querySelector and querySelectorAll vs getElementsByClassName and getElementById in JavaScript

Question

I would like to know what exactly is the difference between querySelector and querySelectorAll against  getElementsByClassName and getElementById   From this link I could gather that with querySelector I can write document querySelector   myclass   to get elements with class myclass and document querySelector   myid   to get element with ID myid  But I can already do that getElementsByClassName and getElementById  Which one should be preferred   Also I work in XPages where the ID is dynamically generated with colon and looks like this view  id1 inputText1  So when I write document querySelector   view  id1 inputText1   it doesn t work  But writing document getElementById  view  id1 inputText1   works  Any ideas why

User · Answer

collecting from Mozilla Documentation   The NodeSelector interface This specification adds two new methods to any objects implementing the Document  DocumentFragment  or Element interfaces   querySelector     Returns the first matching Element node within the node s subtree  If   no matching node is found  null is returned    querySelectorAll     Returns a NodeList containing all matching Element nodes within the   node s subtree  or an empty NodeList if no matches are found    and      Note  The NodeList returned by querySelectorAll   is not live  which   means that changes in the DOM are not reflected in the collection    This is different from other DOM querying methods that return live   node lists

User · Answer

querySelector is of w3c Selector API  getElementBy is of w3c DOM API  IMO the most notable difference is that the return type of querySelectorAll is a static node list and for getElementsBy it s a live node list  Therefore the looping in demo 2 never ends because lis is live and updates itself during each iteration      Demo 1 correct var ul   document querySelectorAll  ul   0       lis   ul querySelectorAll  li    for var i   0  i  lt  lis length   i         ul appendChild document createElement  li           Demo 2 wrong var ul   document getElementsByTagName  ul   0        lis   ul getElementsByTagName  li     for var i   0  i  lt  lis length   i         ul appendChild document createElement  li

User · Answer

querySelector and querySelectorAll are a relatively new APIs  whereas getElementById and  getElementsByClassName have been with us for a lot longer  That means that what you use will mostly depend on which browsers you need to support   As for the    it has a special meaning so you have to escape it if you have to use it as a part of a ID class name

User · Answer

I came to this page purely to find out the better method to use in terms of performance - i e  which is faster   querySelector   querySelectorAll or getElementsByClassName   and I found this  https   jsperf com getelementsbyclassname-vs-queryselectorall 18  It runs a test on the 2 x examples above  plus it chucks in a test for jQuery s equivalent selector as well  my test results were as follows   getElementsByClassName   1 138 018 operations   sec -  lt  lt  lt  clear winner querySelectorAll   39 033 operations   sec jquery select   381 648 operations   sec

User · Answer

querySelector can be a complete CSS 3 -Selector with IDs and Classes and Pseudo-Classes together like this    id class pseudo      or   tag  id  class  class class   with getElementsByClassName you can just define a class  class   with getElementById you can just define an id  id

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look at this  https   codepen io bagdaulet pen bzdKjL   getElementById fastest than querySelector on 25    jquery is slowest  var q   time my script function          for  i   0  i  lt  1000000  i               var w   document querySelector   ll                console log  querySelector    q  ms

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About the differences  there is an important one in the results between querySelectorAll and getElementsByClassName  the return value is different  querySelectorAll will return a static collection  while getElementsByClassName returns a live collection  This could lead to confusion if you store the results in a variable for later use    A variable generated with querySelectorAll will contain the elements that fulfilled the selector at the moment the method was called  A variable generated with getElementsByClassName will contain the elements that fulfilled the selector when it is used  that may be different from the moment the method was called     For example  notice how even if you haven t reassigned the variables aux1 and aux2  they contain different values after updating the classes    x000D   x000D     storing all the elements with class  blue  using the two methods x000D  var aux1   document querySelectorAll   blue    x000D  var aux2   document getElementsByClassName  blue    x000D   x000D     write the number of elements in each array  values match  x000D  console log  Number of elements with querySelectorAll       aux1 length   x000D  console log  Number of elements with getElementsByClassName       aux2 length   x000D   x000D     change one element s class to  blue  x000D  document getElementById  div1   className    blue   x000D   x000D     write the number of elements in each array  values differ  x000D  console log  Number of elements with querySelectorAll       aux1 length   x000D  console log  Number of elements with getElementsByClassName       aux2 length   x000D   red   color red    x000D   green   color green    x000D   blue   color blue    x000D   lt div id  div0  class  blue  gt Blue lt  div gt  x000D   lt div id  div1  class  red  gt Red lt  div gt  x000D   lt div id  div2  class  green  gt Green lt  div gt  x000D   x000D   x000D

User · Answer

Difference between  querySelector  and  querySelectorAll    x000D   x000D    querySelector returns single element x000D  let listsingle   document querySelector  li    x000D  console log listsingle   x000D   x000D   x000D    querySelectorAll returns lit array of elements x000D  let list   document querySelectorAll  li    x000D  console log list   x000D   x000D   x000D    Note   output will be visible in Console x000D   lt ul gt  x000D   lt li class  test  gt ffff lt  li gt  x000D   lt li class  test  gt vvvv lt  li gt  x000D   lt li gt dddd lt  li gt  x000D   lt li class  test  gt ddff lt  li gt  x000D   lt  ul gt  x000D   x000D   x000D

