Every modern web browser includes a powerful suite of developer tools. These tools do a range of things, from inspecting currently-loaded HTML, CSS, and JavaScript to showing which assets the page has requested and how long they took to load. This article explains how to use the basic functions of your browser's devtools.
Note: Before you run through the examples below, open the Beginner's example site that we built during the Getting started with the Web article series. You should have this open as you follow the steps below.
The devtools live inside your browser in a subwindow that looks roughly like this, depending on what browser you are using:
How do you pull it up? Three ways:
Keyboard:
Menu bar:
Firefox: Menu (☰) ➤ More tools ➤ Web Developer Tools
Chrome: More tools ➤ Developer tools
Opera: Developer ➤ Developer tools
Safari: Develop ➤ Show Web Inspector.
Note: The Safari developer tools are not enabled by default. To enable them, go to Safari ➤ Preferences ➤ Advanced, and check the Show Develop menu in menu bar or Enable features for web developers checkbox.
Context menu: Press-and-hold/right-click an item on a webpage (Ctrl-click on the Mac), and choose Inspect Element from the context menu that appears. (An added bonus: this method straight-away highlights the code of the element you right-clicked.)
The developer tools usually open by default to the inspector, which looks something like the following screenshot. This tool shows what the HTML on your page looks like at runtime, as well as what CSS is applied to each element on the page. It also allows you to instantly modify the HTML and CSS and see the results of your changes reflected live in the browser viewport.
If you don't see the inspector:
For a start, right-click (Ctrl-click) an HTML element in the DOM inspector and look at the context menu. The available menu options vary among browsers, but the important ones are mostly the same:
Try editing some of your DOM now. Double-click an element, or right-click it and choose Edit as HTML from the context menu. You can make any changes you'd like, but you cannot save your changes.
By default, the CSS editor displays the CSS rules applied to the currently selected element:
These features are especially handy:
You'll notice several clickable tabs at the top of the CSS Viewer:
The box model view visually represents the current element's box model, so you can see at a glance what padding, border and margin is applied to it, and how big its content is. In Firefox, this is located in the Layout tab; in other browsers, it is in the Computed tab.
In some browsers, the JavaScript details of the selected element can also be viewed in this panel. In Safari, these are unified under the Node tab, but are in separate tabs in Chrome, Opera, and Edge.
Find out more about the Inspector in different browsers:
The JavaScript debugger allows you to watch the value of variables and set breakpoints, places in your code that you want to pause execution and identify the problems that prevent your code from executing properly.
To get to the debugger:
Each browser's JavaScript debugger is divided into three panes. The layout of these differs across browsers; this guide uses Firefox as a reference.
The first pane on the left contains the list of files associated with the page you are debugging. Select the file you want to work with from this list. Click a file to select it and view its contents in the Debugger's center pane.
Set breakpoints where you want to pause execution. In the following image, the highlight on the number 18 shows that the line has a breakpoint set.
The right-hand pane shows a list of the watch expressions you have added and breakpoints you have set.
In the image, the first section, Watch expressions, shows that the listItems variable has been added. You can expand the list to view the values in the array.
The next section, Breakpoints, lists the breakpoints set on the page. In example.js, a breakpoint has been set on the statement listItems.push(inputNewItem.value);
The final two sections only appear when the code is running.
The Call stack section shows you what code was executed to get to the current line. You can see that the code is in the function that handles a mouse click, and that the code is currently paused on the breakpoint.
The final section, Scopes, shows what values are visible from various points within your code. For example, in the image below, you can see the objects available to the code in the addItemClick function.
Find out more about the JavaScript debugger in different browsers:
The JavaScript console is an incredibly useful tool for debugging JavaScript that isn't working as expected. It allows you to run lines of JavaScript against the page currently loaded in the browser, and reports the errors encountered as the browser tries to execute your code.
To access the console in any browser, open the developer tools and select the Console tab. This will give you a window like the following:
To see what happens, try entering the following snippets of code into the console one by one (and then pressing Enter):
alert("hello!");document.querySelector("html").style.backgroundColor = "purple";const loginImage = document.createElement("img");
loginImage.setAttribute(
"src",
"https://raw.githubusercontent.com/mdn/learning-area/master/html/forms/image-type-example/login.png",
);
document.querySelector("h1").appendChild(loginImage);Now try entering the following incorrect versions of the code and see what you get.
alert("hello!);document.cheeseSelector("html").style.backgroundColor = "purple";const loginImage = document.createElement("img");
banana.setAttribute(
"src",
"https://raw.githubusercontent.com/mdn/learning-area/master/html/forms/image-type-example/login.png",
);
document.querySelector("h1").appendChild(loginImage);You'll start to see the kind of errors that the browser returns. Often, these errors are fairly cryptic, but it should be pretty simple to figure these problems out!
Find out more about the JavaScript console in different browsers: