JavaScript Object Id

Question

Do JavaScript objects/variables have some sort of unique identifier? Like Ruby has object_id. I don't mean the DOM id attribute, but rather some sort of memory address of some kind.

Answer 1

If you want to lookup/associate an object with a unique identifier without modifying the underlying object, you can use a WeakMap:

// Note that object must be an object or array, // NOT a primitive value like string, number, etc. var objIdMap=new WeakMap, objectCount = 0; function objectId(object){   if (!objIdMap.has(object)) objIdMap.set(object,++objectCount);   return objIdMap.get(object); }  var o1={}, o2={}, o3={a:1}, o4={a:1}; console.log( objectId(o1) ) // 1 console.log( objectId(o2) ) // 2 console.log( objectId(o1) ) // 1 console.log( objectId(o3) ) // 3 console.log( objectId(o4) ) // 4 console.log( objectId(o3) ) // 3 

Using a WeakMap instead of Map ensures that the objects can still be garbage-collected.

Answer 2

No, objects don't have a built in identifier, though you can add one by modifying the object prototype. Here's an example of how you might do that:

(function() {     var id = 0;      function generateId() { return id++; };      Object.prototype.id = function() {         var newId = generateId();          this.id = function() { return newId; };          return newId;     }; })(); 

That said, in general modifying the object prototype is considered very bad practice. I would instead recommend that you manually assign an id to objects as needed or use a touch function as others have suggested.