add class with JavaScript

Question

I am writing some vanilla JavaScript to create a nice navigation menu   I am stuck on adding an active class   I am getting elements by class name NOT by id  The below code works if substituted with id  however  I need it to apply to more than one element     HTML   lt img class  navButton  id  topArrow  src  images arrows top png    gt   lt img class  navButton  id  rightArrow  src  images arrows right png    gt    JS  var button   document getElementsByClassName  navButton     button onmouseover   function         button setAttribute  class    active        button setAttribute  src    images arrows top o png        No answers containing jQuery please

User · Answer

Here is a method adapted from Jquery 2 1 1 that take a dom element instead of a jquery object  so jquery is not needed   Includes type checks and regex expressions   function addClass element  value           Regex terms     var rclass      t r n f  g          rnotwhite      S  g        var classes          cur          curClass          finalValue          proceed   typeof value      string   amp  amp  value       if   proceed  return element       classes    value        match rnotwhite              cur   element nodeType     1          amp  amp   element className                          element className        replace rclass                                             if   cur  return element       var j   0       while   curClass   classes j                 if  cur indexOf       curClass         lt  0                 cur    curClass                                  only assign if different to avoid unneeded rendering      finalValue   cur trim         if  element className     finalValue             element className   finalValue              return element

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I like to use a custom  foreach  function of sorts for these kinds of things   function Each  objs  func         if   objs length   for   var i   0  ol   objs length  v   objs  0    i  lt  ol  amp  amp  func  v  i       false  v   objs    i          else for   var p in objs   if   func  objs  p    p       false   break       Can t remember where I found the above function  but it has been quite useful    Then after fetching your objects  to elements in this example  just do  Each  elements  function  element         element addEventListener   mouseover   function                 element classList add   active               element setAttribute   class    active             element setAttribute   src    newsource                     Remove class and new src after  mouseover  ends  if you wish      element addEventListener   mouseout   function                 element classList remove   active             element setAttribute   src    originalsource                   classList is a simple way for handling elements  classes  Just needs a shim for a few browsers  If you must use setAttribute you must remember that whatever is set with it will overwrite the previous values   EDIT  Forgot to mention that you need to use attachEvent instead of addEventListener on some IE versions  Test with if   document addEventListener

User · Answer

getElementsByClassName   returns HTMLCollection so you could try this  var button   document getElementsByClassName  navButton   0     Edit  var buttons   document getElementsByClassName  navButton    for i 0 buttons length i        buttons i  onmouseover   function         this className      active    add class      this setAttribute  src    images arrows top o png

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There is build in forEach loop for array in ECMAScript 5th Edition   var buttons   document getElementsByClassName  navButton     Array prototype forEach call buttons function button         button setAttribute  class    active        button setAttribute  src    images arrows top o png

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Simply add a class name to the beginning of the funciton and the 2nd and 3rd arguments are optional and the magic is done for you   function getElementsByClass searchClass  node  tag       var classElements   new Array       if  node    null       node   document     if  tag    null       tag           var els   node getElementsByTagName tag      var elsLen   els length     var pattern   new RegExp         s     searchClass         s          for  i   0  j   0  i  lt  elsLen  i           if  pattern test els i  className            classElements j    els i          j                   return classElements

User · Answer

document getElementsByClassName returns a node list  So you ll have to iterate over the list and bind the event to individual elements  Like this     var buttons   document getElementsByClassName  navButton     for var i   0  i  lt  buttons length    i       buttons i  onmouseover   function             this setAttribute  class    active            this setAttribute  src    images arrows top o png

User · Answer

In your snippet  button is an instance of NodeList  to which you can t attach an event listener directly  nor can you change the elements  className properties directly  Your best bet is to delegate the event  document body addEventListener  mouseover  function e        e   e    window event      var target   e target    e srcElement      if  target tagName toLowerCase        img   amp  amp  target className match   bnavButton b                  target className      active    set class         false    Of course  my guess is that the active class needs to be removed once the mouseout event fires  you might consider using a second delegator for that  but you could just aswell attach an event handler to the one element that has the active class  document body addEventListener  mouseover  function e        e   e    window event      var oldSrc  target   e target    e srcElement      if  target tagName toLowerCase        img   amp  amp  target className match   bnavButton b                  target className      active    set class         oldSrc   target getAttribute  src            target setAttribute  src    images arrows top o png            target onmouseout   function                         target onmouseout   null   remove this event handler  we don t need it anymore             target className   target className replace   bactive b      trim                target setAttribute  src   oldSrc                      false    There is some room for improvements  with this code  but I m not going to have all the fun here  -   Check the fiddle here

[javascript] add class with JavaScript

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