document ready equivalent without jQuery

Question

I have a script that uses   document  ready  but it doesn t use anything else from jQuery  I d like to lighten it up by removing the jQuery dependency   How can I implement my own   document  ready functionality without using jQuery  I know that  using window onload will not be the same  as window onload fires after all images  frames  etc  have been loaded

User · Answer

function onDocReady fn         d readyState    loading    fn   document addEventListener  DOMContentLoaded  fn      function onWinLoad fn        d readyState    complete     fn    window addEventListener  load  fn        onDocReady provides a callback when the HTML dom is ready to fully  access parse manipulate   onWinLoad provides a callback when everything has loaded  images etc    These functions can be called whenever you want  Supports multiple  listeners    Will work in any browser

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The following approach ensures a script functionality only runs when the document is ready  even if the script is loaded asynchronously  Unlike JQuery s   document  ready  DOMContentLoaded will not fire if the script is loaded dynamically afterwards   function ready         if   document body   return setTimeout ready  50           Document is ready here        The same solution applies to any condition we want a script to be executed after  Say we are asynchronously loading JQuery and another custom script that requires JQuery  We don t know in advance which of the two will load first  but we can ensure that our custom script waits for JQuery to be present   function ready         if   window     return setTimeout ready  50           JQuery is ready here

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Three options    If script is the last tag of the body  the DOM would be ready before script tag executes When the DOM is ready   readyState  will change to  complete  Put everything under  DOMContentLoaded  event listener   onreadystatechange    document onreadystatechange   function           if  document readyState     complete             document is ready  Do your stuff here           Source  MDN  DOMContentLoaded  document addEventListener  DOMContentLoaded   function        console log  document is ready  I can sleep now          Concerned about stone age browsers  Go to the jQuery source code and use the ready function  In that case you are not parsing executing the whole library you re are doing only a very small part of it

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Just add this to the bottom of your HTML page      lt script gt      Your Function     lt  script gt    Because  HTML documents are parsed by top-bottom

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Cross-browser  old browsers too  and a simple solution   var docLoaded   setInterval function          if document readyState      complete   return      clearInterval docLoaded                   Your code goes here i e  init             30     Showing alert in jsfiddle

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How about this solution      other onload attached earlier window onload function        alert  test        tmpPreviousFunction window onload   window onload   null      our onload function window onload function        alert  another message           execute previous one    if  tmpPreviousFunction  tmpPreviousFunction

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Edit of the edit of  duskwuff to support Internet nbsp Explorer nbsp 8 too  The difference is a new call to the function test of the regex and the setTimeout with an anonymous function   Also  I set the timeout to 99   function ready f   in  test document readyState  setTimeout function   ready f    99  f

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Here is the smallest code snippet to test DOM ready which works across all browsers  even IE 8    r function        alert  DOM Ready         function r f   in  test document readyState  setTimeout  r   f     9  f      See this answer

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I was recently using this for a mobile site  This is John Resig s simplified version from  Pro JavaScript Techniques   It depends on addEvent   var ready     function        function ready  f         if  ready done   return f         if  ready timer           ready ready push f         else         addEvent  window   load   isDOMReady          ready ready     f          ready timer   setInterval isDOMReady  13                 function isDOMReady         if  ready done   return false       if  document  amp  amp  document getElementsByTagName  amp  amp  document getElementById  amp  amp  document body           clearInterval  ready timer          ready timer   null        for  var i   0  i  lt  ready ready length  i               ready ready i                   ready ready   null        ready done   true               return ready

User · Answer

Try this   function ready callback       if typeof callback      function            document addEventListener  DOMContentLoaded   callback           window addEventListener  load   callback        else          throw new Error  Sorry  I can not run this             ready function        console log  It worked

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We found a quick-and-dirty cross browser implementation of ours that may do the trick for most simple cases with a minimal implementation   window onReady   function onReady fn       document body   fn     setTimeout function    onReady fn    50

