How to check if an object is an array

Question

I m trying to write a function that either accepts a list of strings  or a single string  If it s a string  then I want to convert it to an array with just the one item so I can loop over it without fear of an error  So how do I check if the variable is an array   I ve rounded up the various solutions below and created a jsperf test  They re all fast  so just use Array isArray -- it s well-supported now and works across frames

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Array isArray works fast  but it isn t supported by all versions of browsers  So you could make an exception for others and use universal method       Utils               Utils isArray     isArray  in Array             Array isArray            function  value                return Object prototype toString call value        object Array

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In modern browsers you can do  Array isArray obj    Supported by Chrome 5  Firefox 4 0  IE 9  Opera 10 5 and Safari 5  For backward compatibility you can 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

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The best practice is to compare it using constructor  something like this  if some variable constructor     Array        do something     You can use other methods too like typeOf  converting it to a string and then comparing but comparing it with dataType is always a better approach

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Since I don t like any Object prototype-calls  I searched for another solution  Especially because the solutions of ChaosPandion won t always work  and the solution of MidnightTortoise with isArray   doesn t work with arrays coming from the DOM  like getElementsByTagName   And finally I found an easy and cross-browser solution  which probably also would have worked with Netscape 4      It s just these 4 lines  checking any object h    function isArray h       if  h length  undefined amp  amp h 0   undefined    h length   0 amp  amp h 0    undefined            return true            else  return false        I already tested these arrays  all return true    1  array d getElementsByName  some element       some element  can be a real or unreal element 2  array     3  array  10   4  array new Array    5  array new Array       array push  whatever      Can anybody confirm that this works for all cases  Or does anybody find a case where my solution don t work

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The method given in the ECMAScript standard to find the class of Object is to use the toString method from Object prototype   if  Object prototype toString call  someVar         object Array           alert   Array          Or you could use typeof to test if it is a String   if  typeof someVar      string          someVar     someVar        Or if you re not concerned about performance  you could just do a concat to a new empty Array   someVar      concat  someVar      There s also the constructor which you can query directly   if  somevar constructor name     Array            do something       Check out a thorough treatment from  T J  Crowder s blog  as posted in his comment below   Check out this benchmark to get an idea which method performs better  http   jsben ch   QgYAV  From  Bharath convert string to array using Es6 for the question asked   const convertStringToArray    object    gt       return  typeof object      string     Array object    object      suppose    let m    bla  let n     bla   Meow   let y   convertStringToArray m  let z   convertStringToArray n  console log  check y    JSON stringify y        check y    bla   console log  check y    JSON stringify z        check y    bla   Meow

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first you can check console log typeof Object   if the output is object then  var  data  object ie just destructure the object according to the object keys   and the function can be like this   const abc  str1 str2 null   gt          var result           result push str1  result push str2         return result join

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You can find with push like below    x000D   x000D  function isArray obj   x000D     return  typeof obj push     function   true false  x000D    x000D   x000D  var array new Array    x000D  or x000D  var array   a   b   c    x000D  console log isArray array    x000D   x000D   x000D

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Other methods also exist to check but I prefer the following method  as my best to check  as you can easily check types of other objects     gt  a    1  2    1  2    gt   gt  Object prototype toString call a  slice 8   replace             Array   gt   gt  Object prototype toString call     slice 8 -1     best approach  Array    Explanation  with simple examples on Node REPL  raquo    gt  o     ok   1    ok  1    gt  a    1  2    1  2    gt  typeof o  object   gt  typeof a  object   gt   gt  Object prototype toString call o    object Object    gt  Object prototype toString call a    object Array    gt    Object or Array  raquo    gt  Object prototype toString call o  slice 8   replace             Object   gt   gt  Object prototype toString call a  slice 8   replace             Array   gt    Null or Undefined  raquo    gt  Object prototype toString call undefined  slice 8   replace             Undefined   gt  Object prototype toString call null  slice 8   replace             Null   gt    String  raquo    gt  Object prototype toString call  ok   slice 8   replace             String    Number  raquo    gt  Object prototype toString call 19  slice 8   replace             Number   gt  Object prototype toString call 19 0  slice 8   replace             Number   gt  Object prototype toString call 19 7  slice 8   replace             Number   gt    I appreciate  mpen s suggestion to use -1 in place of regular expression as follows    gt  Object prototype toString call 12  slice 8 -1   Number   gt   gt  Object prototype toString call 12 0  slice 8 -1   Number   gt   gt  Object prototype toString call     slice 8 -1   Array   gt  Object prototype toString call     slice 8 -1   Object   gt   gt  Object prototype toString call     slice 8 -1   String   gt

