Length of a JavaScript object

Question

I have a JavaScript object  Is there a built-in or accepted best practice way to get the length of this object  const myObject   new Object    myObject  quot firstname quot      quot Gareth quot   myObject  quot lastname quot      quot Simpson quot   myObject  quot age quot     21

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This method gets all your object s property names in an array  so you can get the length of that array which is equal to your object s keys  length   Object getOwnPropertyNames   hi   Hi   msg   Message    length       gt  2

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If you need an associative data structure that exposes its size  better use a map instead of an object   x000D   x000D  const myMap   new Map     myMap set  firstname    Gareth    myMap set  lastname    Simpson    myMap set  age   21    console log myMap size      3 x000D   x000D   x000D

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To not mess with the prototype or other code  you could build and extend your own object   function Hash        var length 0      this add   function key  val            if this key     undefined                        length                        this key  val              this length   function            return length            myArray   new Hash    myArray add  lastname    Simpson    myArray add  age   21   alert myArray length        will alert 2   If you always use the add method  the length property will be correct  If you re worried that you or others forget about using it  you could add the property counter which the others have posted to the length method  too   Of course  you could always overwrite the methods  But even if you do  your code would probably fail noticeably  making it easy to debug

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With the ECMAScript  6 in-built Reflect object  you can easily count the properties of an object  Reflect ownKeys targetObject  length  It will give you the length of the target object s own properties  important   Reflect ownKeys target    Returns an array of the target object s own  not inherited  property keys   Now  what does that mean  To explain this  let s see this example  function Person name  age     this name   name    this age   age     Person prototype getIntro  function       return    this name  is   this age  years old       let student   new Person  Anuj   11    console log Reflect ownKeys student  length     2 console log student getIntro       Anuj is 11 years old    You can see here  it returned only its own properties while the object is still inheriting the property from its parent  For more information  refer this  Reflect API

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A variation on some of the above is   var objLength   function obj           var key len 0      for key in obj           len    Number  obj hasOwnProperty key               return len       It is a bit more elegant way to integrate hasOwnProp

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A nice way to achieve this  Internet  Explorer  9  only  is to define a magic getter on the length property  Object defineProperty Object prototype   quot length quot         get  function              return Object keys this  length             And you can just use it like so  var myObj      key    value     myObj length   It would give 1

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x000D   x000D     let myobject         let isempty      Object values myobject       console log isempty   x000D   x000D   x000D

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Here s the most cross-browser solution   This is better than the accepted answer because it uses native Object keys if exists  Thus  it is the fastest for all modern browsers   if   Object keys        Object keys   function  obj            var arr                   key          for  key in obj                if  obj hasOwnProperty key                     arr push key                                   return arr            Object keys obj  length

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The most robust answer  i e  that captures the intent of what you re trying to do while causing the fewest bugs  would be   x000D   x000D  Object size   function obj      var size   0      key    for  key in obj        if  obj hasOwnProperty key   size          return size         Get the size of an object const myObj      var size   Object size myObj   x000D   x000D   x000D   There s a sort of convention in JavaScript that you don t add things to Object prototype  because it can break enumerations in various libraries  Adding methods to Object is usually safe  though   Here s an update as of 2016 and widespread deployment of ES5 and beyond   For IE9  and all other modern ES5  capable browsers  you can use Object keys   so the above code just becomes  var size   Object keys myObj  length   This doesn t have to modify any existing prototype since Object keys   is now built-in  Edit  Objects can have symbolic properties that can not be returned via Object key method  So the answer would be incomplete without mentioning them  Symbol type was added to the language to create unique identifiers for object properties  The main benefit of the Symbol type is the prevention of overwrites  Object keys or Object getOwnPropertyNames does not work for symbolic properties  To return them you need to use Object getOwnPropertySymbols   x000D   x000D  var person        Symbol  name      John Doe      Symbol  age     33     occupation    Programmer      const propOwn   Object getOwnPropertyNames person   console log propOwn length      1  let propSymb   Object getOwnPropertySymbols person   console log propSymb length      2 x000D   x000D   x000D

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Simple one liner   x000D   x000D  console log Object values  id  1  age 23 role number 90   length   x000D   x000D   x000D

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Simple solution     var myObject                   your object goes here     var length   0     for  var property in myObject        if  myObject hasOwnProperty property          length    1                console log length         logs 0 in my example

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The solution work for many cases and cross browser   Code  var getTotal   function collection         var length   collection  length        var isArrayObject    typeof length     number   amp  amp  length  gt   0  amp  amp  length  lt   Math pow 2 53  - 1     Number MAX SAFE INTEGER      if isArrayObject            return collection  length               i  0      for var key in collection            if  collection hasOwnProperty key                 i                         return i       Data Examples      case 1 var a   new Object    a  firstname      Gareth   a  lastname      Simpson   a  age     21     case 2 var b    1 2 3       case 3 var c       c 0    1  c two   2    Usage  getLength a      3 getLength b      3 getLength c      2

