How can I check for undefined in JavaScript

Question

What is the most appropriate way to test if a variable is undefined in JavaScript  I ve seen several possible ways  if  window myVariable   Or if  typeof myVariable      quot undefined quot    Or if  myVariable     This throws an error if undefined  Should this be in Try Catch

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You can use typeof  like this   if  typeof something     undefined

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On the contrary of  Thomas Eding answer   If I forget to declare myVar in my code  then I ll get myVar is not defined   Let s take a real example   I ve a variable name  but I am not sure if it is declared somewhere or not   Then  Anurag s answer will help   var myVariableToCheck    myVar   if  window myVariableToCheck      undefined      console log  Not declared or declared  but undefined         Or you can check it directly  if  window  myVar       undefined       console log  Not declared or declared  but undefined

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Some scenarios illustrating the results of the various answers  http   jsfiddle net drzaus UVjM4    Note that the use of var for in tests make a difference when in a scoped wrapper   Code for reference    function undefined        var definedButNotInitialized      definedAndInitialized   3      someObject             firstProp   1            secondProp  false              undefinedProp not defined              var notDefined       var tests              definedButNotInitialized in window            definedAndInitialized in window            someObject firstProp in window            someObject secondProp in window            someObject undefinedProp in window            notDefined in window              definedButNotInitialized  in window             definedAndInitialized  in window             someObject firstProp  in window             someObject secondProp  in window             someObject undefinedProp  in window             notDefined  in window             typeof definedButNotInitialized     undefined             typeof definedButNotInitialized     typeof undefined            definedButNotInitialized     undefined              definedButNotInitialized               definedButNotInitialized             typeof definedAndInitialized     undefined             typeof definedAndInitialized     typeof undefined            definedAndInitialized     undefined              definedAndInitialized               definedAndInitialized             typeof someObject firstProp     undefined             typeof someObject firstProp     typeof undefined            someObject firstProp     undefined              someObject firstProp               someObject firstProp             typeof someObject secondProp     undefined             typeof someObject secondProp     typeof undefined            someObject secondProp     undefined              someObject secondProp               someObject secondProp             typeof someObject undefinedProp     undefined             typeof someObject undefinedProp     typeof undefined            someObject undefinedProp     undefined              someObject undefinedProp               someObject undefinedProp             typeof notDefined     undefined             typeof notDefined     typeof undefined            notDefined     undefined              notDefined               notDefined              var output   document getElementById  results        var result           for var t in tests            if   tests hasOwnProperty t    continue     bleh          try               result   eval tests t              catch ex                result    Exception--    ex                    console log tests t   result           output innerHTML      n    tests t           result                And results   definedButNotInitialized in window  true definedAndInitialized in window  false someObject firstProp in window  false someObject secondProp in window  false someObject undefinedProp in window  true notDefined in window  Exception--ReferenceError  notDefined is not defined  definedButNotInitialized  in window  false  definedAndInitialized  in window  true  someObject firstProp  in window  false  someObject secondProp  in window  false  someObject undefinedProp  in window  false  notDefined  in window  false typeof definedButNotInitialized     undefined   true typeof definedButNotInitialized     typeof undefined  true definedButNotInitialized     undefined  true   definedButNotInitialized  true    definedButNotInitialized  false typeof definedAndInitialized     undefined   false typeof definedAndInitialized     typeof undefined  false definedAndInitialized     undefined  false   definedAndInitialized  false    definedAndInitialized  true typeof someObject firstProp     undefined   false typeof someObject firstProp     typeof undefined  false someObject firstProp     undefined  false   someObject firstProp  false    someObject firstProp  true typeof someObject secondProp     undefined   false typeof someObject secondProp     typeof undefined  false someObject secondProp     undefined  false   someObject secondProp  true    someObject secondProp  false typeof someObject undefinedProp     undefined   true typeof someObject undefinedProp     typeof undefined  true someObject undefinedProp     undefined  true   someObject undefinedProp  true    someObject undefinedProp  false typeof notDefined     undefined   true typeof notDefined     typeof undefined  true notDefined     undefined  Exception--ReferenceError  notDefined is not defined   notDefined  Exception--ReferenceError  notDefined is not defined    notDefined  Exception--ReferenceError  notDefined is not defined

