Checking if a key exists in a JavaScript object

Question

How do I check if a particular key exists in a JavaScript object or array   If a key doesn t exist  and I try to access it  will it return false  Or throw an error

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key  in obj   Is likely testing only object attribute values that are very different from array keys

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Answer   if   key  in myObj        console log  key exists       else       console log  key doesn t exist         Explanation   The in operator will check if the key exists in the object  If you checked if the value was undefined  if  myObj  key        undefined    you could run into problems because a key could possibly exist in your object with the undefined value   For that reason  it is much better practice to first use the in operator and then compare the value that is inside the key once you already know it exists

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It will return undefined    x000D   x000D  var aa    hello   world    x000D  alert  aa  hello              popup box with  world  x000D  alert  aa  goodbye            popup box with  undefined  x000D   x000D   x000D    undefined is a special constant value   So you can say  e g      note the three equal signs so that null won t be equal to undefined if  aa  goodbye       undefined            do something     This is probably the best way to check for missing keys   However  as is pointed out in a comment below  it s theoretically possible that you d want to have the actual value be undefined   I ve never needed to do this and can t think of a reason offhand why I d ever want to  but just for the sake of completeness  you can use the in operator     this works even if you have   goodbye   undefined  if   goodbye  in aa            do something

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ES6 solution  using Array some and Object keys  It will return true if given key exists in the object or false if it doesn t    x000D   x000D  var obj    foo   one   bar   two    x000D       x000D  function isKeyInObject obj  key    x000D      var res   Object keys obj  some v   gt  v    key   x000D      console log res   x000D    x000D   x000D  isKeyInObject obj   foo    x000D  isKeyInObject obj   something    x000D   x000D   x000D    One-line example    x000D   x000D  console log Object keys  foo   one   bar   two    some v   gt  v     foo     x000D   x000D   x000D

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Quick Answer  How do I check if a particular key exists in a JavaScript object or array  If a key doesn t exist and I try to access it  will it return false  Or throw an error   Accessing directly a missing property using  associative  array style or object style will return an undefined constant  The slow and reliable in operator and hasOwnProperty method As people have already mentioned here  you could have an object with a property associated with an  quot undefined quot  constant   var bizzareObj    valid key   undefined    In that case  you will have to use hasOwnProperty or in operator to know if the key is really there  But  but at what price  so  I tell you    in operator and hasOwnProperty are  quot methods quot  that use the Property Descriptor mechanism in Javascript  similar to Java reflection in the Java language   http   www ecma-international org ecma-262 5 1  sec-8 10  The Property Descriptor type is used to explain the manipulation and reification of named property attributes  Values of the Property Descriptor type are records composed of named fields where each field   s name is an attribute name and its value is a corresponding attribute value as specified in 8 6 1  In addition  any field may be present or absent   On the other hand  calling an object method or key will use Javascript   Get   mechanism  That is a far way faster  Benchmark http   jsperf com checking-if-a-key-exists-in-a-javascript-array   Using in operator var result    quot Impression quot  in array   The result was 12 931 832   0 21  ops sec      92  slower   Using hasOwnProperty var result   array hasOwnProperty  quot Impression quot    The result was 16 021 758   0 45  ops sec     91  slower  Accessing elements directly  brackets style  var result   array  quot Impression quot       undefined  The result was 168 270 439   0 13 ops sec     0 02  slower   Accessing elements directly  object style  var result   array Impression      undefined   The result was 168 303 172   0 20      fastest  EDIT  What is the reason to assign to a property the undefined value  That question puzzles me  In Javascript  there are at least two references for absent objects to avoid problems like this  null and undefined  null is the primitive value that represents the intentional absence of any object value  or in short terms  the confirmed lack of value  On the other hand  undefined is an unknown value  not defined   If there is a property that will be used later with a proper value consider use null reference instead of undefined because in the initial moment the property is confirmed to lack value  Compare  var a    1  null    console log a 1      undefined      output  false  I know the value at position 1 of a   is absent and this was by design  i e    the value is defined   console log a 0      undefined      output  true  I cannot say anything about a 0  value  In this case  the key 0 was not in a     Advice Avoid objects with undefined values  Check directly whenever possible and use null to initialize property values  Otherwise  use the slow in operator or hasOwnProperty   method  EDIT  12 04 2018 - NOT RELEVANT ANYMORE As people have commented  modern versions of the Javascript engines  with firefox exception  have changed the approach for access properties  The current implementation is slower than the previous one for this particular case but the difference between access key and object is neglectable