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There is a standards based replacement DOMContentLoaded that is supported by over 98  of browsers  though not IE8   document addEventListener  DOMContentLoaded   function event         do work       jQuery s native function is much more complicated than just window onload  as depicted below     function bindReady        if   readyBound   return      readyBound   true          Mozilla  Opera and webkit nightlies currently support this event     if   document addEventListener                Use the handy event callback         document addEventListener   DOMContentLoaded   function                document removeEventListener   DOMContentLoaded   arguments callee  false                jQuery ready               false            If IE event model is used       else if   document attachEvent                ensure firing before onload             maybe late but safe also for iframes         document attachEvent  onreadystatechange   function                if   document readyState      complete                      document detachEvent   onreadystatechange   arguments callee                    jQuery ready                                          If IE and not an iframe            continually check to see if the document is ready         if   document documentElement doScroll  amp  amp  window    window top    function                if   jQuery isReady   return               try                      If IE is used  use the trick by Diego Perini                    http   javascript nwbox com IEContentLoaded                  document documentElement doScroll  left                  catch  error                     setTimeout  arguments callee  0                    return                                and execute any waiting functions             jQuery ready                                A fallback to window onload  that will always work     jQuery event add  window   load   jQuery ready

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Edit   Here is a viable replacement for jQuery ready  function ready callback          in case the document is already rendered     if  document readyState   loading   callback           modern browsers     else if  document addEventListener  document addEventListener  DOMContentLoaded   callback          IE  lt   8     else document attachEvent  onreadystatechange   function            if  document readyState   complete   callback               ready function           do something       Taken from https   plainjs com javascript events running-code-when-the-document-is-ready-15   Another good domReady function here taken from https   stackoverflow com a 9899701 175071    As the accepted answer was very far from complete  I stitched together a  ready  function like jQuery ready   based on jQuery 1 6 2 source   var ready    function         var readyList          DOMContentLoaded          class2type               class2type   object Boolean       boolean           class2type   object Number       number           class2type   object String       string           class2type   object Function       function           class2type   object Array       array           class2type   object Date       date           class2type   object RegExp       regexp           class2type   object Object       object        var ReadyObj                Is the DOM ready to be used  Set to true once it occurs          isReady  false             A counter to track how many items to wait for before            the ready event fires  See  6781         readyWait  1             Hold  or release  the ready event         holdReady  function  hold                 if   hold                     ReadyObj readyWait                  else                   ReadyObj ready  true                                        Handle when the DOM is ready         ready  function  wait                    Either a released hold or an DOMready load event and not yet ready             if    wait     true  amp  amp   --ReadyObj readyWait      wait     true  amp  amp   ReadyObj isReady                         Make sure body exists  at least  in case IE gets a little overzealous  ticket  5443                   if    document body                         return setTimeout  ReadyObj ready  1                                          Remember that the DOM is ready                 ReadyObj isReady   true                     If a normal DOM Ready event fired  decrement  and wait if need be                 if   wait     true  amp  amp  --ReadyObj readyWait  gt  0                         return                                       If there are functions bound  to execute                 readyList resolveWith  document    ReadyObj                          Trigger any bound ready events                   if   ReadyObj fn trigger                           ReadyObj  document   trigger   ready    unbind   ready                                                          bindReady  function                 if   readyList                     return                            readyList   ReadyObj  Deferred                    Catch cases where   document  ready   is called after the                browser event has already occurred              if   document readyState      complete                         Handle it asynchronously to allow scripts the opportunity to delay ready                 return setTimeout  ReadyObj ready  1                                  Mozilla  Opera and webkit nightlies currently support this event             if   document addEventListener                        Use the handy event callback                 document addEventListener   DOMContentLoaded   DOMContentLoaded  false                       A fallback to window onload  that will always work                 window addEventListener   load   ReadyObj ready  false                    If IE event model is used               else if   document attachEvent                        ensure firing before onload                     maybe late but safe also for iframes                 document attachEvent   onreadystatechange   DOMContentLoaded                        A fallback to window onload  that will always work                 window attachEvent   onload   ReadyObj ready                        If IE and not a frame                    continually check to see if the document is ready                 var toplevel   false                   try                       toplevel   window frameElement    null                    catch e                      if   document documentElement doScroll  amp  amp  toplevel                         doScrollCheck                                                        Deferred  function                 var    callbacks list                 callbacks                          stored   context   args                   fired                     to avoid firing when already doing so                 firing                     flag to know if the deferred has been cancelled                 cancelled                     the deferred itself                 deferred                              done  f1  f2                           done  function                             if    cancelled                                 var args   arguments                                  i                                  length                                  elem                                  type                                   fired                              if   fired                                      fired   fired                                  fired   0                                                            for   i   0  length   args length  i  lt  length  i                                       elem   args  i                                    type   ReadyObj type  elem                                    if   type      array                                          deferred done apply  deferred  elem                                      else if   type      function                                          callbacks push  elem                                                                                                if    fired                                     deferred resolveWith   fired  0     fired  1                                                                                      return this                                                 resolve with given context and args                     resolveWith  function  context  args                             if    cancelled  amp  amp   fired  amp  amp   firing                                    make sure args are available   8421                              args   args                                    firing   1                              try                                   while  callbacks  0                                           callbacks shift   apply  context  args     shifts a callback  and applies it to document                                                                                             finally                                   fired     context  args                                    firing   0                                                                                  return this                                                 resolve with this as context and given arguments                     resolve  function                             deferred resolveWith  this  arguments                            return this                                                 Has this deferred been resolved                      isResolved  function                             return     firing    fired                                                   Cancel                     cancel  function                             cancelled   1                          callbacks                               return this                                                        return deferred                     type  function  obj                 return obj    null                   String  obj                     class2type  Object prototype toString call obj        object                          The DOM ready check for Internet Explorer     function doScrollCheck             if   ReadyObj isReady                 return                     try                  If IE is used  use the trick by Diego Perini                http   javascript nwbox com IEContentLoaded              document documentElement doScroll  left              catch e                setTimeout  doScrollCheck  1                return                        and execute any waiting functions         ReadyObj ready                 Cleanup functions for the document ready method     if   document addEventListener             DOMContentLoaded   function                 document removeEventListener   DOMContentLoaded   DOMContentLoaded  false                ReadyObj ready                      else if   document attachEvent             DOMContentLoaded   function                    Make sure body exists  at least  in case IE gets a little overzealous  ticket  5443               if   document readyState      complete                      document detachEvent   onreadystatechange   DOMContentLoaded                    ReadyObj ready                                       function ready  fn                Attach the listeners         ReadyObj bindReady             var type   ReadyObj type  fn                Add the callback         readyList done  fn     readyList is result of  Deferred             return ready          How to use    lt script gt      ready function            alert  It works                 ready function            alert  Also works              lt  script gt    I am not sure how functional this code is  but it worked fine with my superficial tests  This took quite a while  so I hope you and others can benefit from it   PS   I suggest compiling it   Or you can use http   dustindiaz com smallest-domready-ever   function r f   in  test document readyState  setTimeout r 9 f  f    r function     code to run        or the native function if you only need to support the new browsers  Unlike jQuery ready  this won t run if you add this after the page has loaded   document addEventListener  DOMContentLoaded  function     fun code to run

