JavaScript isset equivalent

Question

In PHP you can do if isset  array  foo              In JavaScript you often use if array foo          to do the same  but this is not exactly the same statement  The condition will also evaluate to false if array foo does exists but is false or 0  and probably other values as well    What is the perfect equivalent of PHP s isset in JavaScript   In a broader sense  a general  complete guide on JavaScript s handling of variables that don t exist  variables without a value  etc  would be convenient

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To check wether html block is existing or not  I m using this code   if  typeof    selector   html        undefined               selector   is existing        your code here

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If you want to check if an element exists  just use the following code   if  object        if isset  return true   else       else return false     This is sample    x000D   x000D  function switchDiv     x000D      if  document querySelector   divId      x000D          document querySelector   divId   remove    x000D        else   x000D          var newDiv   document createElement  div    x000D          newDiv id    divId   x000D          document querySelector  body   appendChild newDiv   x000D        x000D    x000D   x000D  document querySelector   btn   addEventListener  click   switchDiv   x000D   divId   x000D      background  red  x000D      height  100px  x000D      width  100px  x000D      position  relative  x000D       x000D    x000D   lt body gt  x000D     lt button id  btn  gt Let s Diiiv  lt  button gt  x000D   lt  body gt  x000D   x000D   x000D

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if  var         This is the most concise equivalent of Php s isset

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window isset   function v var        if typeof v var      number    if isNaN v var    return false         if typeof v var      undefined     v var     null   return false      else   return true         plus Tests   https   gist github com daylik 24acc318b6abdcdd63b46607513ae073

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It was really a problem for me when I was accessing a deeper property of an object so I made a function which will return the property value if exist otherwise it will return false  You may use it to save your time     Object on which we want to test var foo         bar            bik                baz   Hello world                          USE  To get value from the object using it properties supplied  Deeper       if found it will return the property value if not found then will return false  You can use this function in two ways WAY - 1  Passing an object as parameter 1 and array of the properties as parameter 2 EG  getValueFromObject foo    bar    bik    baz     WAY - 2   This will work only if  your object available in window object  Passing an STRING as parameter 1 Just similarly how we retrieve value form object using it s properties - difference is only the quote  EG  getValueFromObject  foo bar bik baz       function getValueFromObject object  properties        if typeof object      string                  Here we extract our object and it s properties from the string         properties   object split               object   window properties 0            if typeof object      undefined                 return false                    properties shift              var property   properties 0       properties shift        if object    null  amp  amp  typeof object property       undefined             if typeof object property       object                 if properties length    0                    return getValueFromObject object property   properties        Recursive call to the function               else                   return object property                           else               return object property                   else           return false          console log getValueFromObject  fooo bar bik baz              false console log getValueFromObject  foo bar bik baz               Hello world console log getValueFromObject  foo                           false console log getValueFromObject  foo bar bik                   returns an object   baz   Hello World    console log getValueFromObject foo    bar    bik              returns an object   baz   Hello World    console log getValueFromObject foo    bar    bik    baz       Hello world

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function isset variable        try           return typeof eval variable       undefined         catch  err            return false

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This is a pretty bulletproof solution for testing if a variable exists    var setOrNot   typeof variable     typeof undefined   true   false    Unfortunately  you cannot simply encapsulate it in a function   You might think of doing something like this    function isset variable        return typeof variable     typeof undefined   true   false      However  this will produce a reference error if variable variable has not been defined  because you cannot pass along a non-existing variable to a function        Uncaught ReferenceError  foo is not defined   On the other hand  it does allow you to test whether function parameters are undefined    var a    5    var test   function x  y        console log isset x        console log isset y        test a       OUTPUT      ------------    TRUE    FALSE   Even though no value for y is passed along to function test  our isset function works perfectly in this context  because y is known in function test as an undefined value

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Be careful in ES6  all the previous solutions doesn t work if you want to check a declaration of a let variable and declare it  if it isn t  example  let myTest    text    if typeof myTest      undefined         var myTest    new text      can t be a let because let declare in a scope     you will see a error     Uncaught SyntaxError  Identifier  myTest  has already been declared   The solution was to change it by a var  var myTest    text      I replace let by a var  if typeof myTest      undefined         var myTest    new text       another solution if you can change a let by a var  you need to remove your var  let myTest    text    if typeof myTest      undefined         myTest    new text      I remove the var declaration

