How do I check if an object has a specific property in JavaScript

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How do I check if an object has a specific property in JavaScript   Consider   x     key   1   if   x hasOwnProperty  key             Do this     Is that the best way to do it

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Let s cut through some confusion here  First  let s simplify by assuming hasOwnProperty already exists  this is true of the vast majority of current browsers in use   hasOwnProperty returns true if the attribute name that is passed to it has been added to the object  It is entirely independent of the actual value assigned to it which may be exactly  undefined   Hence   var o      o x   undefined  var a   o hasOwnProperty  x       a is true var b   o x     undefined    b is also true   However   var o       var a   o hasOwnProperty  x       a is now false var b   o x     undefined    b is still true   The problem is what happens when an object in the prototype chain has an attribute with the value of undefined  hasOwnProperty will be false for it  and so will     undefined  Yet  for  in will still list it in the enumeration   The bottom line is there is no cross-browser way  since Internet nbsp Explorer doesn t expose   prototype    to determine that a specific identifier has not been attached to an object or anything in its prototype chain

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If you are searching for a property  then  quot no quot   You want  if   prop  in obj       In general  you should not care whether or not the property comes from the prototype or the object  However  because you used  key  in your sample code  it looks like you are treating the object as a hash  in which case your answer would make sense   All of the hashes keys would be properties in the object  and you avoid the extra properties contributed by the prototype  John Resig s answer was very comprehensive  but I thought it wasn t clear  Especially with when to use  quot  prop  in obj quot

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I m really confused by the answers that have been given - most of them are just outright incorrect  Of course you can have object properties that have undefined  null  or false values  So simply reducing the property check to typeof this property  or  even worse  x key will give you completely misleading results   It depends on what you re looking for  If you want to know if an object physically contains a property  and it is not coming from somewhere up on the prototype chain  then object hasOwnProperty is the way to go  All modern browsers support it   It was missing in older versions of Safari - 2 0 1 and older - but those versions of the browser are rarely used any more    If what you re looking for is if an object has a property on it that is iterable  when you iterate over the properties of the object  it will appear  then doing  prop in object will give you your desired effect   Since using hasOwnProperty is probably what you want  and considering that you may want a fallback method  I present to you the following solution   var obj         a  undefined      b  null      c  false        a  b  c all found for   var prop in obj         document writeln   Object1      prop       function Class        this a   undefined      this b   null      this c   false     Class prototype         a  undefined      b  true      c  true      d  true      e  true     var obj2   new Class        a  b  c  d  e found for   var prop in obj2         document writeln   Object2      prop       function hasOwnProperty obj  prop        var proto   obj   proto      obj constructor prototype      return  prop in obj   amp  amp             prop in proto     proto prop      obj prop       if   Object prototype hasOwnProperty         var hasOwnProperty   function obj  prop            return obj hasOwnProperty prop               a  b  c found in modern browsers    b  c found in Safari 2 0 1 and older for   var prop in obj2         if   hasOwnProperty obj2  prop              document writeln   Object2 w  hasOwn      prop              The above is a working  cross-browser  solution to hasOwnProperty  with one caveat  It is unable to distinguish between cases where an identical property is on the prototype and on the instance - it just assumes that it s coming from the prototype  You could shift it to be more lenient or strict  based upon your situation  but at the very least this should be more helpful

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There is a method   quot hasOwnProperty quot   that exists on an object  but it s not recommended to call this method directly  because it might be sometimes that the object is null or some property exist on the object like    hasOwnProperty  false   So a better way would be   x000D   x000D     Good var obj     bar    here bar desc   console log Object prototype hasOwnProperty call obj   bar         Best const has   Object prototype hasOwnProperty     Cache the lookup once  in module scope  console log has call obj   bar     x000D   x000D   x000D

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A Better approach for iterating on object s own properties  If you want to iterate on object s properties without using hasOwnProperty   check  use for let key of Object keys stud     method  for let key of Object keys stud      console log key      will only log object s Own properties    full Example and comparison with for-in with hasOwnProperty   function Student       this name    quot nitin quot      Student prototype       grade   A     let stud   new Student        for-in approach for let key in stud     if stud hasOwnProperty key        console log key      only outputs  quot name quot            Object keys   approach for let key of Object keys stud      console log key

