For loop for HTMLCollection elements

Question

I m trying to set get id of all elements in an HTMLCollectionOf  I wrote the following code   var list   document getElementsByClassName  events    console log list 0  id   for  key in list        console log key id       But I got the following output in console   event1 undefined   which is not what I expected  Why is the second console output undefined but the first console output is event1

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You can also do like this   let elements   document getElementsByClassName  quot classname quot     for let index in  elements       if index  lt   elements length            console log elements index            x000D   x000D  let elements   document getElementsByClassName  classname    for  let index in elements      if  index  lt   elements length        console log elements index          x000D   lt div class  classname  gt  element 1  lt  div gt   lt div class  classname  gt  element 2  lt  div gt   lt div class  classname  gt  element 3  lt  div gt  x000D   x000D   x000D   or  let elements   document getElementsByClassName  quot classname quot     for let ele of  elements            console log ele       x000D   x000D  let elements   document getElementsByClassName  classname    for  let ele of elements      console log ele     x000D   lt div class  classname  gt  element 1  lt  div gt   lt div class  classname  gt  element 2  lt  div gt   lt div class  classname  gt  element 3  lt  div gt   lt div class  classname  gt  element 4  lt  div gt  x000D   x000D   x000D

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I had a problem using forEach in IE 11 and also Firefox 49  I have found a workaround like this  Array prototype slice call document getElementsByClassName  events    forEach function  key            console log key id

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In response to the original question  you are using for in incorrectly   In your code  key is the index   So  to get the value from the pseudo-array  you d have to do list key  and to get the id  you d do list key  id   But  you should not be doing this with for in in the first place   Summary  added in Dec 2018   Do not ever use for in to iterate a nodeList or an HTMLCollection   The reasons to avoid it are described below   All recent versions of modern browsers  Safari  Firefox  Chrome  Edge  all support for of iteration on DOM lists such nodeList or HTMLCollection   Here s an example   var list   document getElementsByClassName  events    for  let item of list        console log item id       To include older browsers  including things like IE   this will work everywhere   var list  document getElementsByClassName  events    for  var i   0  i  lt  list length  i          console log list i  id     second console output       Explanation For Why You Should Not Use for in  for in is meant for iterating the properties of an object   That means it will return all iterable properties of an object   While it may appear to work for an array  returning array elements or pseudo-array elements   it can also return other properties of the object that are not what you are expecting from the array-like elements   And  guess what  an HTMLCollection or nodeList object can both have other properties that will be returned with a for in iteration   I just tried this in Chrome and iterating it the way you were iterating it will retrieve the items in the list  indexes 0  1  2  etc      but also will retrieve the length and item properties   The for in iteration simply won t work for an HTMLCollection     See http   jsfiddle net jfriend00 FzZ2H  for why you can t iterate an HTMLCollection with for in   In Firefox  your for in iteration would return these items  all the iterable properties of the object    0 1 2 item namedItem   iterator length   Hopefully  now you can see why you want to use for  var i   0  i  lt  list length  i    instead so you just get 0  1 and 2 in your iteration     Following below is an evolution of how browsers have evolved through the time period 2015-2018 giving you additional ways to iterate   None of these are now needed in modern browsers since you can use the options described above   Update for ES6 in 2015  Added to ES6 is Array from   that will convert an array-like structure to an actual array   That allows one to enumerate a list directly like this    use strict    Array from document getElementsByClassName  events    forEach function item       console log item id         Working demo  in Firefox  Chrome  and Edge as of April 2016   https   jsfiddle net jfriend00 8ar4xn2s     Update for ES6 in 2016  You can now use the ES6 for of construct with a NodeList and an HTMLCollection by just adding this to your code   NodeList prototype Symbol iterator    Array prototype Symbol iterator   HTMLCollection prototype Symbol iterator    Array prototype Symbol iterator     Then  you can do   var list   document getElementsByClassName  events    for  var item of list        console log item id       This works in the current version of Chrome  Firefox  and Edge   This works because it attaches the Array iterator to both the NodeList and HTMLCollection prototypes so that when for of iterates them  it uses the Array iterator to iterate them   Working demo  http   jsfiddle net jfriend00 joy06u4e      Second Update for ES6 in Dec 2016  As of Dec 2016  Symbol iterator support has been built-in to Chrome v54 and Firefox v50 so the code below works by itself   It is not yet built-in for Edge   var list   document getElementsByClassName  events    for  let item of list        console log item id       Working demo  in Chrome and Firefox   http   jsfiddle net jfriend00 3ddpz8sp   Third Update for ES6 in Dec 2017  As of Dec  2017  this capability works in Edge 41 16299 15 0 for a nodeList as in document querySelectorAll    but not an HTMLCollection as in document getElementsByClassName   so you have to manually assign the iterator to use it in Edge for an HTMLCollection   It is a total mystery why they d fix one collection type  but not the other   But  you can at least use the result of document querySelectorAll   with ES6 for of syntax in current versions of Edge now   I ve also updated the above jsFiddle so it tests both HTMLCollection and nodeList separately and captures the output in the jsFiddle itself   Fourth Update for ES6 in Mar 2018  Per mesqueeeb  Symbol iterator support has been built-in to Safari too  so you can use for  let item of list  for either document getElementsByClassName   or document querySelectorAll     Fifth Update for ES6 in Apr 2018  Apparently  support for iterating an HTMLCollection with for of will be coming to Edge 18 in Fall 2018     Sixth Update for ES6 in Nov 2018  I can confirm that with Microsoft Edge v18  that is included in the Fall 2018 Windows Update   you can now iterate both an HTMLCollection and a NodeList with for of in Edge   So  now all modern browsers contain native support for for of iteration of both the HTMLCollection and NodeList objects

