Why doesn t document addEventListener load function work in a greasemonkey script

Question

It doesn t give an error  and I put a console log  loaded userscript wifi-autologin    the console log works  but the intended effect of the document addEventListener doesn t happen  After doing a bit more debugging  making it print that the addEventListener was called  I discovered that it wasn t being called   Source of script        UserScript       name        wifi-autologin     namespace   lf-ns     description Hopefully autologins to a captive portal     include         1 1 1 1       version     1     run-at document-end       UserScript    document addEventListener  load   submitAction

User · Answer

To get the value of my drop down box on page load  I use   document addEventListener  DOMContentLoaded  fnName     Hope this helps some one

User · Answer

The problem is WHEN the event is added and EXECUTED via triggering  the document onload property modification can be verified by examining the properties list    When does this execute and modify onload relative to the onload event trigger   document addEventListener  load            before  during or after the load and or render of the page s HTML  This simple scURIple  cut  amp  paste to URL   works  w o alerting as naively expected   data text html charset utf-8       lt html content editable gt  lt head gt             lt script gt                  document addEventListener  load   function    alert 42                  lt  script gt             lt  head gt  lt body gt goodbye universe - hello muiltiverse lt  body gt       lt  html gt    Does loading imply script contents have been executed     A little out of this world expansion     Consider a slight modification   data text html charset utf-8       lt html content editable gt  lt head gt             lt script gt                if confirm  expand mind    document addEventListener  load   function    alert 42                  lt  script gt           lt  head gt  lt body gt goodbye universe - hello muiltiverse lt  body gt       lt  html gt    and whether the HTML has been loaded or not     Rendering is certainly pending since goodbye universe - hello muiltiverse is not seen on screen but  does not the confirm        have to be already loaded to be executed      and so document addEventListener  load                In other words  can you execute code to check for self-loading when the code itself is not yet loaded   Or  another way of looking at the situation  if the code is executable and executed then it has ALREADY been loaded as a done deal and to retroactively check when the transition occurred between not yet loaded and loaded is a priori fait accompli    So which comes first  loading and executing the code or using the code s functionality though not loaded   onload as a window property works because it is subordinate to the object and not self-referential as in the document case  ie  it s the window s contents  via document  that determine the loaded question err situation   PS   When do the following fail to alert        personally experienced gotcha s    caveat  unless loading to the same window is really fast     clobbering is the order of the day so what is really needed below when using the same named window    window open URIstr1  w        addEventListener  load          function    alert 42             window open URIstr2  w                 addEventListener  load                   function    alert 43                      window open URIstr3  w                          addEventListener  load                            function    alert 44                                                                                             alternatively  proceed each successive window open with  alert  press Ok either after   alert shows pending load is done or inspired via divine intervention      data text html charset utf-8       lt html content editable gt  lt head gt  lt  -- tagging fluff -- gt  lt script gt           window open               data text plain  has no DOM or    Window               addEventListener  load   function    alert 42               window open               data text plain  has no DOM but    Window               addEventListener  load   function    alert 4               window open               data text html  lt html gt  lt body gt has DOM and    Window               addEventListener  load   function    alert 2               window open               data text html  lt html gt  lt body gt has DOM and    noWindow               addEventListener  load   function    alert 1                  etc  including where body has onload     in each appropriate open          lt  script gt  lt  -- terminating fluff -- gt  lt  head gt  lt  html gt    which emphasize onload differences as a document or window property   Another caveat concerns preserving XSS  Cross Site Scripting  and SOP  Same Origin Policy rules which may allow loading an HTML URI but not modifying it s content to check for same  If a scURIple is run as a bookmarklet scriplet from the same origin site then there maybe success   ie  From an arbitrary page  this link will do the load but not likely do alert  done            lt a href  javascript window open  view-source http   google ca                         addEventListener   load    function    alert  done         gt  src  vu  lt  a gt    but if the link is bookmarked and then clicked when viewing a google ca page  it does both    test environment      window navigator userAgent       Mozilla 5 0  X11  U  Linux i686  en-US  rv 1 9 0 4  Gecko 2008102920 Firefox 3 0 4  Splashtop-v1 2 17 0

User · Answer

this happened  again around last quarter of 2017   greasemonkey firing too late   after domcontentloaded event already been fired  what to do   i used  run-at document-start   instead of document-end updated firefox to 57   from   https   github com greasemonkey greasemonkey issues 2769  Even as a  private  script writer I m confused why my script isn t working  The most likely problem is that the  DOMContentLoaded  event is fired before the script is run  Now before you come back and say  run-at document-start is set  that directive isn t fully supported at the moment  Due to the very asynchronous nature of WebExtensions there s little guarantee on when something will be executed  When FF59 rolls around we ll have  2663 which will help  It ll actually help a lot of things  debugging too

User · Answer

According to HTML living standard specification  the load event is  Fired at the Window when the document has finished loading  fired at an element containing a resource  e g  img  embed  when its resource has finished loading  I e  load event is not fired on document object  Credit  Why does document addEventListener    load     handler  not work

User · Answer

Apparently  document addEventListener   is unreliable  and hence  my error  Use window addEventListener   with the same parameters  instead

[javascript] Why doesn't document.addEventListener('load', function) work in a greasemonkey script?

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