User · Answer

If you don t have to support very old browsers  here is a way to do it even when your external script is loaded with async attribute   HTMLDocument prototype ready   new Promise function resolve       if document readyState     loading         resolve       else       document addEventListener  DOMContentLoaded   function              resolve                   document ready then function        console log  document ready

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Place your  lt script gt   JavaScript code   lt  script gt  right before the closing  lt  body gt  tag   Admittedly  this might not suit everyone s purposes since it requires changing the HTML file rather than just doing something in the JavaScript file a la document ready  but still

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The setTimeout setInterval solutions presented here will only work in specific circumstances   The problem shows up especially in older Internet Explorer versions up to 8   The variables affecting the success of these setTimeout setInterval solutions are   1  dynamic or static HTML 2  cached or non cached requests 3  size of the complete HTML document 4  chunked or non chunked transfer encoding   the original  native Javascript  code solving this specific issue is here   https   github com dperini ContentLoaded http   javascript nwbox com ContentLoaded  test    this is the code from which the jQuery team have built their implementation

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I simply use   setTimeout function          reference manipulate DOM here       And unlike document addEventListener  DOMContentLoaded    etc as in the very top answer  it works as far back as IE9 -- http   caniuse com  search DOMContentLoaded only indicates as recently as IE11    Interestingly I stumbled upon this setTimeout solution in 2009  Is checking for the readiness of the DOM overkill   which probably could have been worded slightly better  as I meant  is it overkill to use various frameworks  more complicated approaches to check for the readiness of the DOM    My best explanation for why this technique works is that  when the script with such a setTimeout has been reached  the DOM is in the middle of being parsed  so execution of the code within the setTimeout gets deferred until that operation is finished