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Age old thread  but there are new ways to run an equivalent isset    ESNext  Stage 4 December 2019  Two new syntax allow us to vastly simplify the use of isset   functionality   Optional Chaining     Nullish Coalescing Operator      Please read the docs and mind the browser compatibility  Answer See below for explanation  Note I use StandardJS syntax Example Usage    IMPORTANT pass a function to our isset   that returns the value we re    trying to test ES6 arrow function  isset      gt  some     false     Defining objects let some     nested    value   hello          More tests that never throw an error isset      gt  some     true isset      gt  some nested     true isset      gt  some nested value     true isset      gt  some nested deeper value     false     Less compact but still viable except when trying to use  this  context isset function      return some nested deeper value       false  Answer Function        Checks to see if a value is set         param    Function  accessor Function that returns our value     returns  Boolean            Value is not undefined or null     function isset  accessor      try          Note we re seeing if the returned value of our function is not        undefined or null     return accessor       undefined  amp  amp  accessor       null     catch  e           And we re able to catch the Error it would normally throw for        referencing a property of undefined     return false         Explanation PHP Note that in PHP you can reference any variable at any depth - even trying to access a non-array as an array will return a simple true or false     Referencing an undeclared variable isset  some      false   some    hello       Declared but has no depth not an array  isset  some      true isset  some  nested        false   some     nested    gt   hello        Declared as an array but not with the depth we re testing for isset  some  nested        true isset  some  nested    deeper        false  JS In JavaScript  we don t have that freedom  we ll always get an error if we do the same  because JS is immediately attempting to access the value of deeper before we can wrap it in our isset   function so       Common pitfall answer ES6 arrow function  const isset    ref    gt  typeof ref      undefined      Same as above function isset  ref    return typeof ref      undefined        Referencing an undeclared variable will throw an error  so no luck here isset some     Error  some is not defined     Defining a simple object with no properties - so we aren t defining    the property  nested  let some          Simple checking if we have a declared variable isset some     true     Now trying to see if we have a top level property  still valid isset some nested     false     But here is where things fall apart  trying to access a deep property    of a complex object  it will throw an error isset some nested deeper     Error  Cannot read property  deeper  of undefined                   undefined  More failing alternatives     Any way we attempt to access the  deeper  property of  nested  will    throw an error some nested deeper hasOwnProperty  value      Error             undefined  Object hasOwnProperty call some nested deeper   value      Error                                        undefined     Same goes for typeof typeof some nested deeper      undefined     Error                    undefined  And some working alternatives that can get redundant fast     Wrap everything in try   catch try     if  isset some nested deeper                      catch  e      try     if  typeof some nested deeper      undefined                      catch  e         Or by chaining all of the isset which can get long isset some   amp  amp  isset some nested   amp  amp  isset some nested deeper     false                                  returns false so the next isset   is never run  Conclusion All of the other answers - though most are viable     Assume you re only checking to see if the variable is not undefined which is fine for some use cases but can still throw an Error Assume you re only trying to access a top level property  which again is fine for some use cases Force you to use a less than ideal approach relative to PHP s isset   e g  isset some   nested deeper value   Use eval   which works but I personally avoid  I think I covered a lot of it  There are some points I make in my answer that I don t touch upon because they - although relevant - are not part of the question e g  short circuiting   If need be  though  I can update my answer with links to some of the more technical aspects based on demand  I spent waaay to much time on this so hopefully it helps people out  Thank-you for reading

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javascript isset  let test       a        b   0  1          console log test isset  a b         true console log test isset  a b 1       true console log test isset  a b 5       false console log test isset  a c         false console log  abv  isset  0          true

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if     foo  in obj          not set

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I generally use the typeof operator  if  typeof obj foo      undefined          your code here    It will return  quot undefined quot  either if the property doesn t exist or its value is undefined   See also  Difference between undefined and not being defined   There are other ways to figure out if a property exists on an object  like the hasOwnProperty method  if  obj hasOwnProperty  foo           your code here    And the in operator  if   foo  in obj         your code here    The difference between the last two is that the hasOwnProperty method will check if the property exist physically on the object  the property is not inherited   The in operator will check on all the properties reachable up in the prototype chain  e g   var obj     foo   bar     obj hasOwnProperty  foo       true obj hasOwnProperty  toString       false  toString  in obj     true  As you can see  hasOwnProperty returns false and the in operator returns true when checking the toString method  this method is defined up in the prototype chain  because obj inherits form Object prototype

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Try to create function like empty function of PHP in Javascript  May this helps  function empty str     try      if typeof str    quot string quot            str str trim              return   str     undefined  amp  amp  str      quot undefined quot   amp  amp  str     null  amp  amp  str    quot  quot   amp  amp  str   0  amp  amp  str   false      catch ex       return true          console log empty 0    true console log empty null    true console log empty  quot   quot     true console log empty  quot  quot     true console log empty undefined    true console log empty  quot undefined quot     true  var tmp 1  console log empty tmp    false  var tmp  quot Test quot   console log empty tmp    false  var tmp  quot  Test  quot   console log empty tmp    false  var tmp  a 1 b false c 0   console log empty tmp a    false console log empty tmp b    true console log empty tmp c    true console log empty tmp c    true console log empty tmp c d    true

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This solution worked for me     function isset object       return  typeof object     undefined