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vanila js  yourObjName hasOwnProperty key    true   false    If you want to check if the object has at least one property in es2015  Object keys yourObjName  length   true   false

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Checking for undefined-ness is not an accurate way of testing whether a key exists  What if the key exists but the value is actually undefined  var obj     key  undefined    obj  quot key quot       undefined    false  but the key exists   You should instead use the in operator   quot key quot  in obj    true  regardless of the actual value  If you want to check if a key doesn t exist  remember to use parenthesis     quot key quot  in obj     true if  quot key quot  doesn t exist in object   quot key quot  in obj      Do not do this  It is equivalent to  quot false in obj quot   Or  if you want to particularly test for properties of the object instance  and not inherited properties   use hasOwnProperty  obj hasOwnProperty  quot key quot      true  For performance comparison between the methods that are in  hasOwnProperty and key is undefined  see this benchmark

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New awesome solution with JavaScript Destructuring   let obj          key1    value1        key2    value2        key3    value3       let  key1  key2  key3  key4    obj      key1    value1     key2    value2     key3    value3     key4   undefined     Can easily use  if  here on key4 if  key4    console log  key not present         Key not present   Do check other use of JavaScript Destructuring

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const object1       a   something     b   something     c   something      const key    s       Object keys object1  will return array of the object keys   a    b    c    Object keys object1  indexOf key      -1    the key is not there     yep the key is exist

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Three ways to check if a property is present in a javascript object      obj theProperty Will convert value to bool  returns true for all but the false value  theProperty  in obj Will return true if the property exists  no matter its value  even empty  obj hasOwnProperty  theProperty   Does not check the prototype chain   since all objects have the toString method  1 and 2 will return true on it  while 3 can return false on it     Reference       http   book mixu net node ch5 html

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yourArray indexOf yourArrayKeyName    -1  fruit     apple    grapes    banana    fruit indexOf  apple    gt  -1   true    fruit     apple    grapes    banana    fruit indexOf  apple1    gt  -1   false

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We can use - hasOwnProperty call obj  key    The underscore js way -   if   has this options   login         key  login  exists in this options       has   function obj  key      return hasOwnProperty call obj  key

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The accepted answer refers to Object  Beware using the in operator on Array to find data instead of keys     true  in   true    false       - gt  false  Because the keys of the above Array are actually 0 and 1    To test existing elements in an Array  Best way to find if an item is in a JavaScript array

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Optional Chaining operator can also be used for this      sorce  https   developer mozilla org en-US docs Web JavaScript Reference Operators Optional chaining const adventurer       name   Alice     cat        name   Dinah          const dogName   adventurer dog  name  console log dogName      expected output  undefined

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The easiest way to check is    key  in object   for example    var obj       a  1    b  2     a  in obj    true  c  in obj    false   Return value as true implies that key exists in the object

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If you want to check for any key at any depth on an object and account for falsey values consider this line for a utility function   var keyExistsOn    o  k    gt  k split      reduce  a  c    gt  a hasOwnProperty c    a c     1   false  Object assign     o       false   false   true    Results  var obj         test          locals            test              test2  false          test3  NaN          test4  0          test5  undefined          auth                user   hw                     keyExistsOn obj       gt  false keyExistsOn obj   locals test    gt  true keyExistsOn obj   locals test2    gt  true keyExistsOn obj   locals test3    gt  true keyExistsOn obj   locals test4    gt  true keyExistsOn obj   locals test5    gt  true keyExistsOn obj   sdsdf   false keyExistsOn obj   sdsdf rtsd   false keyExistsOn obj   sdsdf 234d   false keyExistsOn obj   2134 sdsdf 234d   false keyExistsOn obj   locals   true keyExistsOn obj   locals    false keyExistsOn obj   locals auth   true keyExistsOn obj   locals autht   false keyExistsOn obj   locals auth    false keyExistsOn obj   locals auth user   true keyExistsOn obj   locals auth userr   false keyExistsOn obj   locals auth user    false keyExistsOn obj   locals auth user   true   Also see this NPM package  https   www npmjs com package has-deep-value