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Comparison Here  in below snippet  is comparison of chosen available browser  quot built-in quot  methods and their execution sequence  Remarks  the document onload  X  is not supported by any modern browser  event is never fired  if you use  lt body onload  quot bodyOnLoad   quot  gt   F  and at the same time window onload  E  then only first one will be executed  because it override second one  event handler given in  lt body onload  quot     quot  gt   F   is wrapped by additional onload function document onreadystatechange  D  not override document  addEventListener  readystatechange       C  probably cecasue onXYZevent-like methods are independent than addEventListener queues  which allows add multiple listeners   Probably nothing happens between execution this two handlers  all scripts write their timestamp in console - but scripts which also have access to div write their timestamps also in body  click  quot Full Page quot  link after script execution to see it   solutions readystatechange  C D  are executed multiple times by browser but for different document states  loading - the document is loading  no fired in snippet  interactive - the document is parsed  fired before DOMContentLoaded complete - the document and resources are loaded  fired before body window onload   x000D   x000D   lt html gt    lt head gt     lt script gt         solution A     console log   timestamp    Date now     A  Head script                 solution B     document addEventListener  DOMContentLoaded        gt          print   timestamp    Date now     B  DOMContentLoaded                    solution C     document addEventListener  readystatechange        gt          print   timestamp    Date now     C  ReadyState    document readyState                        solution D     document onreadystatechange   s  gt   print   timestamp    Date now     D  document onreadystatechange ReadyState    document readyState                  solution E  never executed      window onload        gt          print  E   lt body onload       gt  override this handler                   solution F     function bodyOnLoad           print   timestamp    Date now     F   lt body onload       gt                 infoAboutOnLoad       additional info                   solution X     document onload        gt   print  document onload is never fired               HELPERS      function print txt           console log txt         if mydiv  mydiv innerHTML    txt replace   lt     amp lt    replace   gt     amp gt        lt br gt                   function infoAboutOnLoad           console log  window onload  after  override         document body onload  replace   s  g              console log  body onload  window onload -- gt    document body onload  window onload                            console log  window onload  before override         document body onload  replace   s  g            lt  script gt   lt  head gt    lt body onload  bodyOnLoad    gt     lt div id  mydiv  gt  lt  div gt      lt  -- this script must te at the bottom of  lt body gt  -- gt     lt script gt         solution G     print   timestamp    Date now     G   lt body gt  bottom script       lt  script gt   lt  body gt    lt  html gt  x000D   x000D   x000D

User · Answer

Really  if you care about Internet nbsp Explorer nbsp 9  only  this code would be enough to replace jQuery ready       document addEventListener  DOMContentLoaded   callback       If you worry about Internet nbsp Explorer nbsp 6 and some really strange and rare browsers  this will work   domReady  function  callback           Mozilla  Opera and WebKit     if  document addEventListener            document addEventListener  DOMContentLoaded   callback  false              If Internet Explorer  the event model is used       else if  document attachEvent            document attachEvent  onreadystatechange   function                 if  document readyState      complete                      callback                                         A fallback to window onload  that will always work       else           var oldOnload   window onload          window onload   function                  oldOnload  amp  amp  oldOnload                callback

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Poor man s solution   var checkLoad   function            document readyState      complete    setTimeout checkLoad  11    alert  loaded              checkLoad        View Fiddle  Added this one  a bit better I guess  own scope  and non recursive   function        var tId   setInterval function             if  document readyState     complete   onComplete          11       function onComplete            clearInterval tId               alert  loaded                       View Fiddle

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The ready function in jQuery does a number of things  Frankly  I don t see that point of replacing it unless you have amazingly small output from your website  jQuery is a pretty tiny library  and it handles all sorts of cross-browser things you ll need later   Anyway  there s little point in posting it here  just open up jQuery and look at the bindReady method   It starts by calling either document addEventListener  DOMContentLoaded   or document attachEvent  onreadystatechange   depending on the event model  and goes on from there

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If you are loading jQuery near the bottom of BODY  but are having trouble with code that writes out jQuery  lt func   or jQuery document  ready  lt func    check out jqShim on Github   Rather than recreate its own document ready function  it simply holds onto the functions until jQuery is available then proceeds with jQuery as expected  The point of moving jQuery to the bottom of body is to speed up page load  and you can still accomplish it by inlining the jqShim min js in the head of your template   I ended up writing this code to make moving all the scripts in WordPress to the footer  and just this shim code now sits directly in the header