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typeof SOMETHING       undefined    It s too long to write when used  But we can t package the typeof keyword into a function  because an error will thrown before the function is called  like this   function isdef  var        return  typeof  var       undefined      isdef SOMETHING         thrown error  SOMETHING is not defined   So I figured out a way   function isdef  type        return  type      undefined      isdef typeof SOMETHING     It can work both with individual variables  variables that does not exist at all   or object properties  non-existent properties   And only 7 more characters than PHP isset

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I always use this generic function to prevent errrors on primitive variables as well as arrays and objects   isset   function obj      var i  max i    if obj     undefined  return false    for  i   1  max i   arguments length  i  lt  max i  i          if  obj arguments i       undefined            return false            obj   obj arguments i          return true      console log isset obj                         returns false var obj    huhu   console log isset obj                         returns true obj    hallo  hoi  hoi     console log isset obj   niet                  returns false console log isset obj   hallo                 returns true console log isset obj   hallo    hallo        returns false console log isset obj   hallo    hoi          returns true

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If you are using underscorejs I always use  if     isUndefined data   amp  amp     isNull data            your stuff

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tring to reference non-existing variable throws ReferenceError      before test function is even executed        example  if you do                if   isset  someVar                doStuff  someVar              you get a ReferenceError   if there is no someVar           and isset fn doesn t get executed         if you pass variable name as string  ex  isset   novar           this might work     function isset   strVariableName           try            eval  strVariableName          catch  err              if   err instanceof ReferenceError               return false             return true

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isset  user permissions saveProject   args        function isset string  context            try               var arr   string split                   var checkObj   context    window               for  var i in arr                    if  checkObj arr i       undefined  return false                  checkObj   checkObj arr i                               return true            catch  e                return false

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This simple solution works  but not for deep object check   function isset str        return window str      undefined

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PHP Manual say      isset     Determine if a variable is set and is not NULL   And interface something like this   bool isset   mixed  var    mixed           The parameter  var is the variable to be checked  it can have any number of parameter though   isset   returns TRUE if var exists and has value other than NULL  FALSE otherwise   Some example    foo    bar   var dump isset  foo           - gt  true   baz   null  var dump isset  baz           - gt  false  var dump isset  undefined     - gt  false   As this in mind  Apparently  It s not possible to write exact equivalent of php isset   function  For example when we call like this   if  isset some var         function issset            function definition     Javascript trigger Uncaught ReferenceError  some var is not defined at  file name  line number  The important and remarkable thing about this behavior is that when trying to pass non-existent variables to normal functions  an error is triggered   But in PHP isset   are not actually regular functions but language constructs  That means they re part of the PHP language itself  do not play by the normal rules of functions and can hence get away with not triggering an error for non-existent variables  This is important when trying to figure out whether a variable exists or not  But in javscript  it triggers an error in the first place say function call with non-existent variables   My point is that we can t write it as equivlent javscript function but we can do something like this  if  typeof some var      undefined           your code here     If you want exact same effect PHP also check varable is not NULL  For example   baz   null  var dump isset  baz           - gt  false   So  we can incorporate this into javascript then it look like this   if  typeof some var      undefined   amp  amp  some var     null          your code here

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Provide the object path as a string  then you can break this string into a path and resolve hasOwnProperty at each step while overwriting the object itself with each iteration   If you are coding in ES6 environment  take a look at this stackoverflow Ques    x000D   x000D  var a  x000D   x000D  a     x000D      b    x000D          c   e  x000D        x000D     x000D   x000D  function isset  obj  path    x000D      var stone  x000D   x000D      path   path        x000D   x000D      if  path indexOf          -1    x000D          throw new Error  Unsupported object path notation     x000D        x000D   x000D       x000D      path   path split       x000D       x000D      do   x000D          if  obj     undefined    x000D              return false  x000D            x000D   x000D          stone   path shift    x000D           x000D          if   obj hasOwnProperty stone     x000D              return false  x000D            x000D           x000D          obj   obj stone   x000D           x000D        while  path length   x000D   x000D      return true  x000D    x000D   x000D  console log  x000D      isset a   b      true  x000D      isset a   b c      true  x000D      isset a   b c d      false  x000D      isset a   b c d e      false  x000D      isset a   b c d e f      false x000D     x000D   x000D   x000D

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Reference to SOURCE       module exports   function isset            discuss at  http   locutus io php isset       original by  Kevin van Zonneveld  http   kvz io       improved by  FremyCompany      improved by  Onno Marsman  https   twitter com onnomarsman       improved by  Rafal Kukawski  http   blog kukawski pl         example 1  isset  undefined  true         returns 1  false        example 2  isset   Kevin van Zonneveld           returns 2  true    var a   arguments   var l   a length   var i   0   var undef    if  l     0        throw new Error  Empty isset          while  i     l        if  a i      undef    a i      null          return false           i          return true     phpjs org is mostly retired in favor of locutus Here is the new link http   locutus io php var isset

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I use a function that can check variables and objects  very convenient to work with jQuery      function  isset  variable            if typeof variable      undefined     variable    null              return false          else             if typeof variable      object   amp  amp   variable length                   return false              else                 return true

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