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For those which have lodash included in their project There is a lodash   get method  which tries to get  deep  keys      Gets the value at path of object  If the resolved value is undefined    the defaultValue is returned in its place     x000D   x000D  var object      a       b      c   3         x000D   x000D  console log  x000D      get object   a 0  b c                   gt  3 x000D      get object    a    0    b    c          gt  3 x000D      get object   a b c                      gt  undefined  x000D      get object   a b c    default           gt   default  x000D    x000D   lt script src  https   cdnjs cloudflare com ajax libs lodash js 4 17 4 lodash min js  gt  lt  script gt  x000D   x000D   x000D      This will effectively check if that key  however deep  is defined and will not throw an error which might harm the flow of your program if that key is not defined

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A fast and easy solution is to convert your object to json then you will be able to do this easy task  const allowed           login              register             resetpsw         console log   login  in allowed     returns true  If you use an array the object key will be converted to integers ex 0 1 2 3 etc  therefore  it will always be false

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Here s a helper function I find quite useful  This keyExists key  search  can be used to easily lookup a key within  objects or arrays    Just pass it the key you want to find  and search obj  the object or array  you want to find it in    x000D   x000D  function keyExists key  search    x000D          if   search     search constructor     Array  amp  amp  search constructor     Object     x000D              return false  x000D            x000D          for  var i   0  i  lt  search length  i      x000D              if  search i      key    x000D                  return true  x000D                x000D            x000D          return key in search  x000D        x000D   x000D     How to use it  x000D     Searching for keys in Arrays x000D  console log keyExists  apple     apple    banana    orange         true x000D  console log keyExists  fruit     apple    banana    orange         false x000D   x000D     Searching for keys in Objects x000D  console log keyExists  age     name    Bill    age   29         true x000D  console log keyExists  title     name    Jason    age   29         false x000D   x000D   x000D    It s been pretty reliable and works well cross-browser

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While this doesn t necessarily check if a key exists  it does check for the truthiness of a value  Which undefined and null fall under   Boolean obj foo   This solution works best for me because I use typescript  and using strings like so  foo  in obj or obj hasOwnProperty  foo      to check whether a key exists or not does not provide me with intellisense

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These example can demonstrate the differences between defferent ways  Hope it will help you to pick the right one for your needs      Lets create object  a  using create function  A  function A      A prototype onProtDef 2  A prototype onProtUndef undefined  var a new A    a ownProp   3  a ownPropUndef   undefined      Let s try different methods   a onProtDef     2 a onProtUndef     undefined a ownProp     3 a ownPropUndef     undefined a whatEver     undefined a valueOf        valueOf      native code     a hasOwnProperty  onProtDef       false a hasOwnProperty  onProtUndef       false a hasOwnProperty  ownProp       true a hasOwnProperty  ownPropUndef       true a hasOwnProperty  whatEver       false a hasOwnProperty  valueOf       false   onProtDef  in a     true  onProtUndef  in a     true  ownProp  in a     true  ownPropUndef  in a     true  whatEver  in a     false  valueOf  in a     true  on the prototype chain - Object valueOf   Object keys a        ownProp    ownPropUndef

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If you are using underscore js library then object array operations become simple    In your case   has method can be used  Example   yourArray    age   10      has yourArray   age     returns true   But     has yourArray   invalidKey     returns false

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In  array  world we can look on indexes as some kind of keys  What is surprising the in operator  which is good choice for object  also works with arrays  The returned value for non-existed key is undefined   x000D   x000D  let arr     a   b   c       we have indexes  0 1 2 x000D  delete arr 1                set  empty  at index 1 x000D  arr pop                     remove last item x000D   x000D  console log 0 in arr   arr 0    x000D  console log 1 in arr   arr 1    x000D  console log 2 in arr   arr 2    x000D   x000D   x000D

[javascript] Checking if a key exists in a JavaScript object?

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