What is JavaScript garbage collection

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What is JavaScript garbage collection  What s important for a web programmer to understand about JavaScript garbage collection  in order to write better code

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To the best of my knowledge  JavaScript s objects are garbage collected periodically when there are no references remaining to the object   It is something that happens automatically  but if you want to see more about how it works  at the C   level  it makes sense to take a look at the WebKit or V8 source code  Typically you don t need to think about it  however  in older browsers  like IE 5 5 and early versions of IE 6  and perhaps current versions  closures would create circular references that when unchecked would end up eating up memory   In the particular case that I mean about closures  it was when you added a JavaScript reference to a dom object  and an object to a DOM object that referred back to the JavaScript object   Basically it could never be collected  and would eventually cause the OS to become unstable in test apps that looped to create crashes   In practice these leaks are usually small  but to keep your code clean you should delete the JavaScript reference to the DOM object   Usually it is a good idea to use the delete keyword to immediately de-reference big objects like JSON data that you have received back and done whatever you need to do with it  especially in mobile web development   This causes the next sweep of the GC to remove that object and free its memory

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In computer science  garbage collection  GC  is a form of automatic memory management  The garbage collector  or just collector  attempts to reclaim garbage  or memory used by objects that will never be accessed or mutated again by the application     All JavaScript engines have their own garbage collectors  and they may differ  Most time you do not have to deal with them because they just do what they supposed to do   Writing better code mostly depends of how good do you know programming principles  language and particular implementation

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garbage collection  GC  is a form of automatic memory management by removing the objects that no needed anymore   any process deal with memory follow these steps   1 - allocate your memory space you need  2 - do some processing  3 - free this memory space  there are two main algorithm used to detect which objects no needed anymore   Reference-counting garbage collection  this algorithm reduces the definition of  an object is not needed anymore  to  an object has no other object referencing to it   the object will removed if no reference point to it  Mark-and-sweep algorithm  connect each objects to root source  any object doesn t connect to root or other object  this object will be removed   currently most modern browsers using the second algorithm

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Reference types do not store the object directly into the variable to which it is assigned  so the object variable in the example below  doesn   t actually contain the object instance  Instead  it holds a pointer  or reference  to the location in memory  where the object exists  var object   new Object     if you assign one reference typed variable to another  each variable gets a copy of the pointer  and both still reference to the same object in memory  var object1   new Object    var object2   object1     JavaScript is a garbage-collected language  so you don   t really need to worry about memory allocations when you use reference types  However  it   s best to dereference objects that you no longer need so that the garbage collector can free up that memory  The best way to do this is to set the object variable to null   var object1   new Object       do something object1   null     dereference   Dereferencing objects is especially important in very large applications that use millions of objects   from The Principles of Object-Oriented JavaScript - NICHOLAS C  ZAKAS

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Eric Lippert wrote a detailed blog post about this subject a while back  additionally comparing it to VBScript   More accurately  he wrote about JScript  which is Microsoft s own implementation of ECMAScript  although very similar to JavaScript  I would imagine that you can assume the vast majority of behaviour would be the same for the JavaScript engine of Internet Explorer  Of course  the implementation will vary from browser to browser  though I suspect you could take a number of the common principles and apply them to other browsers   Quoted from that page      JScript uses a nongenerational   mark-and-sweep garbage collector   It   works like this           Every variable which is  in scope    is called a  scavenger    A scavenger   may refer to a number  an object  a   string  whatever   We maintain a list   of scavengers -- variables are moved   on to the scav list when they come   into scope and off the scav list when   they go out of scope    Every now and then the garbage   collector runs    First it puts a    mark  on every object  variable    string  etc     all the memory tracked   by the GC    JScript uses the VARIANT   data structure internally and there   are plenty of extra unused bits in   that structure  so we just set one of   them     Second  it clears the mark on the   scavengers and the transitive closure   of scavenger references   So if a   scavenger object references a   nonscavenger object then we clear the   bits on the nonscavenger  and on   everything that it refers to    I am   using the word  closure  in a   different sense than in my earlier   post     At this point we know that all the   memory still marked is allocated   memory which cannot be reached by any   path from any in-scope variable   All   of those objects are instructed to   tear themselves down  which destroys   any circular references       The main purpose of garbage collection is to allow the programmer not to worry about memory management of the objects they create and use  though of course there s no avoiding it sometimes - it is always beneficial to have at least a rough idea of how garbage collection works   Historical note  an earlier revision of the answer had an incorrect reference to the delete operator  In JavaScript the delete operator removes a property from an object  and is wholly different to delete in C C

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Beware of circular references when DOM objects are involved   Memory leak patterns in JavaScript  Keep in mind that memory can only be reclaimed when there are no active references to the object  This is a common pitfall with closures and event handlers  as some JS engines will not check which variables actually are referenced in inner functions and just keep all local variables of the enclosing functions   Here s a simple example   function init         var bigString   new Array 1000  join  xxx        var foo   document getElementById  foo        foo onclick   function                this might create a closure over  bigString              even if  bigString  isn t referenced anywhere             A naive JS implementation can t collect bigString as long as the event handler is around  There are several ways to solve this problem  eg setting bigString   null at the end of init    delete won t work for local variables and function arguments  delete removes properties from objects  and the variable object is inaccessible - ES5 in strict mode will even throw a ReferenceError if you try to delete a local variable     I recommend to avoid unnecessary closures as much as possible if you care for memory consumption

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Good quote taken from a blog  The DOM component is  garbage collected   as is the JScript component  which means that if you create an object within either component  and then lose track of that object  it will eventually be cleaned up   For example   function makeABigObject     var bigArray   new Array 20000       When you call that function  the JScript component creates an object  named bigArray  that is accessible within the function  As soon as the function returns  though  you  lose track  of bigArray because there s no way to refer to it anymore  Well  the JScript component realizes that you ve lost track of it  and so bigArray is cleaned up--its memory is reclaimed  The same sort of thing works in the DOM component  If you say document createElement  div    or something similar  then the DOM component creates an object for you  Once you lose track of that object somehow  the DOM component will clean up the related

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What is JavaScript garbage collection     check this      What s important for a web programmer to understand about JavaScript garbage collection     in order to write better code    In Javascript you don t care about memory allocation and deallocation  The whole problem is demanded to the Javascript interpreter  Leaks are still possible in Javascript  but they are bugs of the interpreter  If you are interested in this topic you could read more in  www memorymanagement org

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On windows you can use Drip exe to find memory leaks or check if your free mem routine works    It s really simple  just enter a website URL and you will see the memory consumption of the integrated IE renderer  Then hit refresh  if the memory increases  you found a memory leak somewhere on the webpage  But this is also very useful to see if routines for freeing memory work for IE

[javascript] What is JavaScript garbage collection?

Examples related to javascript

Examples related to garbage-collection