How to efficiently count the number of keys properties of an object in JavaScript

Question

What s the fastest way to count the number of keys properties of an object   It it possible to do this without iterating over the object   i e  without doing  var count   0  for  k in myobj  if  myobj hasOwnProperty k   count       Firefox did provide a magic   count   property  but this was removed somewhere around version 4

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To iterate on Avi Flax answer Object keys obj  length is correct for an object that doesnt have functions tied to it  example   obj     lol    what   owo   pfft    Object keys obj  length     should be 2   versus  arr       obj     lol    what   owo   pfft    obj omg   function          each obj  function a           arr push a              Object keys obj  length     should be 3 because it looks like this     obj       lol    what   owo   pfft   omg  function     each obj  function a  arr push a             steps to avoid this    do not put functions in an object that you want to count the number of keys in use a seperate object or make a new object specifically for functions  if you want to count how many functions there are in the file using Object keys obj  length    also yes i used the   or underscore module from nodejs in my example   documentation can be found here http   underscorejs org  as well as its source on github and various other info  And finally a lodash implementation https   lodash com docs size    size obj

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If you are actually running into a performance problem I would suggest wrapping the calls that add remove properties to from the object with a function that also increments decrements an appropriately named  size   property   You only need to calculate the initial number of properties once and move on from there  If there isn t an actual performance problem  don t bother  Just wrap that bit of code in a function getNumberOfProperties object  and be done with it

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For those who have Underscore js included in their project you can do      a     b      size        gt  2   or functional style     size  a     b           gt  2

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For those that have Ext JS 4 in their project you can do   Ext Object getSize myobj     The advantage of this is that it ll work on all Ext compatible browsers  IE6-IE8 included   however  I believe the running time is no better than O n  though  as with other suggested solutions

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The standard Object implementation  ES5 1 Object Internal Properties and Methods  does not require an Object to track its number of keys properties  so there should be no standard way to determine the size of an Object without explicitly or implicitly iterating over its keys   So here are the most commonly used alternatives     1  ECMAScript s Object keys    Object keys obj  length  Works by internally iterating over the keys to compute a temporary array and returns its length    Pros - Readable and clean syntax  No library or custom code required except a shim if native support is unavailable Cons - Memory overhead due to the creation of the array    2  Library-based solutions  Many library-based examples elsewhere in this topic are useful idioms in the context of their library  From a performance viewpoint  however  there is nothing to gain compared to a perfect no-library code since all those library methods actually encapsulate either a for-loop or ES5 Object keys  native or shimmed    3  Optimizing a for-loop  The slowest part of such a for-loop is generally the  hasOwnProperty   call  because of the function call overhead  So when I just want the number of entries of a JSON object  I just skip the  hasOwnProperty   call if I know that no code did nor will extend  Object prototype   Otherwise  your code could be very slightly optimized by making k local  var k  and by using prefix-increment operator    count  instead of postfix   var count   0  for  var k in myobj  if  myobj hasOwnProperty k     count    Another idea relies on caching the hasOwnProperty method   var hasOwn   Object prototype hasOwnProperty  var count   0  for  var k in myobj  if  hasOwn call myobj  k     count    Whether this is faster or not on a given environment is a question of benchmarking  Very limited performance gain can be expected anyway

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If you are using Underscore js you can use   size  thanks  douwe     size obj   Alternatively you can also use   keys which might be clearer for some    keys obj  length   I highly recommend Underscore  its a tight library for doing lots of basic things  Whenever possible they match ECMA5 and defer to the native implementation   Otherwise I support  Avi s answer  I edited it to add a link to the MDC doc which includes the keys   method you can add to non-ECMA5 browsers

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How I ve solved this problem is to build my own implementation of a basic list which keeps a record of how many items are stored in the object  Its very simple  Something like this   function BasicList        var items          this count   0      this add   function index  item             items index    item        this count             this remove   function  index             delete items index         this count--           this get   function index             if  undefined     index          return items        else         return items index

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Google Closure has a nice function for this    goog object getCount obj   look at goog Object Documentation

