Detecting an undefined object property

Question

What s the best way of checking if an object property in JavaScript is undefined

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If you are using Angular    angular isUndefined obj  angular isUndefined obj prop    Underscore js     isUndefined obj     isUndefined obj prop

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propertyName  in obj   - gt  true   false

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There are a few little helpers in the Lodash library  isUndefined - to check if value is undefined    isUndefined undefined       gt  true   isUndefined null       gt  false  has - to check if object contains a property const object      a      b   2        has object   a b        gt  true   has object   a c        gt  false

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The usual way to check if the value of a property is the special value undefined  is   if o myProperty     undefined      alert  myProperty value is the special value  undefined         To check if an object does not actually have such a property  and will therefore return undefined by default when you try and access it   if  o hasOwnProperty  myProperty        alert  myProperty does not exist        To check if the value associated with an identifier is the special value undefined  or if that identifier has not been declared  Note  this method is the only way of referring to an undeclared  note  different from having a value of undefined  identifier without an early error   if typeof myVariable      undefined       alert  myVariable is either the special value  undefined   or it has not been declared        In versions of JavaScript prior to ECMAScript 5  the property named  undefined  on the global object was writeable  and therefore a simple check foo     undefined might behave unexpectedly if it had accidentally been redefined  In modern JavaScript  the property is read-only   However  in modern JavaScript   undefined  is not a keyword  and so variables inside functions can be named  undefined  and shadow the global property    If you are worried about this  unlikely  edge case  you can use the void operator to get at the special undefined value itself   if myVariable     void 0      alert  myVariable is the special value  undefined

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I found this article  7 Tips to Handle undefined in JavaScript  which is showing really interesting things about undefined like  The existence of undefined is a consequence of JavaScript   s permissive nature that allows the usage of   uninitialized variables non-existing object properties or methods out of bounds indexes to access array elements the invocation result of a function that returns nothing

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I believe there are a number of incorrect answers to this topic  Contrary to common belief   undefined  is not a keyword in JavaScript and can in fact have a value assigned to it   Correct Code  The most robust way to perform this test is   if  typeof myVar      undefined     This will always return the correct result  and even handles the situation where myVar is not declared   Degenerate code  DO NOT USE   var undefined   false      Shockingly  this is completely legal  if  myVar     undefined        alert  You have been misled  Run away         Additionally  myVar     undefined will raise an error in the situation where myVar is undeclared

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if  window x       is error safe  Most likely you want if  window x   This check is safe even if x hasn t been declared  var x   - browser doesn t throw an error    Example  I want to know if my browser supports History API  if  window history        history call some function        How this works   window is an object which holds all global variables as its members  and it is legal to try to access a non-existing member  If x hasn t been declared or hasn t been set then window x returns undefined  undefined leads to false when if   evaluates it

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You can use the JavaScript object function like this  var ojb        age  12    if ojb hasOwnProperty  name         console log  property exists and is not undefined       The above method returns true if it got that property or the property is not undefined

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I didn t see  hope I didn t miss it  anyone checking the object before the property  So  this is the shortest and most effective  though not necessarily the most clear    if  obj  amp  amp  obj prop         Do something      If the obj or obj prop is undefined  null  or  falsy   the if statement will not execute the code block  This is usually the desired behavior in most code block statements  in JavaScript

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I provide three ways here for those who expect weird answers   x000D   x000D  function isUndefined1 val        try           val a        catch  e            return  undefined  test e message             return false     function isUndefined2 val        return  val  amp  amp  val         undefined      function isUndefined3 val        const defaultVal           return   input   defaultVal    gt  input     defaultVal  val      function test func       console group  test start    func name       console log func undefined        console log func null        console log func 1        console log func  1         console log func 0        console log func           console log func function               console groupEnd      test isUndefined1   test isUndefined2   test isUndefined3   x000D   x000D   x000D   isUndefined1  Try to get a property of the input value  and check the error message if it exists  If the input value is undefined  the error message would be Uncaught TypeError  Cannot read property  b  of undefined  isUndefined2  Convert the input value to a string to compare with  quot undefined quot  and ensure it s a negative value  isUndefined3  In JavaScript  an optional parameter works when the input value is exactly undefined