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function isArray value        if  value            if  typeof value      object                 return  Object prototype toString call value       object Array                        return false     var ar     ff   tt   alert isArray ar

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The following could be used if you know that your object doesn t have a concat method    x000D   x000D  var arr       x000D  if  typeof arr concat      function     x000D      console log  It s an array    x000D    x000D   x000D   x000D

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You can try this   var arr        or  arr   new Array    var obj        or  arr   new Object     arr constructor prototype hasOwnProperty  push     true  obj constructor prototype hasOwnProperty  push      false

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Thankfully  ECMA 5 introduced Array isArray   back in December 2009  If for some reason  you are using a version of JavaScript older than ECMA 5  please upgrade   If you insist on it  though  then arrays do have certain properties that differentiate them from any other type  Properties that I haven t seen mentioned in any of the other answers  Let s get into some JavaScript politics   An array is an object  typeof         object    but unlike traditional objects  they have a length property  typeof        length      undefined    null is also an object  typeof null      object    but you can t access a property of null because null is not an object  This is a bug in the spec that goes all the way back to the very beginning of JavaScript  when objects had the type tag 0 and null was represented as a literal null pointer 0x00  which caused the interpreter to confuse it with objects    Unfortunately  this doesn t account for    vs  length 0   So we must now turn to the prototype chain            proto       Array prototype  amp  amp           proto       Object prototype   Thus  without Array isArray    this is just about the closest we can get   function is array array       return array     null          amp  amp  typeof array      object           amp  amp  array   proto       Array prototype            1 2 3    length  0         1  0  Infinity  NaN   1     1 2 3      null  undefined   null    undefined    a                length  0     Infinity    NaN        proto    Array prototype    filter is array     Expected         1 2 3    null    undefined           length  0     Infinity    NaN       Actual           1 2 3    null    undefined           length  0     Infinity    NaN      proto    Array prototype      The object maliciously designed to look just like an array actually passes the turing test  However  replacing the prototype chain with the Array prototype chain is enough to make it act just like an array  effectively making it an array  The only thing in the world that can tell such an object is actually not an array  is Array isArray    But for the purposes you would usually be checking if an object is an array  said object should play nice with your code  Even the behavior when you change the length of the array artificially is the same  if the length is longer than the number of elements in the array  you WILL have  empty slots  of that special  implicit undefined  type that is somehow distinct from undefined while also being     undefined  the very same type that is the reason we use typeof obj      undefined  to avoid throwing a ReferenceError because obj     undefined only doesn t throw an error if obj was explicitly defined as undefined   a      proto    Array prototype      Array    a push 5  a     5  a length   5 a     5  empty x 4  b   a map n   gt  n n      25  empty x 4  b push undefined  b push undefined  b     25  empty x 4  undefined  undefined  b 1     undefined b 1      b 5     true Array isArray a     false Array isArray b     true   Don t use is array    though  It s one thing to reinvent the wheel for learning purposes  It s another thing to do it in production code  Don t even use it as a polyfill  Supporting old JS versions means supporting old browsers means encouraging the use of insecure software means putting the user at risk for malware

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Array isArray obj   does not give very helpful results  I ve created a prototype method of Object  that seems to correctly determine whether and object is an array or not  The only edge case that I know of where it fails is when item in the array is set to undefined  Object prototype isArrayLike   function         var length   this length    Object keys this  length      if  length     0    this constructor name      quot String quot           return false      for  i   0  i  lt  length  i                  if  typeof this i       quot undefined quot               return false            return true      var arr     aaa    bbb    ccc    ddd    var arr1     quot 0 quot   aaa    quot 1 quot   bbb   2  ccc   3  ddd    var arr2     quot 0 quot   aaa    quot a quot   bbb   2  ccc   3  ddd    var arr3    quot qwerty quot   var arr4       var arr5    0  aaa   1  bbb   2  ccc   3  ddd     console log  quot arrayLike  quot    arr isArrayLike     console log  quot Array isArray arr   quot    Array isArray arr       arrayLike  true    Array isArray arr   true console log  quot arrayLike1  quot    arr1 isArrayLike     console log  quot Array isArray arr1   quot    Array isArray arr1       arrayLike1  true    Array isArray arr1   false console log  quot arrayLike2  quot    arr2 isArrayLike     console log  quot Array isArray arr2   quot    Array isArray arr2       arrayLike2  false    Array isArray arr2   false console log  quot arrayLike3  quot    arr3 isArrayLike     console log  quot Array isArray arr3   quot    Array isArray arr3       arrayLike3  false    Array isArray arr3   false console log  quot arrayLike4  quot    arr4 isArrayLike     console log  quot Array isArray arr4   quot    Array isArray arr4       arrayLike4  false    Array isArray arr4   true console log  quot arrayLike5  quot    arr5 isArrayLike     console log  quot Array isArray arr5   quot    Array isArray arr5       arrayLike5  false    Array isArray arr5   true