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The jQuery answer was pretty useful to me  With a little refactory it fitted my needs well  I hope it helps anybody else   function onReady   callback        var addListener   document addEventListener    document attachEvent          removeListener    document removeEventListener    document detachEvent         eventName   document addEventListener    DOMContentLoaded     onreadystatechange       addListener call document  eventName  function            removeListener  eventName  arguments callee  false           callback          false

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This cross-browser code will call a function once the DOM is ready   var domReady function func       var scriptText     func             var scriptElement document createElement  script        scriptElement innerText scriptText      document body appendChild scriptElement        Here s how it works    The first line of domReady calls the toString method of the function to get a string representation of the function you pass in and wraps it in an expression that immediately calls the function  The rest of domReady creates a script element with the expression and appends it to the body of the document  The browser runs script tags appended to body after the DOM is ready    For example  if you do this  domReady function   alert        the following will appended to the body element     lt script gt  function    alert         lt  script gt    Note that this works only for user-defined functions  The following won t work  domReady alert

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For IE9    function ready fn      if  document readyState     loading        fn        else       document addEventListener  DOMContentLoaded   fn

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This was a good https   stackoverflow com a 11810957 185565 poor man s solution  One comment considered a counter to bail out in case of emergency  This is my modification   function doTheMagic counter      alert  It worked on     counter         wait for document ready then call handler function var checkLoad   function counter      counter      if  document readyState     complete   amp  amp  counter lt 1000        var fn   function     checkLoad counter          setTimeout fn 10       else doTheMagic counter      checkLoad 0

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This question was asked quite a long time ago   For anyone just seeing this question  there is now a site called  you might not need jquery   which breaks down - by level of IE support required - all the functionality of jquery and provides some alternative  smaller libraries   IE8 document ready script according to you might not need jquery  function ready fn        if  document readyState     loading           fn        else if  document addEventListener          document addEventListener  DOMContentLoaded   fn       else         document attachEvent  onreadystatechange   function                 if  document readyState     loading                   fn

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function f     if document readyState     loading   f      else document addEventListener  DOMContentLoaded   f      function      console log  The Document is ready