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Like most JavaScript problems  there are many solutions   You could extend the Object that for better or worse works like many other languages  Dictionary    first class citizens   Nothing wrong with that  but another option is to construct a new Object that meets your specific needs    function uberject obj       this  count   0      for var param in obj           this param    obj param           this  count             uberject prototype getLength   function        return this  count      var foo   new uberject  bar 123 baz 456    alert foo getLength

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What about something like this --  function keyValuePairs         this length   0      function add key  value    this key    value  this length          function remove key    if  this hasOwnProperty key     delete this key   this length--

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Use    x000D   x000D  var myArray   new Object    x000D  myArray  firstname      Gareth   x000D  myArray  lastname      Simpson   x000D  myArray  age     21  x000D  obj   Object keys myArray  length  x000D  console log obj  x000D   x000D   x000D

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You can always do Object getOwnPropertyNames myObject  length to get the same result as    length would give for normal array

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For some cases it is better to just store the size in a separate variable  Especially  if you re adding to the array by one element in one place and can easily increment the size  It would obviously work much faster if you need to check the size often

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Below is a version of James Coglan s answer in CoffeeScript for those who have abandoned straight JavaScript     Object size    obj  - gt    size   0   size   for own key of obj   size

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If you are using AngularJS 1 x you can do things the AngularJS way by creating a filter and using the code from any of the other examples such as the following      Count the elements in an object app filter  lengthOfObject   function       return function  obj         var size   0  key      for  key in obj          if  obj hasOwnProperty key   size             return size          Usage  In your controller    scope filterResult    filter  lengthOfObject    scope object    Or in your view    lt any ng-expression  object   lengthOfObject  gt  lt  any gt

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x000D   x000D  var myObject   new Object    x000D  myObject  firstname      Gareth   x000D  myObject  lastname      Simpson   x000D  myObject  age     21  x000D   x000D  var size   JSON stringify myObject  length  x000D   x000D  document write size   x000D   x000D   x000D     x000D   x000D  JSON stringify myObject  x000D   x000D   x000D

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palmsey  In fairness to the OP  the JavaScript documentation  actually explicitly refer to using variables of type Object in this manner as  quot associative arrays quot     And in fairness to  palmsey he was quite correct  They aren t associative arrays  they re definitely objects    - doing the job of an associative array  But as regards to the wider point  you definitely seem to have the right of it according to this rather fine article I found  JavaScript    Associative Arrays    Considered Harmful But according to all this  the accepted answer itself is bad practice    Specify a prototype size   function for Object   If anything else has been added to Object  prototype  then the suggested code will fail   lt script type  quot text javascript quot  gt  Object prototype size   function        var len   this length   --this length   -1      for  var k in this        len      return len    Object prototype size2   function        var len   this length   --this length   -1      for  var k in this        len      return len    var myArray   new Object    myArray  quot firstname quot      quot Gareth quot   myArray  quot lastname quot      quot Simpson quot   myArray  quot age quot     21  alert  quot age is  quot    myArray  quot age quot     alert  quot length is  quot    myArray size      lt  script gt   I don t think that answer should be the accepted one as it can t be trusted to work if you have any other code running in the same execution context  To do it in a robust fashion  surely you would need to define the size method within myArray and check for the type of the members as you iterate through them

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Simply use this to get the length   Object keys myObject  length

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You can simply use Object keys obj  length on any object to get its length  Object keys returns an array containing all of the object keys  properties  which can come in handy for finding the length of that object using the length of the corresponding array  You can even write a function for this  Let s get creative and write a method for it as well  along with a more convienient getter property     x000D   x000D  function objLength obj  x000D    x000D    return Object keys obj  length  x000D    x000D   x000D  console log objLength  a 1  b  summit   c  nonsense      x000D   x000D     Works perfectly fine x000D  var obj   new Object    x000D  obj  fish     30  x000D  obj  nullified content     null  x000D  console log objLength obj    x000D   x000D     It also works your way  which is creating it using the Object constructor x000D  Object prototype getLength   function     x000D     return Object keys this  length  x000D    x000D  console log obj getLength     x000D   x000D     You can also write it as a method  which is more efficient as done so above x000D   x000D  Object defineProperty Object prototype   length    get function    x000D      return Object keys this  length  x000D       x000D  console log obj length   x000D   x000D     probably the most effictive approach is done so and demonstrated above which sets a getter property called  length  for objects which returns the equivalent value of getLength this  or this getLength   x000D   x000D   x000D

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I m not a JavaScript expert  but it looks like you would have to loop through the elements and count them since Object doesn t have a length method   var element count   0  for  e in myArray     if  myArray hasOwnProperty e   element count         palmsey  In fairness to the OP  the JavaScript documentation actually explicitly refer to using variables of type Object in this manner as  associative arrays

