Console messages

Most of the Web Console is occupied by the message display pane:

Each message is displayed as a separate row:

By default, the console is cleared each time you navigate to a new page or reload the current page. To override this behavior, check "Enable persistent logs" in the Settings.

Message categories

Network

Network requests are logged with a line that looks like this:

Viewing network request details

If you click on the message, you'll be redirected to the Network panel where the request is selected and more request and response details are shown within the side panel at the right side. In versions prior to Firefox 43 this information was shown in a separate popup.

Starting in Firefox 48, you can access much of this detail inline, without leaving the Web Console. Network request entries get a disclosure arrow next to them. Click this to see:

• Headers: request and response headers
• Response: the response body
• Cookies: any cookies that were sent along with the request
• Call Stack (new in Firefox 50): for requests initiated by JavaScript, such as XMLHttpRequest or Fetch, you can see the call stack at the point the request was made.

JS

JavaScript messages look like this:

"Learn more" link

JavaScript errors contain a "Learn more" link that takes you to the JavaScript error reference containing additional advice for fixing issues:

Source maps

From Firefox 50, the Web Console understands source maps. This means that if your JavaScript sources are compressed, you can supply a source map for them. Then any messages or errors your source generates will show up in the Web Console with a link back to the original source, not the compressed version.

At the moment source map support is disabled by default. To enable it, visit "about:config", find the preference "devtools.sourcemap.locations.enabled" and set it to true.

Be aware that support is still experimental. It may have bugs and may slow down console output. Bug 1289570 tracks any work that needs to be done before source map support can be enabled by default.

CSS

CSS messages look like this:

Reflow events

The Web Console also logs reflow events under the CSS category. A reflow is the name given to the operation in which the browser calculates the layout of all or part of the page. Reflows occur when a change has happened to a page that the browser thinks affects the layout. Many events can trigger reflows, including: resizing the browser window, activating pseudoclasses like :hover, or manipulating the DOM in JavaScript.

Because reflows can be computationally expensive and directly affect the user interface, they can have a big impact on the responsiveness of a website or web app. By logging reflow events the Web Console can give you insight into when reflow events are being triggered, how long they take to execute and, if the reflows are synchronous reflows triggered from JavaScript, which code triggered them.

Reflow events are logged as "Reflow" messages, as distinct from CSS errors or warnings. By default, they are disabled. You can enable them by clicking the "CSS" button in the toolbar and selecting "Reflows".

Each message is labeled "reflow" and shows the time taken to execute the reflow:

If the reflow is a synchronous reflow triggered from JavaScript, it also shows a link to the line of code that triggered the reflow:

Click the link to open the file in the Debugger.

Synchronous and asynchronous reflows

If a change is made that invalidates the current layout - for example, the browser window is resized or some JavaScript modifies an element's CSS - the layout is not recalculated immediately. Instead, the reflow happens asynchronously, the next time the browser decides it needs to be done (generally, the next time the browser repaints). In this way the browser can save up a collection of invalidating changes and recalculate their effect at once.

However, if some JavaScript code reads a style that has been modified, then the browser must perform a synchronous reflow in order to calculate the computed style to return. For example, code like this will cause an immediate, synchronous, reflow, when it calls window.getComputedStyle(thing).height:

var thing = document.getElementById("the-thing");
thing.style.display = "inline-block";
var thingHeight = window.getComputedStyle(thing).height;

Because of this, it's a good idea to avoid interleaving write and read calls to an element's styles when manipulating the DOM, because every time you read back a style that has been invalidated by a previous write call, you force a synchronous reflow.

Security

Security warnings and errors look like this:

The security messages shown in the Web Console help developers find potential or actual vulnerabilities in their sites. Additionally, many of these messages help educate developers because they end with a “Learn More” link that takes you to a page with background information and advice for mitigating the issue.

The complete list of security messages is as follows:

Bug 863874 is the meta-bug for logging relevant security messages to the Web Console. If you have more ideas for useful features like the ones discussed here, or are interested in contributing, check out the metabug and its dependencies.

Logging

The Logging category includes messages logged using the Console API.

The Web console supports the following Console API messages:

• assert()
• clear() (new in Firefox 48)
• count()
• dir()
• dirxml()
• error()
• exception()
• info()
• log()
• table()
• time()
• timeEnd()
• trace()
• warn()

The console prints a stack trace for all error messages, like this:

function foo() {
  console.error("it explodes");
}

function bar() {
  foo();
}

function doStuff() {
 bar();
}

doStuff();

Server

From Firefox 43, the Web Console can display messages sent from the server. This enables you to use the Web Console to debug server-side code.

It uses the Chrome Logger protocol. Briefly, the way it works is:

• your server-side code - Python, PHP, Node.js, ... - includes a library that provides a console API
• your server-side code uses this API to log messages
• the server-side library creates JSON objects from the messages and encodes them for transmission
• the messages are transmitted to the client as a response header named X-ChromeLogger-Data
• the Web Console decodes these headers and displays them

To find a suitable library for your server code, see the Chrome Logger documentation.

Command line input/output

Commands sent to the browser using the Web Console's command line, and the corresponding responses, are logged using lines like this:

The dark gray bar indicates that these are input/output messages, while the direction of the arrow discriminates between input and output.

Filtering and searching

Filtering by category

You can use the toolbar along the top to constrain the results displayed.

To see only messages of particular categories, click the button labeled with that category ("Net", "CSS", and so on). Clicking the main part of the button toggles that category on or off, while clicking the arrow on the right gives you more fine-grained filter options within that category:

• Net
  • Errors
  • Warnings
  • XHR
  • Log
• CSS
  • Errors
  • Warnings
  • Reflows
• JS
  • Errors
  • Warnings
• Security
  • Errors
  • Warnings
• Logging
  • Errors
  • Warnings
  • Info
  • Log
  • Shared Workers
  • Service Workers
  • Add-on or Chrome Workers
• Server
  • Errors
  • Warnings
  • info
  • Log

Filtering by text

To see only messages that contain a specific string, type in the text box labeled "Filter output".

Clearing the log

Finally, you can use this toolbar to clear the log. Before Firefox 48, this is a button labeled "Clear" on the right of the toolbar. From Firefox 48 it's a trashcan icon on the left.