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On Edge  if  NodeList prototype forEach      NodeList prototype forEach   function fn  scope        for var i   0  len   this length  i  lt  len    i          fn call scope  this i   i  this

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Alternative to Array from is to use Array prototype forEach call  forEach  Array prototype forEach call htmlCollection  i   gt    console log i       map  Array prototype map call htmlCollection  i   gt    console log i       ect

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Easy workaround that I always use  let list   document getElementsByClassName  events    let listArr   Array from list    After this you can run any desired Array methods on the selection  listArr map item   gt  console log item id   listArr forEach item   gt  console log item id   listArr reverse

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You can t use for in on NodeLists or HTMLCollections  However  you can use some Array prototype methods  as long as you  call   them and pass in the NodeList or HTMLCollection as this   So consider the following as an alternative to jfriend00 s for loop   var list  document getElementsByClassName  events       forEach call list  function el        console log el id         There s a good article on MDN that covers this technique  Note their warning about browser compatibility though            passing a host object  like a NodeList  as   this to a native method  such as forEach  is not guaranteed to work in   all browsers and is known to fail in some    So while this approach is convenient  a for loop may be the most browser-compatible solution   Update  Aug 30  2014   Eventually you ll be able to use ES6 for of   var list   document getElementsByClassName  events    for  const el of list    console log el id     It s already supported in recent versions of Chrome and Firefox

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As of March 2016  in Chrome 49 0  for   of works for HTMLCollection   this headers   this getElementsByTagName  header     for  var header of this headers        console log header        See here the documentation   But it only works if you apply the following workaround before using the for   of   HTMLCollection prototype Symbol iterator    Array prototype Symbol iterator     The same is necessary for using for   of with NodeList   NamedNodeMap prototype Symbol iterator    Array prototype Symbol iterator     I believe hope for   of will soon work without the above workaround  The open issue is here      https   bugs chromium org p chromium issues detail id 401699   Update  See Expenzor s comment below  This has been fixed as of April 2016  You don t need to add HTMLCollection prototype Symbol iterator    Array prototype Symbol iterator   to iterate over an HTMLCollection with for   of

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if you use oldder varsions of ES   ES5 for example   you can use as any   for  let element of elementsToIterate as any          console log element

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In ES6  you could do something like     collection   or Array from collection    let someCollection   document querySelectorAll someSelector      someCollection  forEach someFn     or Array from collection  forEach someFn    Eg -      navDoms   document getElementsByClassName  nav-container        Array from navDoms  forEach function navDom          implement function operations

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you can add this two lines   HTMLCollection prototype forEach   Array prototype forEach  NodeList prototype forEach   Array prototype forEach    HTMLCollection is return by getElementsByClassName and getElementsByTagName   NodeList is return by querySelectorAll  Like this you can do a forEach   var selections   document getElementsByClassName  myClass        alternative   var selections   document querySelectorAll   myClass        selections forEach function element  i     do your stuffs

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You want to change it to   var list  document getElementsByClassName  events    console log list 0  id     first console output for  key in list       console log list key  id     second console output

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There s no reason to use es6 features to escape for looping if you re on IE9 or above   In ES5  there are two good options  First  you can  borrow  Array s forEach as evan mentions   But even better     Use Object keys    which does have forEach and filters to  own properties  automatically   That is  Object keys is essentially equivalent to doing a for    in with a HasOwnProperty  but is much smoother   var eventNodes   document getElementsByClassName  events    Object keys eventNodes  forEach function  key        console log eventNodes key  id

[javascript] For loop for HTMLCollection elements

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