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Property Object defineProperty Object prototype   length         get  function              var size   0  key          for  key in this              if  this hasOwnProperty key                   size            return size             Use var o    a  1  b  2  c  3   alert o length       lt -- 3 o  foo     123  alert o length       lt -- 4

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The simplest way is like this  Object keys myobject  length  Where myobject is the object of what you want the length of

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Object keys does not return the right result in case of object inheritance  To properly count object properties  including inherited ones  use for-in  For example  by the following function  related question   var objLength    o i 0    gt    for p in o  i    return i     x000D   x000D  var myObject   new Object    myObject  firstname      Gareth   myObject  lastname      Simpson   myObject  age     21   var child   Object create myObject   child  sex      male    var objLength    o i 0    gt    for p in o  i    return i    console log  Object keys myObject     Object keys myObject  length    OK     console log  Object keys child        Object keys child  length    wrong     console log  objLength child          objLength child     OK     x000D   x000D   x000D

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Use Object keys myObject  length to get the length of object array  x000D   x000D  var myObject   new Object    myObject  firstname      Gareth   myObject  lastname      Simpson   myObject  age     21   console log Object keys myObject  length     3 x000D   x000D   x000D

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If you know you don t have to worry about hasOwnProperty checks  you can do this very simply   Object keys myArray  length

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Updated  If you re using Underscore js  recommended  it s lightweight    then you can just do    size  one   1  two   2  three   3      gt  3   If not  and you don t want to mess around with Object properties for whatever reason  and are already using jQuery  a plugin is equally accessible     assocArraySize   function obj           http   stackoverflow com a 6700 11236     var size   0  key      for  key in obj            if  obj hasOwnProperty key   size              return size

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const myObject   new Object    myObject  firstname      Gareth   myObject  lastname      Simpson   myObject  age     21   console log Object keys myObject  length      o p 3

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If you don t care about supporting Internet Explorer 8 or lower  you can easily get the number of properties in an object by applying the following two steps    Run either Object keys   to get an array that contains the names of only those properties that are enumerable or Object getOwnPropertyNames   if you want to also include the names of properties that are not enumerable  Get the  length property of that array      If you need to do this more than once  you could wrap this logic in a function   function size obj  enumerablesOnly        return enumerablesOnly     false           Object getOwnPropertyNames obj  length           Object keys obj  length      How to use this particular function   var myObj   Object create           getFoo          setFoo         myObj Foo   12   var myArr    1 2 5 4 8 15    console log size myObj              Output   1 console log size myObj  true        Output   1 console log size myObj  false       Output   3 console log size myArr              Output   6 console log size myArr  true        Output   6 console log size myArr  false       Output   7   See also this Fiddle for a demo

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If we have the hash      hash      a     b    c    d      we can get the length using the length of the keys which is the length of the hash      keys hash  length

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var myObject   new Object    myObject  firstname      Gareth   myObject  lastname      Simpson   myObject  age     21     Object values myObject  length Object entries myObject  length Object keys myObject  length

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I had a similar need to calculate the bandwidth used by objects received over a websocket  Simply finding the length of the Stringified object was enough for me   websocket on  message   data   gt        dataPerSecond    JSON stringify data  length

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We can find the length of Object by using   x000D   x000D  const myObject       console log Object values myObject  length   x000D   x000D   x000D

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Here is a completely different solution that will only work in more modern browsers  Internet  Explorer  9   Chrome  Firefox 4   Opera 11 60   and Safari 5 1   See this jsFiddle  Setup your associative array class         constructor     AssociativeArray   function            Make the length property work Object defineProperty AssociativeArray prototype   quot length quot         get  function              var count   0          for  var key in this                if  this hasOwnProperty key                   count                      return count             Now you can use this code as follows    var a1   new AssociativeArray    a1  quot prop1 quot      quot test quot   a1  quot prop2 quot     1234  a1  quot prop3 quot      quot something else quot   alert  quot Length of array is  quot    a1 length

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Here s a different version of James Cogan s answer  Instead of passing an argument  just prototype out the Object class and make the code cleaner   Object prototype size   function          var size   0          key      for  key in this            if  this hasOwnProperty key   size              return size      var x         one  1      two  2      three  3     x size       3    jsfiddle example  http   jsfiddle net qar4j 1

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x000D   x000D   lt script gt  x000D  myObj     key1     Hello    key2     Goodbye    x000D  var size   Object keys myObj  length  x000D  console log size   x000D   lt  script gt  x000D   x000D   lt p id  myObj  gt The number of  lt b gt keys lt  b gt  in  lt b gt myObj lt  b gt  are   lt script gt document write size  lt  script gt  lt  p gt  x000D   x000D   x000D    This works for me   var size   Object keys myObj  length

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Here s how and don t forget to check that the property is not on the prototype chain   var element count   0  for var e in myArray      if myArray hasOwnProperty e           element count

[javascript] Length of a JavaScript object

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