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I m not aware of any way to do this  however to keep the iterations to a minimum  you could try checking for the existance of   count   and if it doesn t exist  ie not Firefox  then you could iterate over the object and define it for later use eg   if  myobj   count       undefined      myobj   count             This way any browser supporting   count   would use that  and iterations would only be carried out for those which don t  If the count changes and you can t do this  you could always make it a function   if  myobj   count       undefined      myobj   count     function     return         myobj   count   toString   function     return this          This way anytime you reference myobj   count   the function will fire and recalculate

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To do this in any ES5-compatible environment  such as Node  Chrome  IE 9   Firefox 4   or Safari 5   Object keys obj  length   Browser compatibility Object keys documentation  includes a method you can add to non-ES5 browsers

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I don t think this is possible  at least not without using some internals   And I don t think you would gain much by optimizing this

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I try to make it available to all object like this   Object defineProperty Object prototype   length     get         if   Object keys            Object keys   function  obj                var keys      k              for  k in obj                    if  Object prototype hasOwnProperty call obj  k                         keys push k                                               return keys                       return Object keys this  length         console log   Name   Joe   Age  26  length    returns 2

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You can use   Object keys objectName  length     and   Object values objectName  length

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As stated by Avi Flax https   stackoverflow com a 4889658 1047014  Object keys obj  length   will do the trick for all enumerable properties on your object but to also include the non-enumerable properties you can instead use the Object getOwnPropertyNames  Here s the difference   var myObject   new Object     Object defineProperty myObject   nonEnumerableProp       enumerable  false     Object defineProperty myObject   enumerableProp       enumerable  true      console log Object getOwnPropertyNames myObject  length     outputs 2 console log Object keys myObject  length     outputs 1  console log myObject hasOwnProperty  nonEnumerableProp       outputs true console log myObject hasOwnProperty  enumerableProp       outputs true  console log  nonEnumerableProp  in myObject     outputs true console log  enumerableProp  in myObject     outputs true   As stated here this has the same browser support as Object keys  However  in most cases  you might not want to include the nonenumerables in these type of operations  but it s always good to know the difference

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as answered above  Object keys obj  length  But  as we have now a real Map class in ES6  I would suggest to use it instead of using the properties of an object   const map   new Map    map set  key    value    map size     THE fastest way

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Here are some performance tests for three methods   https   jsperf com get-the-number-of-keys-in-an-object  Object keys   length  20 735 operations per second  Very simple and compatible  Runs fast but expensive because it creates a new array of keys  which that then gets thrown away    return Object keys objectToRead  length    loop through the keys  15 734 operations per second  let size 0  for let k in objectToRead      size     return size    Slightly slower  but nowhere near the memory usage  so probably better if you re interested in optimising for mobile or other small machines  Using Map instead of Object  953 839 338 operations per second  return mapToRead size    Basically  Map tracks its own size so we re just returning a number field  Far  far faster than any other method  If you have control of the object  convert them to maps instead

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You could use this code   if   Object keys        Object keys   function  obj            var keys                   k          for  k in obj                if  Object prototype hasOwnProperty call obj  k                     keys push k                                   return keys             Then you can use this in older browsers as well   var len   Object keys obj  length

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From  https   developer mozilla org en docs Web JavaScript Reference Global Objects Object defineProperty     Object defineProperty obj  prop  descriptor    You can either add it to all your objects   Object defineProperty Object prototype   length         enumerable  false      get  function             return Object keys this  length              Or a single object   var myObj       Object defineProperty myObj   length         enumerable  false      get  function             return Object keys this  length              Example   var myObj       myObj name     John Doe   myObj email    leaked example com   myObj length    output  2   Added that way  it won t be displayed in for  in loops   for var i in myObj         console log i         myObj i        Output   name John Doe email leaked example com   Note  it does not work in  lt  IE9 browsers

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If jQuery above does not work  then try    Object Item  length

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OP didn t specify if the object is a nodeList  if it is then you can just use length method on it directly  Example   buttons   document querySelectorAll   id button     console log  Found     buttons length     on the screen

[javascript] How to efficiently count the number of keys/properties of an object in JavaScript?

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