Pure JavaScript equivalent of jQuery s ready - how to call a function when the page DOM is ready for it

Question

With jQuery  we all know the wonderful  ready   function      document   ready function         However  let s say I want to run a function that is written in standard JavaScript with no library backing it  and that I want to launch a function as soon as the page is ready to handle it  What s the proper way to approach this   I know I can do   window onload  myFunction       Or I can use the body tag    lt body onload  myFunction    gt    Or I can even try at the bottom of the page after everything  but the end body or html tag like    lt script type  text javascript  gt      myFunction     lt  script gt    What is a cross-browser old new -compliant method of issuing one or more functions in a manner like jQuery s   ready

User · Answer

I m not quite sure what you re asking  but maybe this can help   window onload   function           Code           Or   window onload   main   function main           Code

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The good folks at HubSpot have a resource where you can find pure Javascript methodologies for achieving a lot of jQuery goodness - including ready  http   youmightnotneedjquery com  ready  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                    example inline usage   ready function     alert  hello

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I would like to mention some of the possible ways here together with a pure javascript trick which works across all browsers      with jQuery    document  ready function                      shorter jQuery version    function                      without jQuery  doesn t work in older IEs  document addEventListener  DOMContentLoaded   function            your code goes here    false       and here s the trick  works everywhere  function r f   in  test document readyState  setTimeout  r   f     9  f       use like r function        alert  DOM Ready           The trick here  as explained by the original author  is that we are checking the document readyState property  If it contains the string in  as in uninitialized and loading  the first two DOM ready states out of 5  we set a timeout and check again  Otherwise  we execute the passed function   And here s the jsFiddle for the trick which works across all browsers    Thanks to Tutorialzine for including this in their book

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Tested in IE9  and latest Firefox and Chrome and also supported in IE8   document onreadystatechange   function        var state   document readyState    if  state     interactive           init        else if  state     complete           initOnCompleteLoad              Example  http   jsfiddle net electricvisions Jacck   UPDATE - reusable version  I have just developed the following  It s a rather simplistic equivalent to jQuery or Dom ready without backwards compatibility  It probably needs further refinement  Tested in latest versions of Chrome  Firefox and IE  10 11  and should work in older browsers as commented on  I ll update if I find any issues   window readyHandlers       window ready   function ready handler      window readyHandlers push handler     handleState        window handleState   function handleState        if    interactive    complete   indexOf document readyState   gt  -1        while window readyHandlers length  gt  0           window readyHandlers shift                     document onreadystatechange   window handleState    Usage   ready function           your code here       It s written to handle async loading of JS but you might want to sync load this script first unless you re minifying  I ve found it useful in development   Modern browsers also support async loading of scripts which further enhances the experience  Support for async means multiple scripts can be downloaded simultaneously all while still rendering the page  Just watch out when depending on other scripts loaded asynchronously or use a minifier or something like browserify to handle dependencies

