Debug JavaScript | Chrome DevTools | Chrome for Developers

DevTools provides a lot of different tools for different tasks, such as changing CSS, profiling page load performance, and monitoring network requests. The Sources panel is where you debug JavaScript.

Open DevTools and navigate to the Sources panel.

The Page tab with the file tree. Every file that the page requests is listed here.

The Code Editor section. After selecting a file in the Page tab, the contents of that file are displayed here.

The Debugger section. Various tools for inspecting the page's JavaScript.

If your DevTools window is wide, by default, the Debugger is to the right of the Code Editor . In this case, the Scope and Watch tabs join Breakpoints , Call stack , and others as collapsible sections.

Pause the code with a breakpoint

A common method for debugging a problem like this is to insert a lot of console.log() statements into the code, in order to inspect values as the script executes. For example:

function updateLabel() {
  var addend1 = getNumber1();
  console.log('addend1:', addend1);
  var addend2 = getNumber2();
  console.log('addend2:', addend2);
  var sum = addend1 + addend2;
  console.log('sum:', sum);
  label.textContent = addend1 + ' + ' + addend2 + ' = ' + sum;
The console.log() method may get the job done, but breakpoints can get it done faster. A
breakpoint lets you pause your code in the middle of its execution, and examine all values at that
moment in time. Breakpoints have a few advantages over the console.log() method:
With console.log(), you need to manually open the source code, find the relevant code, insert
the console.log() statements, and then reload the page in order to see the messages in the
Console. With breakpoints, you can pause on the relevant code without even knowing how the code is
structured.
In your console.log() statements you need to explicitly specify each value that you want to
inspect. With breakpoints, DevTools shows you the values of all variables at that moment in time.
Sometimes there are variables affecting your code that you're not even aware of.
In short, breakpoints can help you find and fix bugs faster than the console.log() method.
If you take a step back and think about how the app works, you can make an educated guess that the
incorrect sum (5 + 1 = 51) gets computed in the click event listener that's associated to the
Add Number 1 and Number 2 button. Therefore, you probably want to pause the code around the time
that the click listener executes. Event Listener Breakpoints let you do exactly that:
In the Debugger section, click Event Listener Breakpoints to expand the
section. DevTools reveals a list of expandable event categories, such as Animation and
Clipboard.
Next to the Mouse event category, click arrow_right Expand.
DevTools reveals a list of mouse events, such as click and mousedown. Each event has a checkbox next to it.
Check the click checkbox. DevTools is now set up to automatically pause when any click
event listener executes.
Back on the demo, click Add Number 1 and Number 2 again. DevTools pauses the demo and
highlights a line of code in the Sources panel. DevTools should be paused on this line of
code:
function onClick() {
If you're paused on a different line of code, press resume Resume Script Execution until you're paused on the correct line.
Note: If you paused on a different line, you have a browser extension that registers a
click event listener on every page that you visit. You were paused in the extension's click
listener. If you use Incognito Mode to browse in private, which disables all extensions,
you can see that you pause on the correct line of code every time.

