unique object identifier in javascript

Question

I need to do some experiment and I need to know some kind of unique identifier for objects in javascript  so I can see if they are the same  I don t want to use equality operators  I need something like the id   function in python    Does something like this exist

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This one will calculate a HashCode for each object  optimized for string  number and virtually anything that has a getHashCode function  For the rest it assigns a new reference number   function       var   gRefID   0    window getHashCode   function ref            if  ref    null    throw Error  quot Unable to calculate HashCode on a null reference quot                already cached reference id       if  ref hasOwnProperty  quot   refID quot      return ref  quot   refID quot                numbers are already hashcodes       if  typeof ref      quot number quot     return ref              strings are immutable  so we need to calculate this every time       if  typeof ref      quot string quot                     var hash   0  i  chr            for  i   0  i  lt  ref length  i                  chr   ref charCodeAt i               hash     hash  lt  lt  5  - hash    chr              hash    0                        return hash                    virtual call       if  typeof ref getHashCode      quot function quot     return ref getHashCode                generate and return a new reference id       return  ref  quot   refID quot      quot ref quot      gRefID

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For browsers implementing the Object defineProperty   method  the code below generates and returns a function that you can bind to any object you own   This approach has the advantage of not extending Object prototype   The code works by checking if the given object has a   objectID   property  and by defining it as a hidden  non-enumerable  read-only property if not   So it is safe against any attempt to change or redefine the read-only obj   objectID   property after it has been defined  and consistently throws a nice error instead of silently fail   Finally  in the quite extreme case where some other code would already have defined   objectID   on a given object  this value would simply be returned   var getObjectID    function           var id   0        Private ID counter      return function  obj              if obj hasOwnProperty    objectID                     return obj   objectID                else                   id               Object defineProperty obj     objectID                                                Explicitly sets these two attribute values to false                      although they are false by default                                         configurable    false                    enumerable      false                                            This closure guarantees that different objects                     will not share the same id variable                                         get     function    objectID                           return function      return   objectID                            id                      set    function                           throw new Error  Sorry  but  obj   objectID    is read-only                                                       return obj   objectID

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Update My original answer below was written 6 years ago in a style befitting the times and my understanding   In response to some conversation in the comments  a more modern approach to this is as follows   x000D   x000D       function             if   typeof Object id     undefined                  var id   0               Object id   function o                    if   typeof o   uniqueid     undefined                          Object defineProperty o     uniqueid                             value    id                          enumerable  false                             This could go either way  depending on your                             interpretation of what an  id  is                         writable  false                                                            return o   uniqueid                                              var obj     a  1  b  1             console log Object id obj        console log Object id           console log Object id           console log Object id            console log Object id function              for  var k in obj            if  obj hasOwnProperty k                 console log k                          Logged keys are  a  and  b  x000D   x000D   x000D   If you have archaic browser requirements  check here for browser compatibility for Object defineProperty  The original answer is kept below  instead of just in the change history  because I think the comparison is valuable   You can give the following a spin   This also gives you the option to explicitly set an object s ID in its constructor or elsewhere   x000D   x000D       function             if   typeof Object prototype uniqueId     undefined                  var id   0              Object prototype uniqueId   function                     if   typeof this   uniqueid     undefined                          this   uniqueid     id                                    return this   uniqueid                                              var obj1           var obj2   new Object             console log obj1 uniqueId         console log obj2 uniqueId         console log    uniqueId         console log    uniqueId         console log     uniqueId         console log  function       uniqueId     x000D   x000D   x000D   Take care to make sure that whatever member you use to internally store the unique ID doesn t collide with another automatically created member name

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Latest browsers provide a cleaner method for extending Object prototype  This code will make the property hidden from property enumeration  for p in o   For the browsers that implement defineProperty  you can implement uniqueId property like this    function         var id counter   1      Object defineProperty Object prototype     uniqueId             writable  true             Object defineProperty Object prototype   uniqueId             get  function                 if  this   uniqueId    undefined                  this   uniqueId   id counter                return this   uniqueId                            For details  see https   developer mozilla org en JavaScript Reference Global Objects Object defineProperty