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Review A lot of the given answers give a wrong result because they do not distinguish between the case when an object property does not exist and the case when a property has value undefined  Here is proof for most popular solutions   x000D   x000D  let obj       a  666    u  undefined    The  u  property has value  undefined                    The  x  property does not exist    console log   gt  gt  gt  good results     console log  A    u  in obj   x  in obj   console log  B   obj hasOwnProperty  u         obj hasOwnProperty  x      console log   n gt  gt  gt  bad results     console log  C   obj u     undefined           obj x     undefined   console log  D   obj u    undefined            obj x    undefined   console log  E   obj  u       undefined        obj  x       undefined   console log  F   obj  u      undefined         obj  x      undefined   console log  G    obj u                        obj x   console log  H   typeof obj u      undefined   typeof obj x      undefined    x000D   x000D   x000D

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Despite being vehemently recommended by many other answers here  typeof is a bad choice  It should never be used for checking whether variables have the value undefined  because it acts as a combined check for the value undefined and for whether a variable exists  In the vast majority of cases  you know when a variable exists  and typeof will just introduce the potential for a silent failure if you make a typo in the variable name or in the string literal  undefined   var snapshot         if  typeof snaposhot      undefined                             misspelled       this will never run  but it won   t throw an error      var foo         if  typeof foo      undefned                                       misspelled     this will never run  but it won   t throw an error     So unless you   re doing feature detection    where there   s uncertainty whether a given name will be in scope  like checking typeof module      undefined  as a step in code specific to a CommonJS environment   typeof is a harmful choice when used on a variable  and the correct option is to compare the value directly  var foo         if  foo     undefined             Some common misconceptions about this include   that reading an    uninitialized    variable  var foo  or parameter  function bar foo           called as bar    will fail  This is simply not true     variables without explicit initialization and parameters that weren   t given values always become undefined  and are always in scope   that undefined can be overwritten  It   s true that undefined isn   t a keyword  but it is read-only and non-configurable  There are other built-ins you probably don   t avoid despite their non-keyword status  Object  Math  NaN     and practical code usually isn   t written in an actively malicious environment  so this isn   t a good reason to be worried about undefined   But if you are writing a code generator  feel free to use void 0     With how variables work out of the way  it   s time to address the actual question  object properties  There is no reason to ever use typeof for object properties  The earlier exception regarding feature detection doesn   t apply here     typeof only has special behaviour on variables  and expressions that reference object properties are not variables  This  if  typeof foo bar      undefined              is always exactly equivalent to this    if  foo bar     undefined             and taking into account the advice above  to avoid confusing readers as to why you   re using typeof  because it makes the most sense to use     to check for equality  because it could be refactored to checking a variable   s value later  and because it just plain looks better  you should always use     undefined   here as well  Something else to consider when it comes to object properties is whether you really want to check for undefined at all  A given property name can be absent on an object  producing the value undefined when read   present on the object itself with the value undefined  present on the object   s prototype with the value undefined  or present on either of those with a non-undefined value   key  in obj will tell you whether a key is anywhere on an object   s prototype chain  and Object prototype hasOwnProperty call obj   key   will tell you whether it   s directly on the object  I won   t go into detail in this answer about prototypes and using objects as string-keyed maps  though  because it   s mostly intended to counter all the bad advice in other answers irrespective of the possible interpretations of the original question  Read up on object prototypes on MDN for more     unusual choice of example variable name  this is real dead code from the NoScript extension for Firefox     don   t assume that not knowing what   s in scope is okay in general  though  bonus vulnerability caused by abuse of dynamic scope  Project Zero 1225    once again assuming an ES5  environment and that undefined refers to the undefined property of the global object

