debugging > Chrome DevTools

Learn Chrome DevTools - Quick Guide to Debugging JavaScript, HTML and CSS

In this tutorial we are going share everything you need to know about debugging web applications in your Google Chrome web browser. The good news is that you only need to know one tool: the Chrome DevTools. Already installed in your Google Chrome web browser, the Chrome DevTools is technically just one panel in your browser. But this is an absolutely massive and incredibly useful tool!

What are the Chrome DevTools?

The Chrome DevTools is a window / panel that can open up from any website you are viewing inside your Chrome web browser. You can debug any website’s entire JavaScript, HTML and CSS content, also known as “the browser client”, with the DevTools. Think about the DevTools as the inspector of the browser. It tells you everything about what the web browser is doing behind the scenes- From what HTML and CSS is rendered on the screen, to what JavaScript is running, to debugging tools like breakpoints, network requests, and even showing you what cookies the websites you visit are storing without you knowing! With the Chrome DevTools, those secrets are all revealed and made available for you to see and edit!

How to open the Chrome DevTools

To open the Chrome DevTools, right-click on any website and clicking “inspect”, the DevTools will open and highlight the selected DOM element in the Elements panel. You can also use the keyboard shortcut CMD + Option + J on Mac or CTRL + SHIFT + J on windows to open the Console panel.

The Elements Panel

The Elements panel lets you view and edit a page’s DOM. This is different than viewing the page’s underlying HTML returned from the server.

HTML VS DOM

As explained by Google Developers:

In other words, HTML represents initial page content, and the DOM represents current page content. When JavaScript adds, removes, or edits nodes, the DOM becomes different than the HTML.

If you want to view a website’s HTML sent from the server, simply right click on the page and choose “View page source”.

You can access the Elements panel by either right-clicking any element on the page and choosing “inspect”, or just click on the Elements panel once inside the Chrome DevTools.

Now that we have our DOM at our fingertips, let’s explore some options.

Inspecting a DOM node

If you right-clicked on an element and clicked “inspect”, you should now be viewing that element highlighted in the DOM tree. To select another element on the page, click on the Inspect icon and hover over an element on the page and click once on the highlighted area that matches the element you want to inspect.

Why are we inspecting DOM nodes again?

Here are some reasons we inspect DOM nodes:

  • View the current HTML or CSS properties of a particular element
  • Change the HTML or CSS to see what effects our changes have on the page
  • See where a particular element is created inside JavaScript code
  • See how a particular element’s styles are generated from CSS
  • View the URL of a particular HTML image
  • Debug what JavaScript caused a particular element to be modified by setting a DOM breakpoint

Editing the DOM

Editing the DOM will be reflected live on the page you are viewing, so it’s the perfect tool to check how your new updates to HTML or React components would look.

Double-click on a DOM node to edit the properties or the text inside that node. You can start typing right inside the node. This text or properties are reflected on the page. Useful if you want to see what new text copy would look like on the page.

Editing the HTML

If you right-click on an element and select “Edit as HTML”, you are then given a text field to fully edit the HTML content comprising that element. This is useful if you want to add or remove certain HTML elements around that element to observe new behavior. A common use case would be if you are building a flexbox row component and aren’t sure what effects a new component would have on the row. You can easily add new elements inside or around any element on the page with “Edit as HTML”. You can then edit the CSS of this new component to add new properties. We’ll discuss this shortly. Of course, you know that these changes aren’t saved in your application source code, so they are perfect ways to quickly check a new HTML structure without adding and deleting these elements in your code. Simply refresh the page and you can edit the same component again and again.

In addition to editing elements, you can also delete and reorder elements easily. Simply drag and drop on an element to reorder, or click and press the delete key to delete the element. If you ever find yourself on a website with an annoying advertisement, you can use your debugging skills to delete that element from the page. There’s nothing stopping that page from re-adding that element with JavaScript though, so once again you are only editing the current state of the HTML, which we call the DOM.

Editing the CSS

Now that we discussed editing the HTML, let’s talk about the Styles panel on the side of the Elements panel. This panel is quite possibly the most important part of the Elements panel. Not only good for debugging CSS, the Styles panel helps you develop an understanding of how CSS works. Do not underestimate this panel and use it all the time when writing CSS.

Viewing and Editing CSS for an element

With the Elements panel open, inspect a DOM node that you want to check styles for. Over on the right you will see the Styles panel. This might look overwhelming but it is structured in a very specific way that will help you quickly understand what’s going on with element’s styles.

The Styles Panel

The Styles panel itself is organized by CSS rules that match that element. From top to bottom, each rule section is positioned in order of CSS specificity. Check out this article for an explanation of CSS specificity.

For example, let’s say in your CSS file, you have targeted CSS rules for a child element, a parent element, and the body. To keep it simple, let’s say each of those rules set a different color for a paragraph element. The child CSS rule would override the parent and body styles because it carries a higher specificity weight, so that color would be used for the child. So even though there are 3 different rules applying to the paragraph, the child rule wins and the element is set to the color of that rule. But that information about the parent styling isn’t lost. In the Styles panel, you can scroll down to see the other style rules for the parent, and then the body! These are crossed out because they are overridden by a more specific CSS style. Isn’t that cool? Imagine if we were trying to set a style on the body and didn’t know why it wasn’t being used. The Styles panel not only tells what rule is being applied, but it shows us all the possible rules that could be applied!

Even going all the way down, we can see the user agent stylesheet rule, which is the browser’s default set of CSS rules for all html elements.

The JavaScript Console

Next to the elements panel we have the JavaScript Console. This is where you can view logs from your JavaScript, in addition to evaluating JavaScript of your own. The console is also essential for checking errors and warnings in your app, logging variables, and trying out new code.

Sources Panel

The Sources Panel allows you to debug your app's JavaScript by setting breakpoints and inspecting code. These concepts are fairly straightforward but there are a few tricks that will help you debug faster.

Finding JavaScript Code in your Browser

To find a piece of JavaScript code in your browser, you’ve got a couple of options. If you know the name of your file, head on over to the Sources panel and Chrome should tell you name of your command to open the file. That's CMD + P on Mac or CTRL + P on windows. You then should see a dropdown to find the file in your app. If you are writing React code, you may see multiple files of the same name, one will contain the source-map for your React file.

Setting breakpoints

To set a breakpoint, click on the side of the file near the line number, as shown in the gif below. Then, execute the code that will trigger the breakpoint. The line of code is now highlighted, which signals that the code has paused execution on that line. So you can observe certain data in the code at this point by hovering over the variables you want to inspect. This is essential to debugging- quickly viewing the current state of your app at any point will give you great insight into how your app works, useful especially when you have a bug and need to know what's happening.

Examining the call stack

Once a breakpoint is triggered, you can now step through the call stack. As you can see in the gif below, function A calls function B, which calls C and then D. as we click on the functions in the call stack, our breakpoint moves "up" to where the current function was invoked. We step through the call stack to trace a particular function to down to where it was called. This is useful if you want to investigate a chain of events in your code. "What called this function, what were the arguments it used? What called the function above that?"

For an in-depth guide to setting breakpoints, check out the Google Developers Guide .

Debugging React Code

To learn more about debugging React apps and see these concepts in action, check out our course React Beginner Challenges.

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