How do you check if a variable is an array in JavaScript

Question

I would like to check whether a variable is either an array or a single value in JavaScript   I have found a possible solution     if  variable constructor    Array       Is this the best way this can be done

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From w3schools   function isArray myArray        return myArray constructor toString   indexOf  Array    gt  -1

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I personally like Peter s suggestion  https   stackoverflow com a 767499 414784  for ECMAScript 3  For ECMAScript 5  use Array isArray     Comments on the post indicate  however  that if toString   is changed at all  that way of checking an array will fail  If you really want to be specific and make sure toString   has not been changed  and there are no problems with the objects class attribute   object Array  is the class attribute of an object that is an array   then I recommend doing something like this     see if toString returns proper class attributes of objects that are arrays   returns -1 if it fails test   returns true if it passes test and it s an array   returns false if it passes test and it s not an array function is array o           make sure an array has a class attribute of  object Array      var check class   Object prototype toString call          if check class       object Array                     test passed  now check         return Object prototype toString call o        object Array              else                  may want to change return value to something more desirable         return -1             Note that in JavaScript The Definitive Guide 6th edition  7 10  it says Array isArray   is implemented using Object prototype toString call   in ECMAScript 5  Also note that if you re going to worry about toString   s implementation changing  you should also worry about every other built in method changing too  Why use push    Someone can change it  Such an approach is silly  The above check is an offered solution to those worried about toString   changing  but I believe the check is unnecessary

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Via Crockford   function typeOf value        var s   typeof value      if  s      object             if  value                if  value instanceof Array                    s    array                           else               s    null                       return s      The main failing Crockford mentions is an inability to correctly determine arrays that were created in a different context  e g   window  That page has a much more sophisticated version if this is insufficient

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Since the  length property is special for arrays in javascript you can simply say  obj length      obj length    true if obj is an array   Underscorejs and several other libraries use this short and simple trick

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In Crockford s JavaScript The Good Parts  there is a function to check if the given argument is an array   var is array   function  value        return value  amp  amp          typeof value      object   amp  amp          typeof value length      number   amp  amp          typeof value splice      function   amp  amp            value propertyIsEnumerable  length          He explains      First  we ask if the value is truthy  We do this to reject null and other falsy values  Second  we ask if the typeof value is  object   This will be true for objects  arrays  and  weirdly  null  Third  we ask if the value has a length property that is a number  This will always be true for arrays  but usually not for objects  Fourth  we ask if the value contains a splice method  This again will be true for all arrays  Finally  we ask if the length property is enumerable  will length be produced by a for in loop    That will be false for all arrays  This is the most reliable test for arrayness that I have found  It is unfortunate that it is so complicated

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code referred from https   github com miksago Evan js blob master src evan js    var isArray   Array isArray    function obj        return    obj  amp  amp  obj concat  amp  amp  obj unshift  amp  amp   obj callee

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For those who code-golf  an unreliable test with fewest characters   function isArray a      return a map      This is commonly used when traversing flattening a hierarchy   function golf a      return a map    concat apply    a map golf   a     input   1 2  3 4  5  6   7  8  9     output   1  2  3  4  5  6  7  8  9

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Thought I would add another option for those who might already be using the Underscore js library in their script   Underscore js has an isArray   function  see http   underscorejs org  isArray      isArray object        Returns true if object is an Array

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There are multiple solutions with all their own quirks  This page gives a good overview  One possible solution is   function isArray o      return Object prototype toString call o        object Array

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I have created this little bit of code  which can return true types   I am not sure about performance yet  but it s an attempt to properly identify the typeof   https   github com valtido better-typeOf also blogged a little about it here http   www jqui net jquery better-typeof-than-the-javascript-native-typeof   it works  similar to the current typeof   var user    1 2 3  typeOf user      object Array    It think it may need a bit of fine tuning  and take into account things  I have not come across or test it properly  so further improvements are welcomed  whether it s performance wise  or incorrectly re-porting of typeOf

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There are several ways of checking if an variable is an array or not  The best solution is the one you have chosen   variable constructor     Array   This is the fastest method on Chrome  and most likely all other browsers  All arrays are objects  so checking the constructor property is a fast process for JavaScript engines   If you are having issues with finding out if an objects property is an array  you must first check if the property is there   variable prop  amp  amp  variable prop constructor     Array   Some other ways are   Array isArray variable    Update May 23  2019 using Chrome 75  shout out to  AnduAndrici for having me revisit this with his question This last one is  in my opinion the ugliest  and it is one of the slowest fastest  Running about 1 5 the speed as the first example  This guy is about 2-5  slower  but it s pretty hard to tell  Solid to use  Quite impressed by the outcome  Array prototype  is actually an array  you can read more about it here https   developer mozilla org en-US docs Web JavaScript Reference Global Objects Array isArray  variable instanceof Array   This method runs about 1 3 the speed as the first example  Still pretty solid  looks cleaner  if you re all about pretty code and not so much on performance  Note that checking for numbers does not work as variable instanceof Number always returns false  Update  instanceof now goes 2 3 the speed   So yet another update  Object prototype toString call variable        object Array      This guy is the slowest for trying to check for an Array  However  this is a one stop shop for any type you re looking for  However  since you re looking for an array  just use the fastest method above   Also  I ran some test  http   jsperf com instanceof-array-vs-array-isarray 35 So have some fun and check it out   Note   EscapeNetscape has created another test as jsperf com is down  http   jsben ch   QgYAV I wanted to make sure the original link stay for whenever jsperf comes back online