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function isUnset inp      return  typeof inp      undefined       Returns false if variable is set  and true if is undefined   Then use   if  isUnset var          initialize variable here

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You can also use a Proxy  It will work with nested calls  but it will require one extra check  function resolveUnknownProps obj  resolveKey      const handler         get target  key          if           target key      null  amp  amp          typeof target key       object                    return resolveUnknownProps target key   resolveKey           else if   target key             return resolveUnknownProps    resolveKey   true    resolveKey                  return target key                  return new Proxy obj  handler      const user       console log resolveUnknownProps user   isUndefined   personalInfo name something else        isUndefined  true    So you will use it like  const   isUndefined     resolveUnknownProps user   isUndefined   personalInfo name something else  if   isUndefined         Do something

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I m not sure where the origin of using     with typeof came from  and as a convention I see it used in many libraries  but the typeof operator returns a string literal  and we know that up front  so why would you also want to type check it too   typeof x                          some string literal  string    object    undefined  if  typeof x      string              is redundant because we already know typeof returns a string literal if  typeof x     string           sufficient

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Crossposting my answer from related question How can I check for  quot undefined quot  in JavaScript   Specific to this question  see test cases with someObject  lt whatever gt    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   quot 1 quot            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              quot definedButNotInitialized quot  in window             quot definedAndInitialized quot  in window             quot someObject firstProp quot  in window             quot someObject secondProp quot  in window             quot someObject undefinedProp quot  in window             quot notDefined quot  in window             typeof definedButNotInitialized     quot undefined quot             typeof definedButNotInitialized     typeof undefined            definedButNotInitialized     undefined              definedButNotInitialized               definedButNotInitialized             typeof definedAndInitialized     quot undefined quot             typeof definedAndInitialized     typeof undefined            definedAndInitialized     undefined              definedAndInitialized               definedAndInitialized             typeof someObject firstProp     quot undefined quot             typeof someObject firstProp     typeof undefined            someObject firstProp     undefined              someObject firstProp               someObject firstProp             typeof someObject secondProp     quot undefined quot             typeof someObject secondProp     typeof undefined            someObject secondProp     undefined              someObject secondProp               someObject secondProp             typeof someObject undefinedProp     quot undefined quot             typeof someObject undefinedProp     typeof undefined            someObject undefinedProp     undefined              someObject undefinedProp               someObject undefinedProp             typeof notDefined     quot undefined quot             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     quot  n quot    tests t     quot    quot    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  quot definedButNotInitialized quot  in window  false  quot definedAndInitialized quot  in window  true  quot someObject firstProp quot  in window  false  quot someObject secondProp quot  in window  false  quot someObject undefinedProp quot  in window  false  quot notDefined quot  in window  false typeof definedButNotInitialized     quot undefined quot   true typeof definedButNotInitialized     typeof undefined  true definedButNotInitialized     undefined  true   definedButNotInitialized  true    definedButNotInitialized  false typeof definedAndInitialized     quot undefined quot   false typeof definedAndInitialized     typeof undefined  false definedAndInitialized     undefined  false   definedAndInitialized  false    definedAndInitialized  true typeof someObject firstProp     quot undefined quot   false typeof someObject firstProp     typeof undefined  false someObject firstProp     undefined  false   someObject firstProp  false    someObject firstProp  true typeof someObject secondProp     quot undefined quot   false typeof someObject secondProp     typeof undefined  false someObject secondProp     undefined  false   someObject secondProp  true    someObject secondProp  false typeof someObject undefinedProp     quot undefined quot   true typeof someObject undefinedProp     typeof undefined  true someObject undefinedProp     undefined  true   someObject undefinedProp  true    someObject undefinedProp  false typeof notDefined     quot undefined quot   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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Here is my situation  I am using the result of a REST call  The result should be parsed from JSON to a JavaScript object  There is one error I need to defend  If the arguments to the REST call were incorrect as far as the user specifying the arguments wrong  the REST call comes back basically empty  While using this post to help me defend against this  I tried this  if  typeof restResult data 0       quot undefined quot      throw   quot Some error quot      For my situation  if restResult data 0       quot object quot   then I can safely start inspecting the rest of the members  If undefined then throw the error as above  What I am saying is that for my situation  all the previous suggestions in this post did not work  I m not saying I m right and everyone is wrong  I am not a JavaScript master at all  but hopefully this will help someone