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Do not do this object hasOwnProperty key    It s really bad because these methods may be shadowed by properties on the object in question - consider   hasOwnProperty  false   - or  the object may be a null object  Object create null    The best way is to do Object prototype hasOwnProperty call object  key  or  const has   Object prototype hasOwnProperty     Cache the lookup once  in module scope     Or    import has from  has      https   www npmjs com package has        console log has call object  key

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if  x key     undefined    Armin Ronacher seems to have already beat me to it  but   Object prototype hasOwnProperty   function property        return this property      undefined      x     key   1    if  x hasOwnProperty  key          alert  have key        if   x hasOwnProperty  bar          alert  no bar         A safer  but slower solution  as pointed out by Konrad Rudolph and Armin Ronacher would be   Object prototype hasOwnProperty   function property        return typeof this property       undefined

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For testing simple objects  use  if  obj x      undefined   If you don t know what object type it is  use  if  obj hasOwnProperty x    All other options are slower    Details A performance evaluation of 100 000 000 cycles under Node js to the five options suggested by others here  function hasKey1 k o    return  x in obj     function hasKey2 k o    return  obj x      function hasKey3 k o    return  obj x      undefined     function hasKey4 k o    return  typeof obj x        undefined      function hasKey5 k o    return  obj hasOwnProperty x       The evaluation tells us that unless we specifically want to check the object s prototype chain as well as the object itself  we should not use the common form  if  X in Obj      It is between 2 to 6 times slower depending on the use case hasKey1 execution time  4 51 s hasKey2 execution time  0 90 s hasKey3 execution time  0 76 s hasKey4 execution time  0 93 s hasKey5 execution time  2 15 s  Bottom line  if your Obj is not necessarily a simple object and you wish to avoid checking the object s prototype chain and to ensure x is owned by Obj directly  use if  obj hasOwnProperty x       Otherwise  when using a simple object and not being worried about the object s prototype chain  using if  typeof obj x        undefined      is the safest and fastest way  If you use a simple object as a hash table and never do anything kinky  I would use if  obj x      as I find it much more readable

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OK  it looks like I had the right answer unless if you don t want inherited properties   if  x hasOwnProperty  key      Here are some other options to include inherited properties   if  x key     Quick and dirty  but it does the same thing as below   if  x key     undefined

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Showing how to use this answer const object   key1   data   key2   data2     Object keys object  includes  key1     returns true  We can use indexOf as well  I prefer includes

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You can also use the ES6 Reflect object   x     key   1   Reflect has  x   key       returns true   Documentation on MDN for Reflect has can be found here      The static Reflect has   method works like the in operator as a function

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Here is another option for a specific case     If you want to test for a member on an object and want to know if it has been set to something other than      false null undefined 0      then you can use  var foo       foo bar    quot Yes  this is a proper value  quot   if    foo bar           member is set  do something