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If you are interested in finding out whether a variable has been declared regardless of its value  then using the in operator is the safest way to go  Consider this example      global scope var theFu     theFu has been declared  but its value is undefined typeof theFu      undefined    But this may not be the intended result for some cases  since the variable or property was declared but just not initialized  Use the in operator for a more robust check    theFu  in window     true  theFoo  in window     false   If you are interested in knowing whether the variable hasn t been declared or has the value undefined  then use the typeof operator  which is guaranteed to return a string   if  typeof myVar      undefined     Direct comparisons against undefined are troublesome as undefined can be overwritten    window undefined    foo    foo     undefined    true   As  CMS pointed out  this has been patched in ECMAScript 5th ed   and undefined is non-writable   if  window myVar  will also include these falsy values  so it s not very robust    false 0    NaN null undefined   Thanks to  CMS for pointing out that your third case - if  myVariable  can also throw an error in two cases  The first is when the variable hasn t been defined which throws a ReferenceError       abc was never declared  if  abc           ReferenceError  abc is not defined      The other case is when the variable has been defined  but has a getter function which throws an error when invoked  For example      or it s a property that can throw an error Object defineProperty window   myVariable          get  function     throw new Error  W00t             set  undefined      if  myVariable           Error  W00t

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2020 Update  One of my reasons for preferring a typeof check  namely  that undefined can be redefined  became irrelevant with the mass adoption of ECMAScript 5  The other  that you can use typeof to check the type of an undeclared variable  was always niche  Therefore  I d now recommend using a direct comparison in most situations   myVariable     undefined   Original answer from 2010  Using typeof is my preference  It will work when the variable has never been declared  unlike any comparison with the    or     operators or type coercion using if   undefined  unlike null  may also be redefined in ECMAScript 3 environments  making it unreliable for comparison  although nearly all common environments now are compliant with ECMAScript 5 or above    if  typeof someUndeclaredVariable     undefined            Works    if  someUndeclaredVariable     undefined            Throws an error

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var x      if  x     undefined            alert   I am declared  but not defined               if  typeof y      undefined             alert   I am not even declared                   One more thing to understand  typeof     undefined  also checks         for if a variable is declared  but no value is assigned  In other         words  the variable is declared  but not defined             Will repeat above logic of x for typeof      undefined      if  x     undefined            alert   I am declared  but not defined                  So typeof      undefined  works for both  but x     undefined         only works for a variable which is at least declared             Say if I try using typeof     undefined  not in quotes  for         a variable which is not even declared  we will get run a         time error          if  z     undefined            alert   I am neither declared nor defined                  I got this error for z ReferenceError  z is not defined

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if  typeof foo     undefined         Do something      Note that strict comparison       is not necessary in this case  since typeof will always return a string

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I use it as a function parameter and exclude it on function execution that way I get the  real  undefined  Although it does require you to put your code inside a function  I found this while reading the jQuery source   undefined   2    function  undefined       console log undefined      prints out undefined       and for comparison     if  undeclaredvar     undefined  console log  it works           Of course you could just use typeof though  But all my code is usually inside a containing function anyways  so using this method probably saves me a few bytes here and there

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Since none of the other answers helped me  I suggest doing this  It worked for me in Internet nbsp Explorer nbsp 8   if  typeof variable name value      undefined            variable name is undefined

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In this article I read that frameworks like Underscore js use this function   function isUndefined obj       return obj     void 0

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The most reliable way I know of checking for undefined is to use void 0   This is compatible with newer and older browsers  alike  and cannot be overwritten like window undefined can in some cases   if  myVar     void 0         yup it s undefined