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A simple way to check if a key exists is to use in  if  key in obj         Do something   else        Create key    const obj       0   abc     1   def     const hasZero   0 in obj  console log hasZero     true

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Use   To check if property is undefined   if  typeof something      undefined         alert  undefined        To check if property is not undefined   if  typeof something      undefined         alert  not undefined

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In JavaScript  there are truthy and falsy expressions  If you want to check if the property is undefined or not  there is a straight way of using an if condition as given   Using truthy falsy concept   if  ob someProp       console log  someProp is falsy      However  there are several more approaches to check the object has property or not  but it seems long to me  Here are those   Using     undefined check in if condition  if ob someProp     undefined       console log  someProp is undefined       Using typeof  typeof acts as a combined check for the value undefined and for whether a variable exists  if typeof ob someProp      undefined        console log  someProp is undefined       Using hasOwnProperty method  The JavaScript object has built in the hasOwnProperty function in the object prototype  if  ob hasOwnProperty  someProp         console log  someProp is undefined      Not going in deep  but the 1st way looks shortened and good to me  Here are the details on truthy falsy values in JavaScript and undefined is the falsy value listed in there  So the if condition behaves normally without any glitch  Apart from the undefined  values NaN  false  Obviously       empty string  and number 0 are also the falsy values   Warning  Make sure the property value does not contain any falsy value  otherwise the if condition will return false  For such a case  you can use the hasOwnProperty method

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In JavaScript there is null and there is undefined   They have different meanings   undefined means that the variable value has not been defined  it is not known what the value is  null means that the variable value is defined and set to null  has no value    Marijn Haverbeke states  in his free  online book  quot Eloquent JavaScript quot   emphasis mine    There is also a similar value  null  whose meaning is  this value is defined  but it does not have a value   The difference in meaning between undefined and null is mostly academic  and usually not very interesting  In practical programs  it is often necessary to check whether something  has a value   In these cases  the expression something    undefined may be used  because  even though they are not exactly the same value  null    undefined will produce true   So  I guess the best way to check if something was undefined would be  if  something    undefined   Object properties should work the same way  var person         name   quot John quot       age  28      sex   quot male quot      alert person name       quot John quot  alert person fakeVariable      undefined

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Version for the use of dynamic variables Did you know   x000D   x000D  var boo   lala    function check      if this  foo             console log  foo is here       else          console log  have no foo               if this  boo             console log  boo is here       else          console log  have no boo               check    x000D   x000D   x000D

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This is probably the only explicit form of determining if the existing property-name has an explicit and intended value of undefined  which is  nonetheless  a JavaScript type   quot propertyName quot  in containerObject  amp  amp   quot  quot  containerObject  quot propertyName quot       quot undefined quot    gt  gt  true   false  This expression will only return true if the property name of the given context exists  truly  and only if its intended value is explicitly undefined  There will be no false positives like with empty or blank strings zeros nulls or empty arrays and alike  This does exactly that  Checks i e   makes sure the property name exists  otherwise it would be a false positive   than it explicitly checks if its value is undefined e g  of an undefined JavaScript type in it s string representation form  literally  quot undefined quot   therefore    instead of     because no further conversion is possible  And this expression will only return true if both  that is all conditions are met  E g  if the property-name doesn t exist  - it will return false  Which is the only correct return since nonexistent properties can t have values  not even an undefined one  Example  containerObject     propertyName  void  quot anything quot     gt  gt  Object   propertyName  undefined       Now the testing   quot propertyName quot  in containerObject  amp  amp   quot  quot  containerObject  quot propertyName quot       quot undefined quot    gt  gt  true     Which makes sure that nonexistent property will not return a false positive    unless it is previously defined       quot foo quot  in containerObject  amp  amp   quot  quot  containerObject  quot foo quot       quot undefined quot    gt  gt  false

