How does jQuery work when there are multiple elements with the same ID value

Question

I fetch data from Google s AdWords website which has multiple elements with the same id   Could you please explain why the following 3 queries doesn t result with the same answer  2    Live Demo  HTML    lt div gt       lt span id  a  gt 1 lt  span gt       lt span id  a  gt 2 lt  span gt       lt span gt 3 lt  span gt   lt  div gt    JS     function         var w      div        console log     a   length                 1 - Why      console log    body  a   length            2     console log     a   w  length              2

User · Answer

Everybody says  Each id value must be used only once within a document   but what we do to get the elements we need when we have a stupid page that has more than one element with same id  If we use JQuery   duplicatedId  selector we get the first element only  To achieve selecting the other elements you can do something like this      id duplicatedId     You will get a collection with all elements with id duplicatedId

User · Answer

If you have multiple elements with same id or same name  just assign same class to those multiple elements and access them by index  amp  perform your required operation      lt div gt           lt span id  a  class  demo  gt 1 lt  span gt           lt span id  a  class  demo  gt 2 lt  span gt           lt span gt 3 lt  span gt       lt  div gt    JQ         demo   0   val  First span          demo   1   val  Second span

User · Answer

you can simply write    span a   length to get the length   Here is the Solution for your code   console log    span a   length     try JSfiddle   https   jsfiddle net vickyfor2007 wcc0ab5g 2

User · Answer

Having 2 elements with the same ID is not valid html according to the W3C specification   When your CSS selector only has an ID selector  and is not used on a specific context   jQuery uses the native document getElementById method  which returns only the first element with that ID   However  in the other two instances  jQuery relies on the Sizzle selector engine  or querySelectorAll  if available   which apparently selects both elements  Results may vary on a per browser basis   However  you should never have two elements on the same page with the same ID  If you need it for your CSS  use a class instead     If you absolutely must select by duplicate ID  use an attribute selector       id  a        Take a look at the fiddle  http   jsfiddle net P2j3f 2   Note  if possible  you should qualify that selector with a tag selector  like this      span id  a

User · Answer

There should only be one element with a given id   If you re stuck with that situation  see the 2nd half of my answer for options    How a browser behaves when you have multiple elements with the same id  illegal HTML  is not defined by specification   You could test all the browsers and find out how they behave  but it s unwise to use this configuration or rely on any particular behavior     Use classes if you want multiple objects to have the same identifier    lt div gt       lt span class  a  gt 1 lt  span gt       lt span class  a  gt 2 lt  span gt       lt span gt 3 lt  span gt   lt  div gt     function         var w      div        console log     a   length                 2     console log    body  a   length            2     console log     a   w  length              2       If you want to reliably look at elements with IDs that are the same because you can t fix the document  then you will have to do your own iteration as you cannot rely on any of the built in DOM functions   You could do so like this   function findMultiID id        var results           var children      div   get 0  children      for  var i   0  i  lt  children length  i              if  children i  id    id                results push children i                        return results       Or  using jQuery      div     filter function    return this id     a         jQuery working example  http   jsfiddle net jfriend00 XY2tX    As to Why you get different results  that would have to do with the internal implementation of whatever piece of code was carrying out the actual selector operation   In jQuery  you could study the code to find out what any given version was doing  but since this is illegal HTML  there is no guarantee that it will stay the same over time   From what I ve seen in jQuery  it first checks to see if the selector is a simple id like  a and if so  just used document getElementById  a     If the selector is more complex than that and querySelectorAll   exists  jQuery will often pass the selector off to the built in browser function which will have an implementation specific to that browser   If querySelectorAll   does not exist  then it will use the Sizzle selector engine to manually find the selector which will have it s own implementation   So  you can have at least three different implementations all in the same browser family depending upon the exact selector and how new the browser is   Then  individual browsers will all have their own querySelectorAll   implementations   If you want to reliably deal with this situation  you will probably have to use your own iteration code as I ve illustrated above

User · Answer

jQuery s id selector only returns one result  The descendant and multiple selectors in the second and third statements are designed to select multiple elements  It s similar to   Statement 1  var length   document getElementById  a   length       Yields one result   Statement 2  var length   0  for  i 0  i lt document body childNodes length  i          if  document body childNodes item i  id     a             length                 Yields two results   Statement 3  var length   document getElementById  a   length   document getElementsByTagName  div   length       Also yields two results

User · Answer

From the id Selector jQuery page   Each id value must be used only once within a document  If more than one element has been assigned the same ID  queries that use that ID will only select the first matched element in the DOM  This behavior should not be relied on  however  a document with more than one element using the same ID is invalid   Naughty Google  But they don t even close their  lt html gt  and  lt body gt  tags I hear  The question is though  why Misha s 2nd and 3rd queries return 2 and not 1 as well

[html] How does jQuery work when there are multiple elements with the same ID value?

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