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I personally use   myVar     undefined   Warning  Please note that     is used over    and that myVar has been previously declared  not defined      I do not like typeof myVar      undefined   I think it is long winded and unnecessary   I can get the same done in less code    Now some people will keel over in pain when they read this  screaming   Wait  WAAITTT    undefined can be redefined    Cool  I know this  Then again  most variables in Javascript can be redefined  Should you never use any built-in identifier that can be redefined   If you follow this rule  good for you  you aren t a hypocrite   The thing is  in order to do lots of real work in JS  developers need to rely on redefinable identifiers to be what they are  I don t hear people telling me that I shouldn t use setTimeout because someone can  window setTimeout   function          alert  Got you now          Bottom line  the  it can be redefined  argument to not use a raw     undefined is bogus     If you are still scared of undefined being redefined  why are you blindly integrating untested library code into your code base  Or even simpler  a linting tool      Also  like the typeof approach  this technique can  detect  undeclared variables    if  window someVar     undefined        doSomething        But both these techniques leak in their abstraction  I urge you not to use this or even   if  typeof myVar      undefined         doSomething        Consider   var iAmUndefined    To catch whether or not that variable is declared or not  you may need to resort to the in operator   In many cases  you can simply read the code O o    if   myVar  in window        doSomething        But wait  There s more  What if some prototype chain magic is happening     Now even the superior in operator does not suffice   Okay  I m done here about this part except to say that for 99  of the time      undefined  and     cough     typeof  works just fine  If you really care  you can read about this subject on its own

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Update 2018-07-25  It s been nearly five years since this post was first made  and JavaScript has come a long way   In repeating the tests in the original post  I found no consistent difference between the following test methods    abc     undefined abc     void 0 typeof abc     undefined  typeof abc      undefined    Even when I modified the tests to prevent Chrome from optimizing them away  the differences were insignificant   As such  I d now recommend abc     undefined for clarity   Relevant content from chrome   version    Google Chrome  67 0 3396 99  Official Build   64-bit   cohort  Stable  Revision  a337fbf3c2ab8ebc6b64b0bfdce73a20e2e2252b-refs branch-heads 3396   790  OS  Windows JavaScript  V8 6 7 288 46 User Agent  Mozilla 5 0  Windows NT 10 0  Win64  x64  AppleWebKit 537 36  KHTML  like Gecko  Chrome 67 0 3396 99 Safari 537 36   Original post 2013-11-01  In Google Chrome  the following was ever so slightly faster than a typeof test   if  abc     void 0           Undefined     The difference was negligible   However  this code is more concise  and clearer at a glance to someone who knows what void 0 means  Note  however  that abc must still be declared   Both typeof and void were significantly faster than comparing directly against undefined   I used the following test format in the Chrome developer console   var abc  start    new Date    for  var i   0  i  lt  10000000  i          if  TEST            void 1          end    new Date    end - start    The results were as follows   Test    abc     undefined      abc     void 0      typeof abc     undefined  ------ --------------------------------------------------------------------- x10M        13678 ms               9854 ms                 9888 ms   x1       1367 8 ns              985 4 ns                988 8 ns   Note that the first row is in milliseconds  while the second row is in nanoseconds   A difference of 3 4 nanoseconds is nothing   The times were pretty consistent in subsequent tests

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x has not been defined before if  typeof x      undefined        Evaluates to true without errors        These statements execute     if  x     undefined       Throws a ReferenceError

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If it is undefined  it will not be equal to a string that contains the characters  undefined   as the string is not undefined   You can check the type of the variable   if  typeof something      undefined         Sometimes you don t even have to check the type  If the value of the variable can t evaluate to false when it s set  for example if it s a function   then you can just evalue the variable  Example   if  something      something param

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Personally  I always use the following   var x  if  x     undefined          Do something here   else        Do something else here     The window undefined property is non-writable in all modern browsers  JavaScript 1 8 5 or later   From Mozilla s documentation  https   developer mozilla org en-US docs Web JavaScript Reference Global Objects undefined  I see this  One reason to use typeof   is that it does not throw an error if the variable has not been defined   I prefer to have the approach of using  x     undefined    because it fails and blows up in my face rather than silently passing failing if x has not been declared before  This alerts me that x is not declared  I believe all variables used in JavaScript should be declared

[javascript] How can I check for "undefined" in JavaScript?

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