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var a       b        console log a constructor name     Array    console log b constructor name     Object

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jQuery also offers an   isArray   method    x000D   x000D  var a     A    AA    AAA    x000D   x000D  if   isArray a     x000D    alert  a is an array     x000D    else   x000D    alert  a is not an array     x000D    x000D   lt script src  https   ajax googleapis com ajax libs jquery 2 1 1 jquery min js  gt  lt  script gt  x000D   x000D   x000D

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var length   16                                   Number  var lastName    Johnson                           String  var cars     Saab    Volvo    BMW                 Array  var x    firstName  John   lastName  Doe      Object prototype myCheck  function     if  this constructor     Array             alert  array             else if  this constructor     Object                    alert  object             else if  this constructor     Number                      alert  number             else if  this constructor     String                      alert  string                   cars myCheck     lastName myCheck     length myCheck

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Although there s some solid answers I would prefer a functional approach using a Functor  A Functor is just a fancy way to say that we will be passing a function to a value   The suggestions that I ve seen are passing values to a function   Create a TypeOf helper  const TypeOf   obj   gt  Object prototype toString call obj  slice 8 -1     This is similar to typeof but now returns Array for    and Object for     I like to think of it as a strict typeof  If you re working on the gmail application and performance is a concern then you can do something like this   const TypeOf   obj   gt      Array isArray obj        array        obj     null    catch null edge case   typeof null is an object         null       typeof obj     You could stop here and call it a day  however you could make it a bit more powerful using composition   You get a lot of benefits if you created a TypeBox Functor  again fancy word for passing a function to a value instead of passing a value to a function   Create TypeBox  const TypeBox    predicate  defaultValue    gt      const TypePredicate   value   gt          value       map  cb   gt  predicate value                    TypePredicate cb value                     TypePredicate defaultValue          return TypePredicate      There s a lot going on here  but it s very powerful   The TypeBox function uses a closure and returns our Functor   Closures give you access to Lexical Scope  think of it as a backpack that holds the things you want access to later    Create ArrayBox  const ArrayBox   TypeOf obj   gt  TypeOf obj       Array    obj    obj      ArrayBox is passing our predicate and defaultValue to TypeOf and will be available when we invoke execute ArrayBox name it whatever makes sense for your use case    Now the fun part  If the input is an Array  return it   ArrayBox   foo    bar    value       foo    bar     If the input is not an array  return it in one  ArrayBox  foo   value      foo     What s great about this approach is that it scales  is easy to test  and it uses composition   You can compose the functions in any manner to get the desired result   There s many other ways we could approach this using Either or Monads

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You could is isArray method but I would prefer to check with   Object getPrototypeOf yourvariable      Array prototype

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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          This function has been taken from  JS the good parts  book  works perfect for me

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You can use this function to get data type   var myAr     1 2    checkType myAr    function checkType data     if typeof data     object        if Object prototype toString call data  indexOf  Array      -1          return  array         else        return  object             else       return typeof data         if checkType myAr       array   console log  yes  It is an array

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The best solution I ve seen is a cross-browser replacement for typeof  Check Angus Croll s solution here   The TL DR version is below  but the article is a great discussion of the issue so you should read it if you have time   Object toType   function obj        return      toString call obj  match   s  a-z A-Z      1  toLowerCase             and usage  Object toType  1 2 3       array   all browsers      or to test    var shouldBeAnArray    1 2 3   if Object toType shouldBeAnArray       array      do stuff

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easiest and fastest way to check if an Object is an Array or not   var arr         arr constructor name     Array     return true   or arr constructor    Array   return true   or you can make a utility function  const isArray    obj   gt    obj  amp  amp  obj constructor    Array   usage  isArray arr     return true

