Searching Elements - Browser: Document, Events, Interfaces - Document

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28 Mar 2024

Searching: getElement, querySelector

DOM navigation properties are great when elements are close to each other. What if they are not? How to get an arbitrary element of the page?
There are additional searching methods for that.

document.getElementById or just id

If an element has the id attribute, we can get the element using the method document.getElementById(id), no matter where it is.

For instance:

<div id="elem"><div id="elem-content">Element</div>
</div>

<script>// get the element
  let elem = document.getElementById('elem');

  // make its background red
  elem.style.background = 'red';
</script>

Also, there’s a global variable named by id that references the element:

<div id="elem"><div id="elem-content">Element</div>
</div>

<script>// elem is a reference to DOM-element with id="elem"
  elem.style.background = 'red';

  // id="elem-content" has a hyphen inside, so it can't be a variable name// ...but we can access it using square brackets: window['elem-content']
</script>

… That’s unless we declare a JavaScript variable with the same name, then it takes precedence:

<div id="elem"></div>

<script>let elem = 5; // now elem is 5, not a reference to <div id="elem">

  alert(elem); // 5
</script>

Please don’t use id-named global variables to access elements

This behavior is described in the specification, but it is supported mainly for compatibility.
The browser tries to help us by mixing namespaces of JS and DOM. That’s fine for simple scripts, inlined into HTML, but generally isn’t a good thing. There may be naming conflicts. Also, when one reads JS code and doesn’t have HTML in view, it’s not obvious where the variable comes from.
Here in the tutorial we use id to directly reference an element for brevity, when it’s obvious where the element comes from.
In real life document.getElementById is the preferred method.

The id must be unique

The id must be unique. There can be only one element in the document with the given id.
If there are multiple elements with the same id, then the behavior of methods that use it is unpredictable, e.g. document.getElementById may return any of such elements at random. So please stick to the rule and keep id unique.

Only document.getElementById, not anyElem.getElementById

The method getElementById can be called only on document object. It looks for the given id in the whole document.

querySelectorAll

By far, the most versatile method, elem.querySelectorAll(css) returns all elements inside elem matching the given CSS selector.

Here we look for all <li> elements that are last children:

<ul><li>The</li><li>test</li>
</ul>
<ul><li>has</li><li>passed</li>
</ul>
<script>
  let elements = document.querySelectorAll('ul > li:last-child');

  for (let elem of elements) {alert(elem.innerHTML); // "test", "passed"}
</script>

This method is indeed powerful, because any CSS selector can be used.

Can use pseudo-classes as well

Pseudo-classes in the CSS selector like :hover and :active are also supported. For instance, document.querySelectorAll(':hover') will return the collection with elements that the pointer is over now (in nesting order: from the outermost <html> to the most nested one).

querySelector

The call to elem.querySelector(css) returns the first element for the given CSS selector.
In other words, the result is the same as elem.querySelectorAll(css)[0], but the latter is looking for all elements and picking one, while elem.querySelector just looks for one. So it’s faster and also shorter to write.

matches

Previous methods were searching the DOM.
The elem.matches(css) does not look for anything, it merely checks if elem matches the given CSS-selector. It returns true or false.
The method comes in handy when we are iterating over elements (like in an array or something) and trying to filter out those that interest us.

For instance:

<a href="http://example.com/file.zip">...</a>
<a href="http://ya.ru">...</a>

<script>// can be any collection instead of document.body.childrenfor (let elem of document.body.children) {
    if (elem.matches('a[href$="zip"]')) {alert("The archive reference: " + elem.href );}}
</script>

closest

Ancestors of an element are: parent, the parent of parent, its parent and so on. The ancestors together form the chain of parents from the element to the top.
The method elem.closest(css) looks for the nearest ancestor that matches the CSS-selector. The elem itself is also included in the search.
In other words, the method closest goes up from the element and checks each of parents. If it matches the selector, then the search stops, and the ancestor is returned.