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Something I just came up with   if  item length        This is an array  else       not an array

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This is an old question but having the same problem i found a very elegant solution that i want to share   Adding a prototype to Array makes it very simple  Array prototype isArray   true    Now once if you have an object you want to test to see if its an array all you need is to check for the new property  var box   doSomething     if  box isArray           do something     isArray is only available if its an array

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If you are using Angular  you can use the angular isArray   function  var myArray       angular isArray myArray      returns true  var myObj       angular isArray myObj     returns false   http   docs angularjs org api ng function angular isArray

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I noticed someone mentioned jQuery  but I didn t know there was an isArray   function  It turns out it was added in version 1 3   jQuery implements it as Peter suggests   isArray  function  obj         return toString call obj        object Array         Having put a lot of faith in jQuery already  especially their techniques for cross-browser compatibility  I will either upgrade to version 1 3 and use their function  providing that upgrading doesn   t cause too many problems  or use this suggested method directly in my code   Many thanks for the suggestions

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When I posted this question the version of JQuery that I was using didn t include an isArray function  If it had have I would have probably just used it trusting that implementation to be the best browser independant way to perform this particular type check   Since JQuery now does offer this function  I would always use it       isArray obj      as of version 1 6 2  It is still implemented using comparisons on strings in the form   toString call obj        object Array

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In modern browsers  and some legacy browsers   you can do  Array isArray obj     Supported by Chrome 5  Firefox 4 0  IE 9  Opera 10 5 and Safari 5   If you need to support older versions of IE  you can use es5-shim to polyfill Array isArray  or add the following    only implement if no native implementation is available if  typeof Array isArray      undefined       Array isArray   function obj        return Object prototype toString call obj        object Array             If you use jQuery you can use jQuery isArray obj  or   isArray obj   If you use underscore you can use   isArray obj   If you don t need to detect arrays created in different frames you can also just use instanceof  obj instanceof Array     Note  the arguments keyword that can be used to access the argument of a function isn t an Array  even though it  usually  behaves like one    x000D   x000D  var func   function     x000D    console log arguments             1  2  3  x000D    console log arguments length     3 x000D    console log Array isArray arguments      false     x000D    console log arguments slice      undefined  Array prototype methods not available  x000D    console log  3 4 5  slice        function slice      native code     x000D    x000D  func 1  2  3  x000D   x000D   x000D

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The universal solution is below   Object prototype toString call obj     object Array     Starting from ECMAScript 5  a formal solution is    Array isArray arr    Also  for old JavaScript libs  you can find below solution although it s not accurate enough   var is array   function  value        return value  amp  amp      typeof value      object   amp  amp      typeof value length      number   amp  amp      typeof value splice      function   amp  amp        value propertyIsEnumerable  length          The solutions are from http   www pixelstech net topic 85-How-to-check-whether-an-object-is-an-array-or-not-in-JavaScript

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I was using this line of code   if  variable push          variable is array  since AMAIK only arrays have push   method

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You could also use   if  value instanceof Array      alert  value is Array       else     alert  Not an array        This seems to me a pretty elegant solution  but to each his own   Edit    As of ES5 there is now also    Array isArray value     But this will break on older browsers  unless you are using polyfills  basically    IE8 or similar

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If you re only dealing with EcmaScript 5 and above then you can use the built in Array isArray function  e g    Array isArray           true Array isArray  foo      false Array isArray           false

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I liked the Brian answer   function is array o          make sure an array has a class attribute of  object Array      var check class   Object prototype toString call          if check class       object Array                    test passed  now check         return Object prototype toString call o        object Array          else             may want to change return value to something more desirable         return -1             but you could just do like this   return Object prototype toString call o      Object prototype toString call

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I think using myObj constructor  Object and myArray constructor  Array is the best way   Its almost 20x faster than using toString     If you extend objects with your own constructors and want those creations to be considered  objects  as well than this doesn t work  but otherwise its way faster   typeof is just as fast as the constructor method but typeof      object  returns true which will often be undesirable   http   jsperf com constructor-vs-tostring  one thing to note is that null constructor will throw an error so if you might be checking for null values you will have to first do if testThing   null

[javascript] How do you check if a variable is an array in JavaScript?

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