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Simply anything is not defined in JavaScript  is undefined  doesn t matter if it s a property inside an Object Array or as just a simple variable     JavaScript has typeof which make it very easy to detect an undefined variable   Simply check if typeof whatever      undefined  and it will return a boolean   That s how the famous function isUndefined   in AngularJs v 1x is written   function isUndefined value   return typeof value      undefined       So as you see the function receive a value  if that value is defined  it will return false  otherwise for undefined values  return true    So let s have a look what gonna be the results when we passing values  including object properties like below  this is the list of variables we have   var stackoverflow       stackoverflow javascipt    javascript   var today  var self   this  var num   8  var list    1  2  3  4  5   var y   null    and we check them as below  you can see the results in front of them as a comment   isUndefined stackoverflow     false isUndefined stackoverflow javascipt     false isUndefined today     true isUndefined self     false isUndefined num     false isUndefined list     false isUndefined y     false isUndefined stackoverflow java     true isUndefined stackoverflow php     true isUndefined stackoverflow  amp  amp  stackoverflow css     true   As you see we can check anything with using something like this in our code  as mentioned you can simply use typeof in your code  but if you are using it over and over  create a function like the angular sample which I share and keep reusing as following DRY code pattern   Also one more thing  for checking property on an object in a real application which you not sure even the object exists or not  check if the object exists first   If you check a property on an object and the object doesn t exist  will throw an error and stop the whole application running   isUndefined x css   VM808 2 Uncaught ReferenceError  x is not defined        So simple you can wrap inside an if statement like below   if typeof x      undefined         do something     Which also equal to isDefined in Angular 1 x     function isDefined value   return typeof value      undefined      Also other javascript frameworks like underscore has similar defining check  but I recommend you use typeof if you already not using any frameworks   I also add this section from MDN which has got useful information about typeof  undefined and void 0       Strict equality and undefined  You can use undefined and the strict equality and inequality operators to determine whether a variable has   a value  In the following code  the variable x is not defined  and the   if statement evaluates to true    var x  if  x     undefined          these statements execute   else         these statements do not execute        Note  The strict equality operator rather than the standard equality   operator must be used here  because x    undefined also checks whether   x is null  while strict equality doesn t  null is not equivalent to   undefined  See comparison operators for details         Typeof operator and undefined    Alternatively  typeof can be used    var x  if  typeof x      undefined           these statements execute        One reason to use typeof is that it does not throw an error if the   variable has not been declared       x has not been declared before if  typeof x      undefined        evaluates to true without errors       these statements execute    if  x     undefined       throws a ReferenceError         However  this kind of technique should be avoided  JavaScript is a   statically scoped language  so knowing if a variable is declared can   be read by seeing whether it is declared in an enclosing context  The   only exception is the global scope  but the global scope is bound to   the global object  so checking the existence of a variable in the   global context can be done by checking the existence of a property on   the global object  using the in operator  for instance          Void operator and undefined      The void operator is a third alternative    var x  if  x     void 0          these statements execute       y has not been declared before if  y     void 0          throws a ReferenceError  in contrast to  typeof       more   here

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ECMAScript  10 introduced a new feature - optional chaining which you can use to use a property of an object only when an object is defined like this  const userPhone   user  contactDetails  phone   It will reference to the phone property only when user and contactDetails are defined  Ref  https   developer mozilla org en-US docs Web JavaScript Reference Operators Optional chaining

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Compare with void 0  for terseness   if  foo     void 0    It s not as verbose as if  typeof foo      undefined