For instance:

<h1>Contents</h1>

<div class="contents"><ul class="book"><li class="chapter">Chapter 1</li><li class="chapter">Chapter 2</li></ul>
</div>

<script>let chapter = document.querySelector('.chapter'); // LI

  alert(chapter.closest('.book')); // ULalert(chapter.closest('.contents')); // DIV

  alert(chapter.closest('h1')); // null (because h1 is not an ancestor)
</script>

getElementsBy*

There are also other methods to look for nodes by a tag, class, etc.
Today, they are mostly history, as querySelector is more powerful and shorter to write.
So here we cover them mainly for completeness, while you can still find them in the old scripts.

elem.getElementsByTagName(tag) looks for elements with the given tag and returns the collection of them. The tag parameter can also be a star "*" for “any tags”.
elem.getElementsByClassName(className) returns elements that have the given CSS class.
document.getElementsByName(name) returns elements with the given name attribute, document-wide. Very rarely used.

For instance:

// get all divs in the document
let divs = document.getElementsByTagName('div');

Let’s find all input tags inside the table:

<table id="table"><tr><td>Your age:</td>

    <td><label><input type="radio" name="age" value="young" checked> less than 18
      </label><label><input type="radio" name="age" value="mature"> from 18 to 50
      </label><label><input type="radio" name="age" value="senior"> more than 60
      </label></td></tr>
</table>

<script>
  let inputs = table.getElementsByTagName('input');

  for (let input of inputs) {alert( input.value + ': ' + input.checked );}
</script>

Don’t forget the "s" letter!

Novice developers sometimes forget the letter "s". That is, they try to call getElementByTagName instead of getElementsByTagName.
The "s" letter is absent in getElementById, because it returns a single element. But getElementsByTagName returns a collection of elements, so there’s "s" inside.

It returns a collection, not an element!

Another widespread novice mistake is to write:

// doesn't work
document.getElementsByTagName('input').value = 5;

That won’t work, because it takes a collection of inputs and assigns the value to it rather than to elements inside it.

We should either iterate over the collection or get an element by its index, and then assign, like this:

// should work (if there's an input)
document.getElementsByTagName('input')[0].value = 5;

Looking for .article elements:

<form name="my-form"><div class="article">Article</div><div class="long article">Long article</div>
</form>

<script>// find by name attributelet form = document.getElementsByName('my-form')[0];

  // find by class inside the formlet articles = form.getElementsByClassName('article');alert(articles.length); // 2, found two elements with class "article"
</script>

Live collections

All methods "getElementsBy*" return a live collection. Such collections always reflect the current state of the document and “auto-update” when it changes.

In the example below, there are two scripts.

The first one creates a reference to the collection of <div>. As of now, its length is 1.
The second scripts runs after the browser meets one more <div>, so its length is 2.

<div>First div</div>

<script>let divs = document.getElementsByTagName('div');alert(divs.length); // 1
</script>

<div>Second div</div>

<script>
  alert(divs.length); // 2
</script>

In contrast, querySelectorAll returns a static collection. It’s like a fixed array of elements.
If we use it instead, then both scripts output 1:

<div>First div</div>

<script>let divs = document.querySelectorAll('div');alert(divs.length); // 1
</script>

<div>Second div</div>

<script>
  alert(divs.length); // 1
</script>

Now we can easily see the difference. The static collection did not increase after the appearance of a new div in the document.

Summary

There are 6 main methods to search for nodes in DOM:
By far the most used are querySelector and querySelectorAll, but getElement(s)By* can be sporadically helpful or found in the old scripts.

Besides that:

There is elem.matches(css) to check if elem matches the given CSS selector.
There is elem.closest(css) to look for the nearest ancestor that matches the given CSS-selector. The elem itself is also checked.

And let’s mention one more method here to check for the child-parent relationship, as it’s sometimes useful:

elemA.contains(elemB) returns true if elemB is inside elemA (a descendant of elemA) or when elemA==elemB.

Tasks

Search for elements
importance: 4

Here’s the document with the table and form.
How to find?…

The table with id="age-table".
All label elements inside that table (there should be 3 of them).
The first td in that table (with the word “Age”).
The form with name="search".
The first input in that form.
The last input in that form.

Open the page table.html in a separate window and make use of browser tools for that.

Original Content at: https://javascript.info/searching-elements-dom

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