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In your case you may use concat method of Array which can accept single objects as well as array  and even combined    function myFunc stringOrArray      var arr      concat stringOrArray      console log arr      arr forEach function item  i          console log i       item           myFunc  one string     myFunc   one string    second    third       concat seems to be one of the oldest methods of Array  even IE 5 5 knows it well

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This is my attempt to improve on this answer taking into account the comments   var isArray   myArray  amp  amp  myArray constructor     Array    It gets rid of the if else  and accounts for the possibility of the array being null or undefined

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This function will turn almost anything into an array   function arr x        if x     null    x     undefined            return               if Array isArray x             return x            if isString x     isNumber x             return  x             if x Symbol iterator      undefined    x length     undefined            return Array from x             return  x      function isString x        return Object prototype toString call x        object String      function isNumber x        return Object prototype toString call x        object Number       It uses some newer browser features so you may want to polyfill this for maximum support   Examples    gt  arr null       gt  arr undefined      gt  arr 3 14    3 14    gt  arr 1 0    Infinity    gt  gen   function      yield 1  yield 2  yield 3     Function  gen   gt  arr gen      1  2  3    gt  arr  4 5 6     4  5  6    gt  arr  foo      foo      N B  strings will be converted into an array with a single element instead of an array of chars  Delete the isString check if you would prefer it the other way around   I ve used Array isArray here because it s the most robust and also simplest

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there is a difference between checkout it s prototype and Array isArray   function isArray obj       return Object getPrototypeOf obj      Array prototype     this function  will directly check if an obj is an array  but for this Proxy object   var arr    1 2 3   var proxy   new Proxy arr      console log Array isArray proxy      true   Array isArray will take it as Array

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https   developer mozilla org en-US docs JavaScript Reference Global Objects Array isArray  Array isArray   Array isArray    function  vArg        return Object prototype toString call vArg        object Array

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Array isArray is the way to go about this  For example   var arr     tuna    chicken    pb amp j    var obj    sandwich   tuna   chips   cape cod        Returns true Array isArray arr       Return false Array isArray obj

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I know this is an old question  but i found the shortest answer now    x000D   x000D  var x    1 2 3  x000D  console log x map 1 0  x000D   x000D   x000D

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Imagine you have this array below    var arr    1 2 3 4 5     Javascript  new and older browsers    function isArray arr      return arr constructor toString   indexOf  Array    gt  -1      or  function isArray arr      return arr instanceof Array      or   function isArray arr      return Object prototype toString call arr        object Array        then call it like this   isArray arr     Javascript  IE9   Ch5   FF4   Saf5   Opera10 5     Array isArray arr     jQuery     isArray arr     Angular   angular isArray arr     Underscore and Lodash     isArray arr

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I know  that people are looking for some kind of raw javascript approach  But if you want think less about  take a look here  http   underscorejs org  isArray    isArray object     Returns true if object is an Array    function    return   isArray arguments           gt  false   isArray  1 2 3      gt  true

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I have updated the jsperf fiddle with two alternative methods as well as error checking   It turns out that the method defining a constant value in the  Object  and  Array  prototypes is faster than any of the other methods   It is a somewhat surprising result    x000D   x000D     Initialisation    x000D  Object prototype isArray   function     x000D    return false  x000D     x000D  Array prototype isArray   function     x000D    return true  x000D     x000D  Object prototype  isArray   false  x000D  Array prototype  isArray   true  x000D   x000D  var arr     1    2    x000D  var noarr    1   x000D   x000D     Method 1  function     x000D  if  arr isArray    document write  arr is an array according to function lt br  gt     x000D  if   noarr isArray    document write  noarr is not an array according to function lt br  gt     x000D     Method 2  value  -      FASTEST          x000D  if  arr  isArray  document write  arr is an array according to member value lt br  gt     x000D  if   noarr  isArray  document write  noarr is not an array according to member value lt br  gt     x000D   x000D   x000D    These two methods do not work if the variable takes the undefined value  but they do work if you are certain that they have a value   With regards to checking with performance in mind if a value is an array or a single value  the second method looks like a valid fast method   It is slightly faster than  instanceof  on Chrome  twice as fast as the second best method in Internet Explorer  Opera and Safari  on my machine

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This is the fastest among all methods  all browsers supported    function isArray obj       return   obj  amp  amp  obj constructor     Array

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If the only two kinds of values that could be passed to this function are a string or an array of strings  keep it simple and use a typeof check for the string possibility   function someFunc arg        var arr    typeof arg     string      arg    arg

