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, then there’s a global variable by the name from that id.

We can use it to immediately access the element no matter where it is:

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

<script>
  alert(elem); // DOM-element with id="elem"
  alert(window.elem); // accessing global variable like this also works

  // for elem-content things are a bit more complex
  // that has a dash inside, so it can't be a variable name
  alert(window['elem-content']); // ...but accessible using square brackets [...]
</script>

The 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. Good for very simple scripts, but there may be name conflicts. Also, when we look in JS and don’t have HTML in view, it’s not obvious where the variable comes from.

If we declare a variable with the same name, it takes precedence:

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

<script>
  let elem = 5;

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

The better alternative is to use a special method document.getElementById(id).

For instance:

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

<script>
  let elem = document.getElementById('elem');

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

Here in the tutorial we’ll often use id to directly reference an element, but that’s only to keep things short. In real life document.getElementById is the preferred method.

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.

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 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 handy when we are iterating over elements (like in array or something) and trying to filter 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.children
  for (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 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 1</li>
  </ul>
</div>

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

  alert(chapter.closest('.book')); // UL
  alert(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>

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

// 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 attribute
  let form = document.getElementsByName('my-form')[0];

  // find by class inside the form
  let 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.

1. The first one creates a reference to the collection of <div>. As of now, its length is 1.
2. 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 getElementBy* 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.