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You can get an array all undefined with path using the following code    function getAllUndefined object             function convertPath arr  key                var path                   for  var i   1  i  lt  arr length  i                       path    arr i     - gt                              path    key              return path                      var stack               var saveUndefined              function getUndefiend obj  key                 var t   typeof obj              switch  t                    case  object                       if  t     null                            return false                                            break                  case  string                   case  number                   case  boolean                   case  null                       return false                  default                      return true                            stack push key               for  k in obj                    if  obj hasOwnProperty k                         v   getUndefiend obj k   k                       if  v                            saveUndefined push convertPath stack  k                                                                      stack pop                        getUndefiend                   object                         return saveUndefined          jsFiddle link

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There is a nice and elegant way to assign a defined property to a new variable if it is defined or assign a default value to it as a fallback if it   s undefined  var a   obj prop    defaultValue   It   s suitable if you have a function  which receives an additional configuration property  var yourFunction   function config       this config   config           this yourConfigValue   config yourConfigValue    1     console log this yourConfigValue      Now executing yourFunction  yourConfigValue 2        gt  2  yourFunction        gt  1  yourFunction  otherProperty 5        gt  1

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If you do  if  myvar    undefined          alert  var does not exists or is not initialized        it will fail when the variable myvar does not exists  because myvar is not defined  so the script is broken and the test has no effect   Because the window object has a global scope  default object  outside a function  a declaration will be  attached  to the window object   For example   var myvar    test     The global variable myvar is the same as window myvar or window  myvar    To avoid errors to test when a global variable exists  you better use   if window myvar    undefined          alert  var does not exists or is not initialized        The question if a variable really exists doesn t matter  its value is incorrect  Otherwise  it is silly to initialize variables with undefined  and it is better use the value false to initialize  When you know that all variables that you declare are initialized with false  you can simply check its type or rely on  window myvar to check if it has a proper valid value  So even when the variable is not defined then  window myvar is the same for myvar   undefined or myvar   false or myvar   0   When you expect a specific type  test the type of the variable  To speed up testing a condition you better do   if   window myvar    typeof window myvar     string          alert  var does not exists or is not type of string        When the first and simple condition is true  the interpreter skips the next tests    It is always better to use the instance object of the variable to check if it got a valid value  It is more stable and is a better way of programming      y

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Reading through this  I m amazed I didn t see this  I have found multiple algorithms that would work for this   Never Defined  If the value of an object was never defined  this will prevent from returning true if it is defined as null or undefined  This is helpful if you want true to be returned for values set as undefined  if obj prop     void 0  console log  The value has never been defined      Defined as undefined Or never Defined  If you want it to result as true for values defined with the value of undefined  or never defined  you can simply use     undefined  if obj prop     undefined  console log  The value is defined as undefined  or never defined      Defined as a falsy value  undefined null  or never defined   Commonly  people have asked me for an algorithm to figure out if a value is either falsy  undefined  or null  The following works   if obj prop    false    obj prop     null    obj prop     undefined        console log  The value is falsy  null  or undefined

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I use if  this variable  to test if it is defined  A simple if  variable   recommended in a previous answer  fails for me  It turns out that it works only when a variable is a field of some object  obj someField to check if it is defined in the dictionary  But we can use this or window as the dictionary object since any variable is a field in the current window  as I understand it  Therefore here is a test   x000D   x000D  if  this abc       alert  defined     else      alert  undefined     abc    abc   if  this abc       alert  defined     else      alert  undefined    x000D   x000D   x000D   It first detects that variable abc is undefined and it is defined after initialization

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The solution is incorrect  In JavaScript   null    undefined   will return true  because they both are  casted  to a boolean and are false  The correct way would be to check  if  something     undefined    which is the identity operator

