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document updated 12 years ago, on Nov 14, 2011
How do you 1) assign and 2) check for the value of "undefined" or "null" or "NaN" in various languages?

Perl 
my $a = undef;

if (defined($a)) {
   ...
}
Javascript 
var a;
    
if (typeof(a) == "undefined") {
    ...
}

undefined = "Variable shouldn't be assignable, but before ECMA5, it was";
var a = null;

if (a === null && a !== null) {
    ...
}
SQL 
SELECT * FROM TableName WHERE colname IS NOT NULL;