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The simplest thing to do in the absence of a framework that does all the cross-browser compatibility for you is to just put a call to your code at the end of the body   This is faster to execute than an onload handler because this waits only for the DOM to be ready  not for all images to load   And  this works in every browser    lt  doctype html gt   lt html gt   lt head gt   lt  head gt   lt body gt  Your HTML here   lt script gt     self executing function here  function           your page initialization code here       the DOM will be available here         lt  script gt   lt  body gt   lt  html gt      For modern browsers  anything from IE9 and newer and any version of Chrome  Firefox or Safari   if you want to be able to implement a jQuery like   document  ready   method that you can call from anywhere  without worrying about where the calling script is positioned   you can just use something like this   function docReady fn           see if DOM is already available     if  document readyState      complete     document readyState      interactive                call on next available tick         setTimeout fn  1         else           document addEventListener  DOMContentLoaded   fn                 Usage   docReady function            DOM is loaded and ready for manipulation here         If you need full cross browser compatibility  including old versions of IE  and you don t want to wait for window onload  then you probably should go look at how a framework like jQuery implements its   document  ready   method   It s fairly involved depending upon the capabilities of the browser   To give you a little idea what jQuery does  which will work wherever the script tag is placed    If supported  it tries the standard   document addEventListener  DOMContentLoaded   fn  false     with a fallback to   window addEventListener  load   fn  false     or for older versions of IE  it uses   document attachEvent  onreadystatechange   fn     with a fallback to   window attachEvent  onload   fn     And  there are some work-arounds in the IE code path that I don t quite follow  but it looks like it has something to do with frames     Here is a full substitute for jQuery s  ready   written in plain javascript    function funcName  baseObj           The public function name defaults to window docReady        but you can pass in your own object and own function name and those will be used        if you want to put them in a different namespace     funcName   funcName     docReady       baseObj   baseObj    window      var readyList           var readyFired   false      var readyEventHandlersInstalled   false          call this when the document is ready        this function protects itself against being called more than once     function ready             if   readyFired                   this must be set to true before we start calling callbacks             readyFired   true              for  var i   0  i  lt  readyList length  i                         if a callback here happens to add new ready handlers                     the docReady   function will see that it already fired                    and will schedule the callback to run right after                    this event loop finishes so all handlers will still execute                    in order and no new ones will be added to the readyList                    while we are processing the list                 readyList i  fn call window  readyList i  ctx                                allow any closures held by these functions to free             readyList                            function readyStateChange             if   document readyState      complete                  ready                            This is the one public interface        docReady fn  context          the context argument is optional - if present  it will be passed        as an argument to the callback     baseObj funcName    function callback  context            if  typeof callback      function                 throw new TypeError  callback for docReady fn  must be a function                         if ready has already fired  then just schedule the callback            to fire asynchronously  but right away         if  readyFired                setTimeout function    callback context     1               return            else                  add the function and context to the list             readyList push  fn  callback  ctx  context                         if document already ready to go  schedule the ready function to run         if  document readyState      complete                 setTimeout ready  1             else if   readyEventHandlersInstalled                   otherwise if we don t have event handlers installed  install them             if  document addEventListener                       first choice is DOMContentLoaded event                 document addEventListener  DOMContentLoaded   ready  false                      backup is window load event                 window addEventListener  load   ready  false                 else                      must be IE                 document attachEvent  onreadystatechange   readyStateChange                   window attachEvent  onload   ready                             readyEventHandlersInstalled   true                      docReady   window     The latest version of the code is shared publicly on GitHub at https   github com jfriend00 docReady  Usage      pass a function reference docReady fn       use an anonymous function docReady function            code here         pass a function reference and a context    the context will be passed to the function as the first argument docReady fn  context       use an anonymous function with a context docReady function context           code here that can use the context argument that was passed to docReady    ctx       This has been tested in   IE6 and up Firefox 3 6 and up Chrome 14 and up Safari 5 1 and up Opera 11 6 and up Multiple iOS devices Multiple Android devices   Working implementation and test bed  http   jsfiddle net jfriend00 YfD3C     Here s a summary of how it works    Create an IIFE  immediately invoked function expression  so we can have non-public state variables  Declare a public function docReady fn  context  When docReady fn  context  is called  check if the ready handler has already fired   If so  just schedule the newly added callback to fire right after this thread of JS finishes with setTimeout fn  1   If the ready handler has not already fired  then add this new callback to the list of callbacks to be called later  Check if the document is already ready   If so  execute all ready handlers  If we haven t installed event listeners yet to know when the document becomes ready  then install them now  If document addEventListener exists  then install event handlers using  addEventListener   for both  DOMContentLoaded  and  load  events   The  load  is a backup event for safety and should not be needed  If document addEventListener doesn t exist  then install event handlers using  attachEvent   for  onreadystatechange  and  onload  events  In the onreadystatechange event  check to see if the document readyState      complete  and if so  call a function to fire all the ready handlers  In all the other event handlers  call a function to fire all the ready handlers  In the function to call all the ready handlers  check a state variable to see if we ve already fired   If we have  do nothing   If we haven t yet been called  then loop through the array of ready functions and call each one in the order they were added   Set a flag to indicate these have all been called so they are never executed more than once  Clear the function array so any closures they might be using can be freed    Handlers registered with docReady   are guaranteed to be fired in the order they were registered   If you call docReady fn  after the document is already ready  the callback will be scheduled to execute as soon as the current thread of execution completes using setTimeout fn  1    This allows the calling code to always assume they are async callbacks that will be called later  even if later is as soon as the current thread of JS finishes and it preserves calling order

User · Answer

Ready  function ready fn  var d document  d readyState   loading   d addEventListener  DOMContentLoaded  fn  fn       Use like  ready function          some code         For self invoking code   function fn  var d document  d readyState   loading   d addEventListener  DOMContentLoaded  fn  fn      function           Some Code here       DOM is avaliable       var h1s   document querySelector  h1             Support  IE9

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Your method  placing script before the closing body tag    lt script gt     myFunction    lt  script gt   lt  body gt   lt  html gt    is a reliable way to support old and new browsers

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If you are doing VANILLA plain JavaScript without jQuery  then you must use  Internet nbsp Explorer nbsp 9 or later    document addEventListener  DOMContentLoaded   function event           Your code to run since DOM is loaded and ready       Above is the equivalent of jQuery  ready     document  ready function         console log  Ready           Which ALSO could be written SHORTHAND like this  which jQuery will run after the ready even occurs     function         console log  ready           NOT TO BE CONFUSED with BELOW  which is not meant to be DOM ready    DO NOT use an IIFE like this that is self executing    Example    function           Your page initialization code here  - WRONG       The DOM will be available here   - WRONG         This IIFE will NOT wait for your DOM to load   I m even talking about latest version of Chrome browser

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Here s a cleaned-up  non-eval-using version of Ram-swaroop s  works in all browsers  variety--works in all browsers   function onReady yourMethod      var readyStateCheckInterval   setInterval function         if  document  amp  amp  document readyState      complete        Or  interactive        clearInterval readyStateCheckInterval         yourMethod               10        use like onReady function     alert  hello           It does wait an extra 10 ms to run  however  so here s a more complicated way that shouldn t   function onReady yourMethod      if  document readyState      complete        Or also compare to  interactive      setTimeout yourMethod  1      Schedule to run immediately       else       readyStateCheckInterval   setInterval function           if  document readyState      complete        Or also compare to  interactive          clearInterval readyStateCheckInterval           yourMethod                   10             Use like onReady function     alert  hello             Or onReady functionName     See also How to check if DOM is ready without a framework

User · Answer

document ondomcontentready function     should do the trick  but it doesn t have full browser compatibility   Seems like you should just use jQuery min

[javascript] Pure JavaScript equivalent of jQuery's $.ready() - how to call a function when the page/DOM is ready for it

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