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I m surprised I haven t seen this suggestion yet  but it gets even more specificity than testing with typeof  Use Object getOwnPropertyDescriptor   if you need to know whether an object property was initialized with undefined or if it was never initialized     to test someObject someProperty var descriptor   Object getOwnPropertyDescriptor someObject   someProperty     if  typeof descriptor      undefined          was never initialized   else if  typeof descriptor value      undefined       if  descriptor get    descriptor set           is an accessor property  defined via getter and setter     else          is initialized with  undefined        else        is initialized with some other value

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There is a very easy and simple way  You can use optional chaining  x    prop  name  quot sajad quot     console log x prop  name     Output is   quot sajad quot  console log x prop  lastName     Output is  undefined  or if x prop  lastName     The result of this  if  statement is false and is not throwing an error  You can use optional chaining even for functions or arrays  As of mid-2020 this is not universally implemented  Check the documentation at https   developer mozilla org en-US docs Web JavaScript Reference Operators Optional chaining

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Going through the comments  for those who want to check both is it undefined or its value is null     Just in JavaScript var s     Undefined if  typeof s     undefined     s     null       alert  either it is undefined or value is null       If you are using jQuery Library then jQuery isEmptyObject   will suffice for both cases   var s     Undefined jQuery isEmptyObject s      Will return true   s   null     Defined as null jQuery isEmptyObject s      Will return true     Usage if  jQuery isEmptyObject s         alert  Either variable s is undefined or its value is null      else        alert  variable s has value     s      s    something      Defined with some value jQuery isEmptyObject s      Will return false

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What does this mean   undefined object property    Actually it can mean two quite different things  First  it can mean the property that has never been defined in the object and  second  it can mean the property that has an undefined value  Let s look at this code   var o     a  undefined     Is o a undefined  Yes  Its value is undefined  Is o b undefined  Sure  There is no property  b  at all  OK  see now how different approaches behave in both situations   typeof o a     undefined     true typeof o b     undefined     true o a     undefined    true o b     undefined    true  a  in o    true  b  in o    false   We can clearly see that typeof obj prop     undefined  and obj prop     undefined are equivalent  and they do not distinguish those different situations  And  prop  in obj can detect the situation when a property hasn t been defined at all and doesn t pay attention to the property value which may be undefined   So what to do   1  You want to know if a property is undefined by either the first or second meaning  the most typical situation    obj prop     undefined    IMHO  see  final fight  below   2  You want to just know if object has some property and don t care about its value    prop  in obj   Notes    You can t check an object and its property at the same time  For example  this x a     undefined or this typeof x a     undefined  raises ReferenceError  x is not defined if x is not defined  Variable undefined is a global variable  so actually it is window undefined in browsers   It has been supported since ECMAScript 1st Edition and since ECMAScript 5 it is read only  So in modern browsers it can t be redefined to true as many authors love to frighten us with  but this is still a true for older browsers    Final fight  obj prop     undefined vs typeof obj prop     undefined   Pluses of obj prop     undefined    It s a bit shorter and looks a bit prettier The JavaScript engine will give you an error if you have misspelled undefined   Minuses of obj prop     undefined    undefined can be overridden in old browsers   Pluses of typeof obj prop     undefined     It is really universal  It works in new and old browsers    Minuses of typeof obj prop     undefined      undefned   misspelled  here is just a string constant  so the JavaScript engine can t help you if you have misspelled it like I just did    Update  for server-side JavaScript    Node js supports the global variable undefined as global undefined  it can also be used without the  global  prefix   I don t know about other implementations of server-side JavaScript

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The issue boils down to three cases    The object has the property and its value is not undefined  The object has the property and its value is undefined  The object does not have the property    This tells us something I consider important   There is a difference between an undefined member and a defined member with an undefined value   But unhappily typeof obj foo does not tell us which of the three cases we have  However we can combine this with  foo  in obj to distinguish the cases                                     typeof obj x      undefined       x  in obj  1                        x 1      false                          false 2       x    function             true                           false 3                                 true                           true   Its worth noting that these tests are the same for null entries too                                    typeof obj x      undefined       x  in obj                        x null      false                          false   I d argue that in some cases it makes more sense  and is clearer  to check whether the property is there  than checking whether it is undefined  and the only case where this check will be different is case 2  the rare case of an actual entry in the object with an undefined value   For example  I ve just been refactoring a bunch of code that had a bunch of checks whether an object had a given property   if  typeof blob x     undefined       fn blob x       Which was clearer when written without a check for undefined   if   x  in blob     fn blob x       But as has been mentioned these are not exactly the same  but are more than good enough for my needs

