What s the best way to loop through a set of elements in JavaScript

Question

In the past and with most my current projects I tend to use a for loop like this   var elements   document getElementsByTagName  div    for  var i 0  i lt elements length  i          doSomething elements i        I ve heard that using a  reverse while  loop is quicker but I have no real way to confirm this   var elements   document getElementsByTagName  div         length   elements length   while length--        doSomething elements length        What is considered as best practice when it comes to looping though elements in JavaScript  or any array for that matter

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I too advise to use the simple way  KISS  -   -- but some optimization could be found  namely not to test the length of an array more than once   var elements   document getElementsByTagName  div    for  var i 0  im elements length  im gt i  i          doSomething elements i

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I know this question is old -- but here s another  extremely simple solution        var elements   Array from document querySelectorAll  div       Then it can be used like any  standard array

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I think you have two alternatives   For dom elements such as  jQuery and like frameworks give you a good method of iteration   The second approach is the for loop

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Here s a nice form of a loop I often use  You create the iterated variable from the for statement and you don t need to check the length property  which can be expensive specially when iterating through a NodeList  However  you must be careful  you can t use it if any of the values in array could be  falsy   In practice  I only use it when iterating over an array of objects that does not contain nulls  like a NodeList   But I love its syntactic sugar   var list     a 1 b 2    a 3 b 5    a 8 b 2    a 4 b 1    a 0 b 8     for  var i 0  item  item   list i   i           Look no need to do list i  in the body of the loop   console log  Looping  index    i   item    item       Note that this can also be used to loop backwards  as long as your list doesn t have a   -1   property   var list     a 1 b 2    a 3 b 5    a 8 b 2    a 4 b 1    a 0 b 8     for  var i   list length - 1  item  item   list i   i--      console log  Looping  index    i   item   item       ES6 Update  for   of gives you the name but not the index  available since ES6   for  let item of list        console log  Looping  index     Sorry       item    item

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At the risk of getting yelled at  i would get a javascript helper library like jquery or prototype they encapsulate the logic in nice methods - both have an  each method iterator to do it - and they both strive to make it cross-browser compatible  EDIT  This answer was posted in 2008  Today much better constructs exist  This particular case could be solved with a  forEach

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I know that you don t want to hear that  but  I consider the best practice is the most readable in this case  As long as the loop is not counting from here to the moon  the performance-gain will not be uhge enough

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Note that in some cases  you need to loop in reverse order  but then you can use i-- too    For example somebody wanted to use the new getElementsByClassName function to loop on elements of a given class and change this class  He found that only one out of two elements was changed  in FF3   That s because the function returns a live NodeList  which thus reflects the changes in the Dom tree  Walking the list in reverse order avoided this issue   var menus   document getElementsByClassName  style2    for  var i   menus length - 1  i  gt   0  i--      menus i  className    style1       In increasing index progression  when we ask the index 1  FF inspects the Dom and skips the first item with style2  which is the 2nd of the original Dom  thus it returns the 3rd initial item

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I prefer the for loop as it s more readable  Looping from length to 0 would be more efficient than looping from 0 to length  And using a reversed while loop is more efficient than a foor loop as you said  I don t have the link to the page with comparison results anymore but I remember that the difference varied on different browsers  For some browser the reversed while loop was twice as fast  However it makes no difference if you re looping  small  arrays  In your example case the length of elements will be  small

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I think using the first form is probably the way to go  since it s probably by far the most common loop structure in the known universe  and since I don t believe the reverse loop saves you any time in reality  still doing an increment decrement and a comparison on each iteration      Code that is recognizable and readable to others is definitely a good thing

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I had a very similar problem earlier with document getElementsByClassName     I didn t know what a nodelist was at the time   var elements   document getElementsByTagName  div    for  var i 0  i lt elements length  i          doSomething elements i        My issue was that I expected that elements would be an array  but it isn t   The nodelist Document getElementsByTagName   returns is iterable  but you can t call array prototype methods on it   You can however populate an array with nodelist elements like this   var myElements       for  var i 0  i lt myNodeList length  i                                         var element   myNodeList i       myElements push element        After that you can feel free to call  innerHTML or  style or something on the elements of your array

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I like to use a TreeWalker if the set of elements are children of a root node

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I like doing   var menu   document getElementsByTagName  div    for  var i   0  menu i   i                   There is no call to the length of the array on every iteration

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Also see my comment on Andrew Hedges  test      I just tried to run a test to compare a simple iteration  the optimization I introduced and the reverse do while  where the elements in an array was tested in every loop   And alas  no surprise  the three browsers I tested had very different results  though the optimized simple iteration was fastest in all  -   Test   An array with 500 000 elements build outside the real test  for every iteration the value of the specific array-element is revealed   Test run 10 times   IE6   Results   Simple  984 922 937 984 891 907 906 891 906 906  Average  923 40 ms   Optimized  766 766 844 797 750 750 765 765 766 766  Average  773 50 ms   Reverse do while  3375 1328 1516 1344 1375 1406 1688 1344 1297 1265  Average  1593 80 ms   Note one especially awkward result   Opera 9 52   Results    Simple  344 343 344 359 343 359 344 359 359 359  Average  351 30 ms   Optimized  281 297 297 297 297 281 281 297 281 281  Average     289 00 ms  Reverse do while  391 407 391 391 500 407 407 406 406 406  Average  411 20 ms   FireFox 3 0 1   Results   Simple  278 251 259 245 243 242 259 246 247 256  Average  252 60 ms   Optimized  267 222 223 226 223 230 221 231 224 230  Average  229 70 ms   Reverse do while  414 381 389 383 388 389 381 387 400 379  Average  389 10 ms

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Form of loop provided by Juan Mendez is very useful and practical  I changed it a little bit  so that now it works with - false  null  zero and empty strings too   var items         true      false      null      0             for var i   0  item   item   items i       undefined  i          console log  Index      i      Value      item

[javascript] What's the best way to loop through a set of elements in JavaScript?

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