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Notwithstanding the advice not to modify Object prototype  this can still be really useful for testing  within a limited scope  The author of the accepted answer changed it  but is still setting Object id  which doesn t make sense to me  Here s a snippet that does the job      Generates a unique  read-only id for an object     The  uid is generated for the object the first time it s accessed    function       var id   0    Object defineProperty Object prototype    uid            The prototype getter sets up a property on the instance  Because        the new instance-prop masks this one  we know this will only ever        be called at most once for any given object      get  function            Object defineProperty this    uid             value  id            writable  false          enumerable  false                  return this  uid             enumerable  false               function assert p    if   p  throw Error  Not       var obj       assert obj  uid    0   assert     uid    1   assert     uid    2   assert obj  uid    0       still

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So far as my observation goes  any answer posted here can have unexpected side effects   In ES2015-compatible enviroment  you can avoid any side effects by using WeakMap   const id         gt        let currentId   0      const map   new WeakMap         return  object    gt            if   map has object                 map set object    currentId                      return map get object                 id          gt  1

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jQuery code uses it s own data   method as such id   var id     data object     At the backstage method data creates a very special field in object called  jQuery    now   put there next id of a stream of unique ids like  id   elem  expando       uuid    I d suggest you use the same method as John Resig obviously knows all there is about JavaScript and his method is based on all that knowledge

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For the purpose of comparing two objects  the simplest way to do this would be to add a unique property to one of the objects at the time you need to compare the objects  check if the property exists in the other and then remove it again  This saves overriding prototypes   function isSameObject objectA  objectB       unique ref    unique id     performance now       objectA unique ref    true     isSame   objectB hasOwnProperty unique ref      delete objectA unique ref      return isSame     object1    something true   object2    something true   object3   object1   console log isSameObject object1  object2      false console log isSameObject object1  object3      true

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I ve used code like this  which will cause Objects to stringify with unique strings   Object prototype   defineGetter      id     function          var gid   0      return function            var id   gid            this   proto                      proto    this   proto                 get   id      return id                      return id          call       Object prototype toString   function          return   Object     this   id               the   proto   bits are to keep the   id   getter from showing up in the object   this has been only tested in firefox

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Actually  you don t need to modify the object prototype and add a function there  The following should work well for your purpose   var   next objid 1  function objectId obj        if  obj  null  return null      if  obj   obj id  null  obj   obj id   next objid        return obj   obj id

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If you came here because you deal with class instances like me you can use static vars methods to reference instances by a custom unique id   x000D   x000D  class Person        constructor  name             this name   name          this id   Person ix            Person stack  this id     this          Person ix   0  Person stack       Person byId   id   gt  Person stack  id     let store       store  new Person   joe    id     true  store  new Person   tim    id     true   for  let id in store         console log  Person byId  id   name      x000D   x000D   x000D

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Typescript version of  justin answer  ES6 compatible  using Symbols to prevent any key collision and added into the global Object id for convenience  Just copy paste the code below  or put it into an ObjecId ts file you will import    enableObjectID      declare global       interface ObjectConstructor           id   object  any    gt  number           const uniqueId  symbol   Symbol  The unique id of an object     export function enableObjectID    void       if  typeof Object  id        undefined             return             let id  number   0       Object  id      object  any    gt            const hasUniqueId  boolean     object uniqueId           if   hasUniqueId                object uniqueId      id                     return object uniqueId              Example of usage   console log Object id myObject

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I faced the same problem and here s the solution I implemented with ES6  code let id   0     This is a kind of global variable accessible for every instance   class Animal   constructor name   this name   name  this id   id        foo         Executes some cool stuff    cat   new Animal  Catty      console log cat id     1

[javascript] unique object identifier in javascript

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