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As MDN says in here      use Array isArray or Object prototype toString call to differentiate   regular objects from arrays   Like this    Object prototype toString call arr        object Array    or Array isArray arr

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Here s a code snippet that ll explain an important fact of arrays that should be known early on while learning JS  unlike me     x000D   x000D     this functions puts a string inside an array x000D  var stringInsideArray   function input    x000D    if  typeof input      string     x000D      return  input   x000D      x000D    else if  Array isArray input     x000D      return input  x000D       x000D    else   x000D      throw new Error  Input is not a string     x000D      x000D    x000D   x000D  var output   stringInsideArray  hello    x000D  console log  step one output     output        hello   x000D   x000D     use typeof method to verify output is an object x000D  console log  step two output     typeof output      object x000D   x000D     use Array isArray   method to verify output is an array x000D  console log  step three output     Array isArray output       true x000D   x000D   x000D    Arrays  are in fact  objects   Using the typeof operator  the output of stringInsideArray  hello   proves that   hello   is really an object  This baffled me for the longest time because I assumed that arrays would be a JavaScript data type     There are only 7 JS data types and arrays are NOT one of them   To answer your question  using the Array isArray   method determines that the output is an array

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I do this in a very simple way  Works for me  Any drawbacks   Array prototype isArray   true   a     b     a isArray     true b isArray      undefined - gt  false

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I know this is an old question but here is a solution that I came up with and have been using for my projects     function isArray  o        return typeof o      object   amp  amp  o length     undefined     isArray         false isArray 1      false isArray  str       false isArray function          false  isArray         true   The only pitfall is that it will give a false positive if your object happens to have a length property   isArray  length 0       true   If you are okay with that drawback and know your pure objects won t have that property  it s a clean solution and should be faster than the Object prototype toString call method

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I would make a function to test the type of object you are dealing with      x000D   x000D  function whatAmI me   return Object prototype toString call me  split   W   2     x000D   x000D     tests x000D  console log  x000D    whatAmI   aiming         x000D    whatAmI  living 4 breathing 4    x000D    whatAmI function ing   return ing   to the global window      x000D    whatAmI  going to do with you    x000D     x000D   x000D     output  Array Object Function String x000D   x000D   x000D    then you can write a simple if statement     if whatAmI myVar       Array           do array stuff   else      could also check  if whatAmI myVar       String    here to be sure        do string stuff

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There is a nice example in Stoyan Stefanov s book JavaScript Patterns which suppose to handle all possible problems as well as utilize ECMAScript 5 method Array isArray     So here it is   if  typeof Array isArray      undefined         Array isArray   function  arg            return Object prototype toString call arg        object Array               By the way  if you are using jQuery  you can use it s method   isArray

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There s just one line solution for this question  x instanceof Array   where x is the variable it will return true if x is an array and false if it is not

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Simple function to check this   function isArray object        return object constructor     Array

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Here s what I use   function isArray input      if  input instanceof Array    Object prototype toString call input        object Array              return true      else return false

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You can also check with array s  length property  When you will try to access length property of an array  it will return a number 0 for empty array  while if you try to access the length property of object it will return undefined   if Object prototype toString call arrayList        object Array        console log  Array

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Here s my lazy approach   if  Array prototype array      undefined      Array prototype array    true             var test           wat        console log test array      true      true console log wat array      true       false   I know it s sacrilege to  mess with  the prototype  but it appears to perform significantly better than the recommended toString method   Note  A pitfall of this approach is that it wont work across iframe boundaries  but for my use case this is not an issue

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Exotic one  You want to check if the parameter is string or not - so try  x   x       x000D   x000D  let isStr   x  gt  x   x     x000D   x000D  console log  isStr        x000D  console log  isStr   aa   bb       x000D  console log  isStr        x000D  console log  isStr  abc      x000D   x000D   x000D

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A    1 2 3  console log A map     map    In search for shortest version here is what I got so far   Note  there is no perfect function that will always detect all possible combinations  It is better to know all abilities and limitations of your tools than expect a magic tool

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A simple function for testing if an input value is an array is the following   function isArray value      return Object prototype toString call value        object Array        This works cross browser  and with older browsers  This is pulled from T J  Crowders  blog post

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You can check the type of your variable whether it is an array with   var myArray      if myArray instanceof Array

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I would first check if your implementation supports isArray   if  Array isArray      return Array isArray v     You could also try using the instanceof operator  v instanceof Array

[javascript] How to check if an object is an array?

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