How to check if a variable is both null and or undefined in JavaScript

Question

Possible Duplicate    Detecting an undefined object property in JavaScript   How to determine if variable is   39 undefined  39  or   39 null  39    Is there a standard function to check for null  undefined  or blank variables in JavaScript      In my code  I have a condition that looks like  if  variable     null  amp  amp  variable     undefined        But instead of doing it in two steps  i e checking if it is not defined and not null  Is there a one step checking that replaces this check

User · Answer

You can wrap it in your own function   function isNullAndUndef variable         return  variable     null  amp  amp  variable     undefined

User · Answer

A variable cannot be both null and undefined at the same time   However  the direct answer to your question is   if  variable    null    One    not two   There are two special clauses in the  abstract equality comparison algorithm  in the JavaScript spec devoted to the case of one operand being null and the other being undefined  and the result is true for    and false for      Thus if the value of the variable is undefined  it s not    null  and if it s not null  it s obviously not    null   Now  the case of an identifier not being defined at all  either as a var or let  as a function parameter  or as a property of the global context is different  A reference to such an identifier is treated as an error at runtime  You could attempt a reference and catch the error   var isDefined   false  try      variable     isDefined   true    catch  x       I would personally consider that a questionable practice however  For global symbols that may or may be there based on the presence or absence of some other library  or some similar situation  you can test for a window property  in browser JavaScript    var isJqueryAvailable   window jQuery    null    or  var isJqueryAvailable    jQuery  in window

[javascript] How to check if a variable is both null and /or undefined in JavaScript

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