Number of elements in a javascript object

Question

Is there a way to get  from somewhere  the number of elements in a javascript object    i e  constant-time complexity    I cant find a property or method that retrieve that information  So far I can only think of doing an iteration through the whole collection  but that s linear time  It s strange there is no direct access to the size of the object  dont you think   EDIT  I m talking about the Object object  not objects in general    var obj   new Object

User · Answer

if you are already using jQuery in your build just do this     yourObject  length   It works nicely for me on objects  and I already had jQuery as a dependancy

User · Answer

To do this in any ES5-compatible environment  Object keys obj  length    Browser support from here   Doc on Object keys here  includes method you can add to non-ECMA5 browsers

User · Answer

function count        var c  0      for var p in this  if this hasOwnProperty p    c      return c     var O  a  1  b  2  c  3    count call O

User · Answer

AFAIK  there is no way to do this reliably  unless you switch to an array   Which honestly  doesn t seem strange - it s seems pretty straight forward to me that arrays are countable  and objects aren t   Probably the closest you ll get is something like this     Monkey patching on purpose to make a point Object prototype length   function       var i   0    for   var p in this   i      return i     alert   foo  bar   bar   baz   length         alerts 3   But this creates problems  or at least questions   All user-created properties are counted  including the  length function itself   And while in this simple example you could avoid it by just using a normal function  that doesn t mean you can stop other scripts from doing this   so what do you do   Ignore function properties   Object prototype length   function       var i   0    for   var p in this             if    function     typeof this p    continue        i          return i     alert   foo  bar   bar   baz   length         alerts 2   In the end  I think you should probably ditch the idea of making your objects countable and figure out another way to do whatever it is you re doing

User · Answer

Although JS implementations might keep track of such a value internally  there s no standard way to get it   In the past  Mozilla s Javascript variant exposed the non-standard   count    but it has been removed with version 1 8 5   For cross-browser scripting you re stuck with explicitly iterating over the properties and checking hasOwnProperty     function countProperties obj        var count   0       for var prop in obj            if obj hasOwnProperty prop                 count             return count      In case of ECMAScript 5 capable implementations  this can also be written as  Kudos to Avi Flax   function countProperties obj        return Object keys obj  length      Keep in mind that you ll also miss properties which aren t enumerable  eg an array s length    If you re using a framework like jQuery  Prototype  Mootools   whatever-the-newest-hype  check if they come with their own collections API  which might be a better solution to your problem than using native JS objects

User · Answer

The concept of number length dimensionality doesn t really make sense for an Object  and needing it suggests you really want an Array to me   Edit  Pointed out to me that you want an O 1  for this  To the best of my knowledge no such way exists I m afraid

[javascript] Number of elements in a javascript object

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