything to go by. Or is this just a quirk of Safari?

In this particular case, we should use the strict Equals Operator (===) 
than just the Equals Operator (==), because undefined == null, for 
instance, produces true, whereas undefined === null yields false.

Note: Douglas Crockford's advice is to never use [what he calls] the 
evil twins (== and !=). Instead, we should always use === and !==, 
although Crockford's advice is a tad exaggerated when dealing with the 
typeof operator which always returns a string value.

Another important thing: if we access a missing array element, we will 
get the undefined value, not null. So, the OP's code might be rewritten to:

   var missingElem = myArray[27];
   if (typeof missingElem == 'undefined') {
     alert('this is a missing element in myArray');
   }


-- 
Joao Rodrigues (J.R.)

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Thread

Undefined array element becomes defined and null Tim Streater <timstreater@greenbee.net> - 2011-12-18 23:37 +0000
  Re: Undefined array element becomes defined and null Stefan Weiss <krewecherl@gmail.com> - 2011-12-19 03:17 +0100
    Re: Undefined array element becomes defined and null Tim Streater <timstreater@greenbee.net> - 2011-12-19 09:17 +0000
    Re: Undefined array element becomes defined and null Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2011-12-19 13:13 +0100
      Re: Undefined array element becomes defined and null "Evertjan." <exjxw.hannivoort@interxnl.net> - 2011-12-19 16:42 +0000
  Re: Undefined array element becomes defined and null "J.R." <groups_jr-1@yahoo.com.br> - 2011-12-19 00:48 -0200

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