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Performance Today 2020 12 17 I perform tests on MacOs HighSierra 10 13 6 on Chrome v87  Safari v13 1 2 and Firefox v83 for chosen solutions  Results I compare only solutions A-F because they give valid result for all cased used in snippet in details section  For all browsers  solution based on in  A  is fast or fastest solution  E  is fastest for chrome for big objects and fastest for firefox for small arrays if key not exists solution  F  is fastest     gt 10x than other solutions  for small arrays solutions  D E  are quite fast solution based on losash has  B  is slowest   Details I perform 4 tests cases   when object has 10 fields and searched key exists - you can run it HERE when object has 10 fields and searched key not exists - you can run it HERE when object has 10000 fields and searched key exists - you can run it HERE when object has 10000 fields and searched key exists - you can run it HERE  Below snippet presents differences between solutions A B C D E F G H I J K  x000D   x000D     SO https   stackoverflow com q 135448 860099      src  https   stackoverflow com a 14664748 860099 function A x      return  key  in x       src  https   stackoverflow com a 11315692 860099 function B x      return   has x   key         src  https   stackoverflow com a 40266120 860099 function C x      return Reflect has  x   key         src  https   stackoverflow com q 135448 860099 function D x      return x hasOwnProperty  key         src  https   stackoverflow com a 11315692 860099 function E x      return Object prototype hasOwnProperty call x   key         src  https   stackoverflow com a 136411 860099 function F x      function hasOwnProperty obj  prop          var proto   obj   proto      obj constructor prototype        return  prop in obj   amp  amp               prop in proto     proto prop      obj prop          return hasOwnProperty x  key         src  https   stackoverflow com a 135568 860099 function G x      return typeof x key       undefined        src  https   stackoverflow com a 22740939 860099 function H x      return x key     undefined       src  https   stackoverflow com a 38332171 860099 function I x      return   x key       src  https   stackoverflow com a 41184688 860099 function J x      return   x  key         src  https   stackoverflow com a 54196605 860099 function K x      return Boolean x key         --------------------    TEST    --------------------  let x1     key   1   let x2     key    1    let x3     key   true   let x4     key        let x5     key        let x6     key       gt      let x7     key        let x8     key   0   let x9     key   false   let x10    key   undefined   let x11    nokey   1      let b  x  gt  x   1 0   console log    1 2 3 4 5 6 7 8 9 10 11      A B C D E F G H I J K   map f  gt        console log         f name    b f x1      b f x2      b f x3      b f x4      b f x5      b f x6      b f x7      b f x8      b f x9      b f x10       b f x11               console log   nLegend  Columns  cases     console log  1   key   1     console log  2   key    1      console log  3   key   true     console log  4   key          console log  5   key          console log  6   key       gt        console log  7   key          console log  8   key   0     console log  9   key   false     console log  10  key   undefined     console log  11  no-key     x000D   lt script src  https   cdnjs cloudflare com ajax libs lodash js 4 17 20 lodash min js  integrity  sha512-90vH1Z83AJY9DmlWa8WkjkV79yfS2n2Oxhsi2dZbIv0nC4E6m5AbH8Nh156kkM7JePmqD6tcZsfad1ueoaovww    crossorigin  anonymous  gt   lt  script gt     This shippet only presents functions used in performance tests - it not perform tests itself  x000D   x000D   x000D   And here are example results for chrome

User · Answer

Yes it is     I think you can also do Object prototype hasOwnProperty call x   key   which should also work if x has a property called hasOwnProperty     But that tests for own properties   If you want to check if it has an property that may also be inhered you can use typeof x foo     undefined

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If the key you are checking is stored in a variable  you can check it like this   x     key   1   y    key   x y

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Note  the following is nowadays largely obsolete thanks to strict mode  and hasOwnProperty  The correct solution is to use strict mode and to check for the presence of a property using obj hasOwnProperty  This answer predates both these things  at least as widely implemented  yes  it is that old   Take the following as a historical note     Bear in mind that undefined is  unfortunately  not a reserved word in JavaScript if you   re not using strict mode  Therefore  someone  someone else  obviously  could have the grand idea of redefining it  breaking your code   A more robust method is therefore the following   if  typeof x attribute       undefined     On the flip side  this method is much more verbose and also slower   -   A common alternative is to ensure that undefined is actually undefined  e g  by putting the code into a function which accepts an additional parameter  called undefined  that isn   t passed a value  To ensure that it   s not passed a value  you could just call it yourself immediately  e g     function  undefined            your code         if  x attribute     undefined              mode code

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You can use the following approaches- var obj    a 1  console log  a  in obj                   1 console log obj hasOwnProperty  a        2 console log Boolean obj a              3  The difference between the following approaches are as follows-  In the first and third approach we are not just searching in object but its prototypal chain too  If the object does not have the property  but the property is present in its prototype chain it is going to give true    x000D   x000D  var obj         a  2        proto      b  2     console log  b  in obj  console log Boolean obj b   x000D   x000D   x000D    The second approach will check only for its own properties  Example -   x000D   x000D  var obj         a  2        proto      b  2     console log obj hasOwnProperty  b    x000D   x000D   x000D    The difference between the first and the third is if there is a property which has value undefined the third approach is going to give false while first will give true    x000D   x000D  var obj         b   undefined    console log Boolean obj b   console log  b  in obj   x000D   x000D   x000D