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Object hasOwnProperty o   propertyname     This doesn t look up through the prototype chain  however

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Introduced in ECMAScript 6  we can now deal with undefined in a new way using Proxies  It can be used to set a default value to any properties which doesn t exist so that we don t have to check each time whether it actually exists  var handler       get  function target  name        return name in target   target name     N A           var p   new Proxy     handler   p name    Kevin   console log  Name     p name     Age    p age     Gender    p gender   Will output the below text without getting any undefined  Name  Kevin   Age  N A   Gender  N A

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All the answers are incomplete  This is the right way of knowing that there is a property  defined as undefined   var hasUndefinedProperty   function hasUndefinedProperty obj  prop     return   prop in obj   amp  amp   typeof obj prop      undefined         Example  var a     b   1  e   null    a c   a d   hasUndefinedProperty a   b       false  b is defined as 1 hasUndefinedProperty a   c       true  c is defined as undefined hasUndefinedProperty a   d       false  d is undefined hasUndefinedProperty a   e       false  e is defined as null     And now    delete a c   hasUndefinedProperty a   c       false  c is undefined  Too bad that this been the right answer and is buried in wrong answers  gt   lt  So  for anyone who pass by  I will give you undefined s for free   var undefined   undefined      undefined      a                         undefined    a                           undefined    a                           undefined  function                      undefined void 0                         undefined eval                           undefined 1  a                           undefined  a  a                          undefined  true  a                       undefined

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if  somevariable    undefined      alert  the variable is not defined         You can also make it into a function  as shown here   function isset varname     return typeof window varname       undefined

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Also  the same things can be written shorter  if   variable          Do it if the variable is undefined    or if  variable          Do it if the variable is defined

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From lodash js  var undefined  function isUndefined value      return value     undefined     It creates a local variable named undefined which is initialized with the default value -- the real undefined  then compares value with the variable undefined   Update 9 9 2019 I found Lodash updated its implementation  See my issue and the code  To be bullet-proof  simply use  function isUndefined value      return value     void 0

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I would like to show you something I m using in order to protect the undefined variable   Object defineProperty window   undefined          This forbids anyone to change the window undefined value therefore destroying the code based on that variable  If using  use strict   anything trying to change its value will end in error  otherwise it would be silently ignored

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if   typeof  something       undefined      This worked for me while the others didn t

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In recent JavaScript release there is new chaining operator introduced  which is most probably best way to check if property exists else it will give you undefined see example below   const adventurer       name   Alice     cat        name   Dinah          const dogName   adventurer dog  name  console log dogName      expected output  undefined  console log adventurer someNonExistentMethod          expected output  undefined  We can replace this old syntax if  response  amp  amp  response data  amp  amp  response data someData  amp  amp  response data someData someMoreData      with this neater syntax if  response  data  someData  someMoreData      This syntax is not supported in IE  Opera  safari  amp  samsund android for more detail you can check this URL https   developer mozilla org en-US docs Web JavaScript Reference Operators Optional chaining

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In the article Exploring the Abyss of Null and Undefined in JavaScript  I read that frameworks like Underscore js use this function   function isUndefined obj       return obj     void 0

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I m assuming you re going to also want to check for it being either undefined or null  If so  I suggest  myVar    null This is one of the only times a double equals is very helpful as it will evaluate to true when myVar is undefined or null  but it will evaluate to false when it is other falsey values such as 0  false      and NaN  This the actual the source code for Lodash s isNil method

[javascript] Detecting an undefined object property

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