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Works in all known browsers  tested via BrowserStack   IE6   Safari 1   Chrome 1   Opera  etc  Uses DOMContentLoaded  with fallbacks to document documentElement doScroll   and window onload      https   github com Kithraya DOMContentLoaded v1 2 6   MIT License     DOMContentLoaded version    quot 1 2 6 quot    function DOMContentLoaded      quot use strict quot            var ael    addEventListener   rel    removeEventListener   aev    attachEvent   dev    detachEvent       var alreadyRun   false     for use in the idempotent function ready           funcs   arguments              old versions of JS return   object Object   for null      function type obj    return  obj     null     null    Object prototype toString call obj  slice 8 -1  toLowerCase         function microtime     return   new Date                   document readyState      complete  reports correctly in every browser I have tested  including IE          But IE6 to 10 don t return the correct readyState values as per the spec          readyState is sometimes  interactive   even when the DOM isn t accessible in IE6 7 so checking for the onreadystatechange event like jQuery does is not optimal         readyState is complete at basically the same time as  window onload   they re functionally equivalent  within a few tenths of a second          Accessing undefined properties of a defined object  document  will not throw an error  in case readyState is undefined                       Check for IE  lt  11 via conditional compilation         values  5   IE5  5 5   IE5 5  5 6 5 7  IE6 7  5 8  IE8  9  IE9  10  IE10  11    IE11 older doc mode   undefined  IE11   NOT IE     var jscript version   Number  new Function  quot    cc on return   jscript version       quot          NaN              check if the DOM has already loaded     if  document readyState      complete     ready null   return        here we send null as the readyTime  since we don t know when the DOM became ready           if  jscript version  lt  9    doIEScrollCheck    return       For IE lt 9 poll document documentElement doScroll    no further actions are needed                        Chrome  Edge  Firefox  IE9   Opera 9   Safari 3 1   Android Webview  Chrome for Android  Edge Mobile           Firefox for Android 4   Opera for Android  iOS Safari  Samsung Internet  etc  support addEventListener         And IE9  supports  DOMContentLoaded                        if  document ael             document ael   quot DOMContentLoaded quot   ready  false            window ael   quot load quot   ready  false      fallback to the load event in case addEventListener is supported  but not DOMContentLoaded       else      if  aev in window    window aev   onload   ready              Old Opera has a default of window attachEvent being falsy  so we use the in operator instead            https   dev opera com blog window-event-attachevent-detachevent-script-onreadystatechange              Honestly if somebody is using a browser so outdated AND obscure  like Opera 7 where neither addEventListener             nor  quot DOMContLoaded quot  is supported  they deserve to wait for the full page              I CBA testing whether readyState      interactive  is truly interactive in browsers designed in 2003  I just assume it isn t  like in IE6-8                     else      fallback to queue window onload that will always work        addOnload ready                          This function allows us to preserve any original window onload handlers  in super old browsers where this is even necessary           while keeping the option to chain onloads  and dequeue them           function addOnload fn    var prev   window onload     old window onload  which could be set by this function  or elsewhere                     we add a function queue list to allow for dequeueing             addOnload queue is the queue of functions that we will run when when the DOM is ready         if   type  addOnload queue        array     addOnload queue                   if   type prev       function     addOnload queue push  prev         add the previously defined event handler                            if  typeof fn      function     addOnload queue push fn             window onload   function        iterate through the queued functions             for  var i   0  i  lt  addOnload queue length  i      addOnload queue i                                   remove a queued window onload function from the chain  simplified             function dequeueOnload fn    var q   addOnload queue  i   0                  sort through the queued functions in addOnload queue until we find  fn          if  type  q        array               if found  remove from the queue             for    i  lt  q length  i         fn     q i     q splice i  1    0       void   fn     q i     q splice i  1    0                             function ready ev       idempotent event handler function         if  alreadyRun   return  alreadyRun   true                       this time is when the DOM has loaded  or if all else fails  when it was actually possible to inference the DOM has loaded via a  load  event             perhaps this should be  null  if we have to inference readyTime via a  load  event  but this functionality is better          var readyTime   microtime                      detach       detach any event handlers                                     run the functions         for  var i 0  i  lt  funcs length  i       var func   funcs i                            if  type func       function                     func call document     readyTime    ev     null   null   readyTime    funcExecuteTime   microtime      func                       jquery calls  ready  with  this  being set to document  so we ll do the same                                             function detach             if  document rel                  document rel   quot DOMContentLoaded quot   ready   window rel   quot load quot   ready             else         if  dev in window    window dev   quot onload quot   ready              else               dequeueOnload ready                                                                                           function doIEScrollCheck        for use in IE  lt  9 only          if   window frameElement                     we re in an  lt iframe gt  or similar                the document documentElemeent doScroll technique does not work if we re not at the top-level  parent document               try   window attachEvent  quot onload quot   ready     catch  e         attach to onload if were in an  lt iframe gt  in IE as there s no way to tell otherwise                          return                     try               document documentElement doScroll  left        when this statement no longer throws  the DOM is accessible in old IE           catch error                setTimeout function                      document readyState      complete     ready     doIEScrollCheck                   50               return                    ready             Usage   lt script gt  DOMContentLoaded function e    console log e       lt  script gt

User · Answer

I use this   document addEventListener  DOMContentLoaded   function event           Do work       Note  This probably only works with newer browsers  especially these  http   caniuse com  feat domcontentloaded

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It is worth looking in Rock Solid addEvent   and http   www braksator com how-to-make-your-own-jquery   Here is the code in case the site goes down  function addEvent obj  type  fn        if  obj addEventListener            obj addEventListener type  fn  false           EventCache add obj  type  fn             else if  obj attachEvent            obj  e  type fn    fn          obj type fn    function     obj  e  type fn   window event              obj attachEvent   on  type  obj type fn             EventCache add obj  type  fn             else           obj  on  type    obj  e  type fn            var EventCache   function        var listEvents           return           listEvents   listEvents          add   function node  sEventName  fHandler               listEvents push arguments                      flush   function                var i  item              for i   listEvents length - 1  i  gt   0  i   i - 1                   item   listEvents i                   if item 0  removeEventListener                       item 0  removeEventListener item 1   item 2   item 3                                       if item 1  substring 0  2      on                        item 1     on    item 1                                      if item 0  detachEvent                       item 0  detachEvent item 1   item 2                                       item 0  item 1     null                                           Usage addEvent window   unload   EventCache flush   addEvent window   load   function   alert  I m ready