User · Answer

For this answer  I refer to querySelector and querySelectorAll as querySelector  and to getElementById  getElementsByClassName  getElementsByTagName  and getElementsByName as getElement   Main Differences  querySelector  is more flexible  as you can pass it any CSS3 selector  not just simple ones for id  tag  or class  The performance of querySelector changes with the size of the DOM that it is invoked on   To be precise  querySelector  calls run in O n  time and getElement  calls run in O 1  time  where n is the total number of all children of the element or document it is invoked on  This fact seems to be the least well-known  so I am bolding it  getElement  calls return direct references to the DOM  whereas querySelector  internally makes copies of the selected elements before returning references to them  These are referred to as  quot live quot  and  quot static quot  elements respectively  This is NOT strictly related to the types that they return  There is no way I know of to tell if an element is live or static programmatically  as it depends on whether the element was copied at some point  and is not an intrinsic property of the data  Changes to live elements apply immediately - changing a live element changes it directly in the DOM  and therefore the very next line of JS can see that change  and it propagates to any other live elements referencing that element immediately  Changes to static elements are only written back to the DOM after the current script is done executing  These extra copy and write steps have some small  and generally negligible  effect on performance  The return types of these calls vary  querySelector and getElementById both return a single element  querySelectorAll and getElementsByName both return NodeLists  being newer functions that were added after HTMLCollection went out of fashion  The older getElementsByClassName and getElementsByTagName both return HTMLCollections  Again  this is essentially irrelevant to whether the elements are live or static   These concepts are summarized in the following table  Function                 Live    Type             Time Complexity querySelector              N     Element           O n  querySelectorAll           N     NodeList          O n  getElementById             Y     Element           O 1  getElementsByClassName     Y     HTMLCollection    O 1  getElementsByTagName       Y     HTMLCollection    O 1  getElementsByName          Y     NodeList          O 1   Details  Tips  and Examples  HTMLCollections are not as array-like as NodeLists and do not support  forEach    I find the spread operator useful to work around this      document getElementsByClassName  quot someClass quot    forEach    Every element  and the global document  have access to all of these functions except for getElementById and getElementsByName  which are only implemented on document   Chaining getElement  calls instead of using querySelector  will improve performance  especially on very large DOMs  Even on small DOMs and or with very long chains  it is generally faster  However  unless you know you need the performance  the readability of querySelector  should be preferred  querySelectorAll is often harder to rewrite  because you must select elements from the NodeList or HTMLCollection at every step  For example  the following code does not work  document getElementsByClassName  quot someClass quot   getElementsByTagName  quot div quot   because you can only use getElements  on single elements  not collections  For example  document querySelector  quot  someId  someClass div quot   could be written as  document getElementById  quot someId quot   getElementsByClassName  quot someClass quot   0  getElementsByTagName  quot div quot   0  Note the use of  0  to get just the first element of the collection at each step that returns a collection  resulting in one element at the end just like with querySelector   Since all elements have access to both querySelector  and getElement  calls  you can make chains using both calls  which can be useful if you want some performance gain  but cannot avoid a querySelector that can not be written in terms of the getElement  calls   Though it is generally easy to tell if a selector can be written using only getElement  calls  there is one case that may not be obvious  document querySelectorAll  quot  class1 class2 quot   can be rewritten as document getElementsByClassName  quot class1 class2 quot    Using getElement  on a static element fetched with querySelector  will result in an element that is live with respect to the static subset of the DOM copied by querySelector  but not live with respect to the full document DOM    this is where the simple live static interpretation of elements begins to fall apart  You should probably avoid situations where you have to worry about this  but if you do  remember that querySelector  calls copy elements they find before returning references to them  but getElement  calls fetch direct references without copying   Neither API specifies which element should be selected first if there are multiple matches   Because querySelector  iterates through the DOM until it finds a match  see Main Difference  2   the above also implies that you cannot rely on the position of an element you are looking for in the DOM to guarantee that it is found quickly  - the browser may iterate through the DOM backwards  forwards  depth first  breadth first  or otherwise  getElement  will still find elements in roughly the same amount of time regardless of their placement

User · Answer

The main difference between querySelector and getlementbyID Claassname Tagname etc  is if there is more than one elements which satifies the condition querySelector will return only one output whereas getElementBy  will return all the elements   Lets consider an example to make it more clear     lt nav id  primary  class  menu  gt                               lt a class  link  href     gt For Business lt  a gt                               lt a class  link  href     gt Become an Instructor lt  a gt                               lt a class  link  href     gt Mobile Applications lt  a gt                               lt a class  link  href     gt Support lt  a gt                               lt a class  link  href     gt Help lt  a gt      lt  nav gt     Below code will explain the difference    QUERY SELECTOR   document querySelector   link       Output   For Business  element   document querySelectorAll   link      Out All the element with class link    GET ELEMENT   document getElementsByClassName  link      Output   will return all the element with a class  link  but whereas in query selector it will return only one element which encounters first    Inshort if we want to select single element go for queryslector or if we want multiple element go for getElement

User · Answer

I would like to know what exactly is the difference between querySelector and querySelectorAll against getElementsByClassName and getElementById    The syntax and the browser support    querySelector is more useful when you want to use more complex selectors   e g  All list items descended from an element that is a member of the foo class   foo li     document querySelector   view  id1 inputText1   it doesn t work  But writing document getElementById  view  id1 inputText1   works  Any ideas why    The   character has special meaning inside a selector  You have to escape it   The selector escape character has special meaning in a JS string too  so you have to escape that too    document querySelector   view    id1   inputText1

[javascript] querySelector and querySelectorAll vs getElementsByClassName and getElementById in JavaScript

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