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Another relatively simple way is using Object keys  This returns an array which means you get all of the features of an array  var noInfo       var info    something   data     Object keys noInfo  length   returns 0 or false Object keys info  length   returns 1 or true  Although we are in a world with great browser support  Because this question is so old I thought I d add this  This is safe to use as of JavaScript v1 8 5

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Don t over-complicate things when you can do  var isProperty     objectname keyname     quot  quot     true   false   It Is simple and clear for most cases

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Use   x000D   x000D  var x        key   1     if   key  in x      console log  has      x000D   x000D   x000D

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hasOwnProperty  quot can be used to determine whether an object has the specified property as a direct property of that object  unlike the in operator  this method does not check down the object s prototype chain  quot  So most probably  for what seems by your question  you don t want to use hasOwnProperty  which determines if the property exists as attached directly to the object itself   If you want to determine if the property exists in the prototype chain  you may want to use it like  if  prop in object       Do something

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You need to use the method object hasOwnProperty property   It returns true if the object has the property and false if the object doesn t

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JavaScript is now evolving and growing it has now good and best even efficient way to check it Here are some easy ways to check if object has a particular property   Using hasOwnProperty    const hero       name   Batman      hero hasOwnProperty  name             gt  true hero hasOwnProperty  realName         gt  false   Using keyword operator in  const hero       name   Batman       name  in hero           gt  true  realName  in hero       gt  false   Comparing with undefined keyword  const hero       name   Batman      hero name           gt   Batman  hero realName       gt  undefined     So consider this hero realName    undefined      gt  true  which means property does not exists in object  hero name    undefined      gt  false  which means that property exists in object   For more information  check here

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With Underscore js or  even better  Lodash    has x   key     Which calls Object prototype hasOwnProperty  but  a  is shorter to type  and  b  uses  quot a safe reference to hasOwnProperty quot   i e  it works even if hasOwnProperty is overwritten   In particular  Lodash defines   has as     function has object  key          return object   hasOwnProperty call object  key    false             hasOwnProperty   Object prototype hasOwnProperty

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You can use the in operator to check if the property exists on an object   x     key   1   alert  key  in x     You can also loop through all the properties of the object using a for - in loop  and then check for the specific property   for  prop in x        if  prop     key               Do something           You must consider if this object property is enumerable or not  because non-enumerable properties will not show up in a for-in loop  Also  if the enumerable property is shadowing a non-enumerable property of the prototype  it will not show up in Internet nbsp Explorer nbsp 8 and earlier   If you   d like a list of all instance properties  whether enumerable or not  you can use  Object getOwnPropertyNames x     This will return an array of names of all properties that exist on an object   Finally  you can use the typeof operator to directly check the data type of the object property   if  typeof x key     undefined         alert  undefined        If the property does not exist on the object  it will return the string undefined  Else it will return the appropriate property type  However  note that this is not always a valid way of checking if an object has a property or not  because you could have a property that is set to undefined  in which case  using typeof x key would still return true  even though the key is still in the object    Update  You can check if a property exists by comparing to the undefined javascript property   if  x key     undefined        alert  undefined        This should work unless key was specifically set to undefined on the x object

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An ECMAScript 6 solution with reflection  Create a wrapper like      Gets an argument from array or object  The possible outcome  - If the key exists the value is returned  - If no key exists the default value is returned  - If no default value is specified an empty string is returned   param obj    The object or array to be searched   param key    The name of the property or key   param defVal Optional default version of the command-line parameter  default  quot  quot    return The default value in case of an error else the found parameter     function getSafeReflectArg  obj  key  defVal        quot use strict quot      var retVal    typeof defVal      undefined     quot  quot    defVal      if   Reflect has  obj  key             return Reflect get  obj  key           return retVal        getSafeReflectArg

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if  typeof x key     undefined          Because  if  x key    fails if x key resolves to false  for example  x key

[javascript] How do I check if an object has a specific property in JavaScript?

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