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Here s what I use  it s fast and covers all bases I think  works for everything except IE lt 9         gt    function fn             On document ready  commands      console log document readyState          if  document readyState     loading    fn      else  document addEventListener  DOMContentLoaded   fn           This seems to catch all cases    fires immediately if the DOM is already ready  if the DOM is not  loading      but either  interactive  or  complete   if  the DOM is still loading  it sets up an event listener for when the DOM is available  interactive     The DOMContentLoaded event is available in IE9 and everything else  so I personally think it s OK to use this  Rewrite the arrow function declaration to a regular anonymous function if you re not transpiling your code from ES2015  to ES5   If you want to wait until all assets are loaded  all images displayed etc then use window onload instead

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In short  instead of the   document  ready   used in jQuery  we can use a JavaScript method    lt script gt      document addEventListener  DOMContentLoaded   function name  false       function function name            statements         lt  script gt    Thus  when the page is ready i e  DOMContentLoaded only then the function function name   will be invoked

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It is year 2020 and  lt script gt  tag has defer attribute   for example    lt script src  demo defer js  defer gt  lt  script gt    it specifies that the script is executed when the page has finished parsing   https   www w3schools com tags att script defer asp

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It s always good to use JavaScript equivalents as compared to jQuery  One reason is one fewer library to depend on and they are much faster than the jQuery equivalents   One fantastic reference for jQuery equivalents is http   youmightnotneedjquery com    As far as your question is concerned  I took the below code from the above link    Only caveat is it only works with Internet nbsp Explorer nbsp 9 and later   function ready fn        if  document readyState     loading             fn              else           document addEventListener  DOMContentLoaded   fn

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Most minimal and 100  working  I have picked the answer from PlainJS and it s working fine for me  It extends DOMContentLoaded so that it can be accepted at all the browsers     This function is the equivalent of jQuery s   document  ready   method   document addEventListener  DOMContentLoaded   function           do something       However  in contrast to jQuery  this code will only run properly in modern browsers  IE   8  and it won t in case the document is already rendered at the time this script gets inserted  e g  via Ajax   Therefore  we need to extend this a little bit   function run            do something       in case the document is already rendered if  document readyState   loading   run       modern browsers else if  document addEventListener   document addEventListener  DOMContentLoaded   run      IE  lt   8 else document attachEvent  onreadystatechange   function        if  document readyState   complete   run          This covers basically all possibilities and is a viable replacement for the jQuery helper

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If you want to support Internet nbsp Explorer 7   no quirks  compatibility and other pain   last Chrome  last Safari  last Firefox and no iframes - this will be enough   is loaded   false callbacks       loaded   - gt    is loaded   true   for i in  0   callbacks length      callbacks i  call document   callbacks       content loaded   - gt    document removeEventListener  DOMContentLoaded   content loaded  true   loaded    state changed   - gt    if document readyState is  complete      document detachEvent  onreadystatechange   state changed     loaded    if   document addEventListener   document addEventListener  DOMContentLoaded   content loaded  true else   document attachEvent  onreadystatechange   state changed  dom ready    callback  - gt    if is loaded     callback call document   else     callbacks push callback

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Nowadays you should use modules  Put your code into the default function of a module and import the function into a script element  client js  export default function        alert   quot test quot       index html   lt  DOCTYPE html gt   lt html gt     lt head gt       lt meta charset  quot UTF-8 quot  gt       lt title gt test lt  title gt     lt  head gt     lt body gt       lt script type  quot module quot  gt        import main from    client js         main          lt  script gt     lt  body gt   lt  html gt

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Most vanilla JS Ready functions do NOT consider the scenario where the DOMContentLoaded handler is set after  the document is already loaded - Which means the function will never run  This can happen if you look for DOMContentLoaded within an async external script   lt script async src  file js  gt  lt  script gt     The code below checks for DOMContentLoaded only if the document s readyState isn t already interactive or complete   var DOMReady   function callback      document readyState      interactive     document readyState      complete    callback     document addEventListener  DOMContentLoaded   callback        DOMReady function         DOM ready        If you want to support IE aswell   var DOMReady   function callback        if  document readyState      interactive     document readyState      complete             callback          else if  document addEventListener            document addEventListener  DOMContentLoaded   callback           else if  document attachEvent            document attachEvent  onreadystatechange   function                 if  document readyState     loading                     callback                                        DOMReady function          DOM ready

[javascript] $(document).ready equivalent without jQuery

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