Event Listener Breakpoints are just one of many types of breakpoints available in DevTools. It's
worth exploring all the different types, because each type ultimately helps you debug different
scenarios as quickly as possible. See Pause Your Code With Breakpoints to learn when and how to
use each type.
Step through the code
One common cause of bugs is when a script executes in the wrong order. Stepping through your code
lets you walk through your code's execution, one line at a time, and figure out exactly where
it's executing in a different order than you expected. Try it now:
On the Sources panel of DevTools, click step_into Step into next function call to step through the execution of the onClick() function, one line at a time. DevTools highlights the following line of code:
if (inputsAreEmpty()) {
Click step_over Step over next function call.
DevTools executes inputsAreEmpty() without stepping into it. Notice how DevTools skips a few lines of
code. This is because inputsAreEmpty() evaluated to false, so the if statement's block of
code didn't execute.
That's the basic idea of stepping through code. If you look at the code in get-started.js, you can
see that the bug is probably somewhere in the updateLabel() function. Rather than stepping through
every line of code, you can use another type of breakpoint to pause the code closer to the probable
location of the bug.
Set a line-of-code breakpoint
Line-of-code breakpoints are the most common type of breakpoint. When you've got a specific line of
code that you want to pause on, use a line-of-code breakpoint:
Look at the last line of code in updateLabel():
label.textContent = addend1 + ' + ' + addend2 + ' = ' + sum;
To the left of the code you can see the line number of this particular line of code, which is
32. Click 32. DevTools puts a blue icon on top of 32. This means that there is a
line-of-code breakpoint on this line. DevTools now always pauses before this line of code is
executed.
Click resume Resume script execution. The
script continues executing until it reaches line 32. On lines 29, 30, and 31, DevTools shows the values of addend1, addend2, and sum inline next to their declarations.
In this example, DevTools pauses on the line-of-code breakpoint on line 32.
Check variable values
The values of addend1, addend2, and sum look suspicious. They're wrapped in quotes, which
means that they're strings. This is a good hypothesis for the explaining the cause of the bug. Now
it's time to gather more information. DevTools provides a lot of tools for examining variable
values.
Method 1: Inspect the Scope
When you're paused on a line of code, the Scope tab shows you what local and global variables
are defined at this point in execution, along with the value of each variable. It also shows closure variables, when
applicable. When you're not paused on a line of code, the Scope tab is empty.
Double-click a variable value to edit it.
Note: Depending on your layout preference and the width of your DevTools window, the Scope tab may appear as a collapsible section together with Breakpoints and Call stack.
Method 2: Watch expressions
The Watch tab lets you monitor the values of variables over time. Watch isn't just limited to variables. You can store any valid JavaScript
expression in the Watch tab.
Note: Depending on your layout preference and the width of your DevTools window, the Watch tab may appear as a collapsible section together with Breakpoints and Call stack.
Try it now:
Click the Watch tab.
Click add Add watch expression.
Type typeof sum.
Press Enter. DevTools shows typeof sum: "string". The value to the right of the colon is the
result of your expression.
This screenshot shows the Watch tab (bottom-right) after creating the typeof sum watch
expression.
As suspected, sum is being evaluated as a string, when it should be a number. You've now confirmed
that this is the cause of the bug.
Method 3: The Console
In addition to viewing console.log() messages, you can also use the Console to evaluate arbitrary
JavaScript statements. In terms of debugging, you can use the Console to test out potential fixes
for bugs. Try it now:
If you don't have the Console drawer open, press Escape to open it. It opens at the bottom of
your DevTools window.
In the Console, type parseInt(addend1) + parseInt(addend2). This statement works because you
are paused on a line of code where addend1 and addend2 are in scope.
Press Enter. DevTools evaluates the statement and prints out 6, which is the result you expect
the demo to produce.
This screenshot shows the Console drawer after evaluating parseInt(addend1) + parseInt(addend2).
Apply a fix
You've found a fix for the bug. All that's left is to try out your fix by editing the code and
re-running the demo. You don't need to leave DevTools to apply the fix. You can edit JavaScript code
directly within the DevTools UI. Try it now:
Click resume Resume script execution.
In the Code Editor, replace line 31, var sum = addend1 + addend2, with
var sum = parseInt(addend1) + parseInt(addend2).
Press Command + S (Mac) or Control + S (Windows, Linux) to save your change.
Click label_off Deactivate breakpoints.
Its color changes to blue to indicate that it's active. While this is set, DevTools ignores any
breakpoints you've set.
Try out the demo with different values. The demo now calculates correctly.
Caution: This workflow only applies a fix to the code that is running in your browser. It won't
fix the code for all users that visit your page. To do that, you need to fix the code that's on your
servers. You can, however, edit files in DevTools and save them to your sources with Workspaces.
Tip: Starting from Chrome version 105, you can Edit a paused function live.
Next steps
Success: Congratulations! You now know how to make the most of Chrome DevTools when debugging JavaScript. The
tools and methods you learned in this tutorial can save you countless hours.
This tutorial only showed you two ways to set breakpoints. DevTools offers many other ways,
including:
Conditional breakpoints that are only triggered when the condition that you provide is true.
Breakpoints on caught or uncaught exceptions.
XHR breakpoints that are triggered when the requested URL matches a substring that you provide.
See Pause Your Code With Breakpoints to learn when and how to use each type.
There's a couple of code stepping controls that weren't explained in this tutorial. See Step over
line of code to learn more.
  Except as otherwise noted, the content of this page is licensed under the Creative Commons Attribution 4.0 License, and code samples are licensed under the Apache 2.0 License. For details, see the Google Developers Site Policies. Java is a registered trademark of Oracle and/or its affiliates.
  Last updated 2024-05-22 UTC.
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