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?
For this answer, I refer to querySelector and querySelectorAll as querySelector* and to getElementById, getElementsByClassName, getElementsByTagName, and getElementsByName as getElement*.
A lot of this information can be verified in the specification, a lot of it is from various benchmarks I ran when I wrote it. The spec: https://dom.spec.whatwg.org/
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.
The return types of these calls vary. querySelector and getElementById both return a single element. querySelectorAll and getElementsByName both return NodeLists. The older getElementsByClassName and getElementsByTagName both return HTMLCollections. NodeLists and HTMLCollections are both referred to as collections of elements.
Collections can return "live" or "static" collections respectively. This is NOT reflected in the actual types that they return. getElements* calls return live collections, and querySelectorAll returns a static collection. The way that I understand it, live collections contain references to elements in the DOM, and static collections contain copies of elements. Take a look at #Jan Feldmann's comment's below for a different angle as well. I haven't figured out a good way to incorporate it into my answer but it may be a more accurate understanding.
These concepts are summarized in the following table.
Function | Live? | Type | Time Complexity
querySelector | | Element | O(n)
querySelectorAll | N | NodeList | O(n)
getElementById | | 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("someClass")].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("someClass").getElementsByTagName("div")
because you can only use getElements* on single elements, not collections, but if you only wanted one element, then:
document.querySelector("#someId .someClass div")
could be written as:
document.getElementById("someId").getElementsByClassName("someClass")[0].getElementsByTagName("div")[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(".class1.class2")
can be rewritten as
document.getElementsByClassName("class1 class2")
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.
querySelector* and getElementById traverse elements in preorder, depth-first, called "tree order" in the specification. With other getElement* calls it is not clear to me from the specification - they may be the same as tree order, but getElementsByClassName(".someClass")[0] may not reliably give the same result in every browser. getElementById("#someId") should though, even if you have multiple copies of the same id on your page.
I was working on an infinite scroll page when I had to look into this, and I think that is likely to be a common case where performance becomes an issue. Our code had onScroll events with querySelectorAll calls in them. Even if the calls were rate limited, the page would break if you scrolled down far enough, at which point there would be too many calls iterating through too many elements for the browser to keep up. The size of the DOM is relevant in this use case, and so there's a good case for preferring getElement* calls in code that runs on an infinite scroll page.
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")
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.
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:
// storing all the elements with class "blue" using the two methods
var aux1 = document.querySelectorAll(".blue");
var aux2 = document.getElementsByClassName("blue");
// write the number of elements in each array (values match)
console.log("Number of elements with querySelectorAll = " + aux1.length);
console.log("Number of elements with getElementsByClassName = " + aux2.length);
// change one element's class to "blue"
document.getElementById("div1").className = "blue";
// write the number of elements in each array (values differ)
console.log("Number of elements with querySelectorAll = " + aux1.length);
console.log("Number of elements with getElementsByClassName = " + aux2.length);
.red { color:red; }
.green { color:green; }
.blue { color:blue; }
<div id="div0" class="blue">Blue</div>
<div id="div1" class="red">Red</div>
<div id="div2" class="green">Green</div>
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 - <<< clear winner
querySelectorAll = 39,033 operations / sec
jquery select = 381,648 operations / sec
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'
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.
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 < 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 < lis.length ; i++){
ul.appendChild(document.createElement("li"));
}
Difference between "querySelector" and "querySelectorAll"
//querySelector returns single element
let listsingle = document.querySelector('li');
console.log(listsingle);
//querySelectorAll returns lit/array of elements
let list = document.querySelectorAll('li');
console.log(list);
//Note : output will be visible in Console
<ul>
<li class="test">ffff</li>
<li class="test">vvvv</li>
<li>dddd</li>
<li class="test">ddff</li>
</ul>
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 < 1000000; i++) {
var w = document.querySelector('#ll');
}
});
console.log('querySelector: '+q+'ms');
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.
<nav id="primary" class="menu">
<a class="link" href="#">For Business</a>
<a class="link" href="#">Become an Instructor</a>
<a class="link" href="#">Mobile Applications</a>
<a class="link" href="#">Support</a>
<a class="link" href="#">Help</a>
</nav>
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
How do I get only one DOM element by class name? I am guessing that the syntax of getting elements by class name is getElementsByClassName, but I am not sure how many elements it's going to return.
document.getElementsByClassName('className') would always return multiple elements because conceptually Classes are meant to be applied to multiple elements. If you want only the first element in the DOM with that class, you can select the first element out of the array-like HTMLCollection returned.
var elements = document.getElementsByClassName('className');
var requiredElement = elements[0];
Else, if you really want to select only one element. Then you need to use 'id' as conceptually it is used as an identifier for unique elements in a Web Page.
// HTML
<div id="myElement"></div>
// JS
var requiredElement = document.getElementById('myElement');
Clarifications :
getElementsByClassName returns a Node List and not an array
You do not need jQuery
What you were looking for was probably document.querySelector :
How do I get only one DOM element by class name?
var firstElementWithClass = document.querySelector('.myclass');
Also see: https://developer.mozilla.org/en/docs/Web/API/Document/querySelector
You can use jQuery:
$('.className').eq(index)
Or you can use the javascript built-in method:
var elements = document.getElementsByClassName('className');
This returns a nodeList of elements that you can iterate and make what you want.
If you want only one, you can access the elements in the javascript version like this:
elements[index]
To anyone looking for a one liner
var first = document.getElementsByClassName('test')[0];
Use NodeList.item().
For example:
var first = getElementsByClassName('test').item(0);
You can try with this:
document.getElementsByClassName("className")[index]
I tried using let modal = document.getElementsByClassName('modal') to select an element with the class modal. It only worked after using node selection to select the first result: let modal = document.getElementsByClassName('modal')[0]. I know the method Document.getElementsByClassName() returns child elements which have all of the given class names, but there's only one element in my HTML with that class. I confirmed this in my browser's dev tools by using var x = document.getElementsByClassName('modal').length and logging the value of x to the console (it returned 1 as expected).
Could someone explain why node selection is needed in this case?
Edit: My question is different than the one marked as a duplicate. In that question, they are asking the difference between methods than return a single element and those that return an array-like collection of elements. I'm already aware getElementsByClassName returns an array-like collection of elements, whereas the other methods return one element. My question is why do you need to specify the index in a case where all elements of a class are returned but there's only one element with a class (so one item, the correct item, is returned).
document.getElementsByClassName will return a list of elements with the given class name. Even if there is only one element with that class name it will be in a Node List which is why you have to use the [0]
It is needed because getElementsByClassName Returns an HTMLCollection and not a single element.
To get the item without using [0], use a query selector instead, this will give you the item instead of a collection of items.
let modal = document.querySelector('.modal')
console.log(modal)
document.getElementsByClassName
will return array of element who has this class
I want to get the control id so that I can attach event to this id,code for this is:
var t=document.getElementsByName('test');
alert(t);
this 't' here returns the Object HTMLCollection but when I run this code on jsfiddle i got Object NodeList and by using t[0].id I got the required id.
I have some requirement so I don't want to use document.getElementById();
Can any one tell me why this is happening and how can I get the id of control through Object HTMLCollection?
So it looks like you have two questions:
1) Can any one tell me why this is happening
and
2) how can I get the id of control through Object HTMLCollection?
First I think you need to understand WHAT an HTMLCollection is. Please read the answer to this stackoverflow question and pay careful attention to what is written, specifically
getElementsByTagName is method of the DOM interface. It accepts a tag
name as input and returns a NodeList (some browsers chose to return
HTMLCollection instead, which is OK, since it is a superset of
NodeList).
So the two share most properties, especially basic properties like id. I recommend reading up on HTMLCollection and NodeList on MDN.
This also contains the answer to your question as to WHY this happens
getElementsByTagName is method of the DOM interface. It accepts a tag
name as input and returns a NodeList (some browsers chose to return
HTMLCollection instead, which is OK, since it is a superset of
NodeList).
Essentially, the answer is simply that different browsers behave differently (when it comes to web development, you will find this is true in MANY ways).
So onto a more deailed answer to the second part of your question. ASSUMING that you have HTML elements with the name 'test' and ASSUMING you want the first one, all you have to do is reference the first element of the returned array, whether it is a NodeList or an HTMLCollection
var element = document.getElementsByName('test')[0];
If you want to make sure you got elements back, just get the array and check that it has > 0 elements
var element;
var elements = document.getElementsByName('test');
if (elements.length > 0)
{
element = elements[0];
}
The method you are using will return an array, so to answer your question you will need to do:
var element = document.getElementsByName('test')[0];
i have created a NodeList with nodeList = document.createDocumentFragment().childNodes;,
then i have set nodeList[0] = document.getElementById("foo");
everything works fine, the only problem is, that nodeList.length stays 0
i have allready tried to change an Array to a NodeList. Also i have tried to use a fragment, appended children and changed them afterwards and returned the childnodes.
i cannot just append the original children, because they get removed somewere else then.
what am i missing?
yes i know i could just use an Array, but i want to NodeList element
I am not sure how duplicate works so i will add answer here
from documentation https://developer.mozilla.org/en-US/docs/Web/API/DocumentFragment?redirectlocale=en-US&redirectslug=DOM%2FDocumentFragment DocumentFragment has child property as read only so you cant write to it, instead use appendChild();
var nodeList = document.createDocumentFragment();
nodeList.appendChild(document.getElementById("foo"))