22. Finding Elements in the DOM

As you saw in previous chapter, your DOM is nothing more than a tree-like structure made up of all the elements that exist in your HTML document:

That detail is only sort of important. What is important is that you have all of these HTML elements floating around that you want to access and read data from or modify. There are many ways to find these HTML elements. After all, these elements are arranged in a tree-like structure, and if there is one thing computer scientists like to do, it is figuring out crazy ways to run up and down a tree to find something.

I won’t subject you to that torture...just yet. In this chapter, you are going to learn how to use two built-in functions called querySelector and querySelectorAll to solve a good chunk of all your DOM searching needs.

Onwards!

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<div id="main">
    <div class="pictureContainer">
        <img class="theImage" src="smiley.png" height="300"
          width="150" />
    </div>
    <div class="pictureContainer">
        <img class="theImage" src="tongue.png" height="300"
          width="150" />
    </div>
    <div class="pictureContainer">
        <img class="theImage" src="meh.png" height="300"
          width="150" />
    </div>
    <div class="pictureContainer">
        <img class="theImage" src="sad.png" height="300"
          width="150" />
    </div>
</div>

In this example, you have one div with an id of main, and then you have four div and img elements each with a class value of pictureContainer and theImage respectively. In the next few sections, we’ll set the querySelector and querySelectorAll functions loose on this HTML and see what happens.

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var element = document.querySelector("CSS selector");

The querySelector function takes an argument, and this argument is a CSS selector for the element you wish to find. What gets returned by querySelector is the first element it finds—even if other elements exist—that could get targeted by the selector. This function is pretty stubborn like that.

Taking the HTML from our earlier example, if we wanted to access the div whose id is main, you would write the following:

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var element = document.querySelector("#main");

Because main is the id, the selector syntax for targeting it would be #main. Similarly, let’s specify the selector for the pictureContainer class:

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var element = document.querySelector(".pictureContainer");

What gets returned is the first div whose class value is pictureContainer. The other div elements with the class value of pictureContainer will simply be ignored.

The selector syntax is not modified or made special because you are in JavaScript. The exact syntax you would use for selectors in your stylesheet or style region can be used!

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var element = document.querySelectorAll("CSS selector");

With the exception of the number of elements returned, everything I described about querySelector above applies to querySelectorAll as well. That important detail changes how you end up actually using the querySelectorAll function. What gets returned is not a single element. Instead, what gets returned is an array-like container of elements!

Continuing to use the HTML from earlier, here is what our JavaScript would look like if we wanted to use querySelectorAll to help us display the src attribute of all the img elements that contain the class value theimage:

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var images = document.querySelectorAll(".theImage");

for (var i = 0; i < images.length; i++) {
    var image = images[i];
    alert(image.getAttribute("src"));
}

See? This is pretty straightforward. The only thing you need to do is remember how to work with Arrays (Chapter 13, “When Primitives Behave Like Objects”). The one weird thing is the mysterious getAttribute function. If you aren’t familiar with getAttribute and how to read values from elements, that’s totally okay. We’ll look at all that really soon.

If you wanted to target all of the img elements without having to specify the class value, here is what our querySelectorAll call could look like:

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var images = document.querySelectorAll("img");

If you wanted to target only the image whose src attribute is set to meh.png, you can do the following:

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var images = document.querySelectorAll("img[src='meh.png']");

Note that I just specified an attribute selector¹ as my argument to querySelectorAll. Pretty much any complex expression you can specify for a selector in your CSS document is fair game for specifying as an argument to either querySelector or querySelectorAll.

1. http://bit.ly/kirupaAttribute

There are some caveats that you should be aware of:

• Not all pseudo-class selectors are allowed. A selector made up of :visited or :link is ignored and no elements are found.

• How crazy you can get with the selectors you provide depends on the browser’s CSS support. Internet Explorer 8 supports querySelector and querySelectorAll. It doesn’t support CSS3. Given that situation, using anything more recent than the selectors defined in CSS 2 will not work when used with querySelector and querySelectorAll on IE8. Chances are, this doesn’t apply to you because you are probably supporting more recent versions of browsers where this IE8 issue isn’t even on the radar.

• The selector you specify only applies to the descendants of the starting element you are beginning your search from. The starting element itself is not included. Not all querySelector and querySelectorAll calls need to be made from a document.