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---
title: Browser Console
slug: Tools/Browser_Console
translation_of: Tools/Browser_Console
---
<div>{{ToolsSidebar}}</div><p>The Browser Console is like the <a href="/en-US/docs/Tools/Web_Console" title="/en-US/docs/Tools/Web_Console">Web Console</a>, but applied to the whole browser rather than a single content tab.</p>
<p>So it logs the same sorts of information as the Web Console - network requests, JavaScript, CSS, and security errors and warnings, and messages explicitly logged by JavaScript code. However, rather than logging this information for a single content tab, it logs information for all content tabs, for add-ons, and for the browser's own code.</p>
<p>Similarly, you can execute JavaScript expressions using the Browser Console. But while the Web Console executes code in the page window scope, the Browser Console executes them in the scope of the browser's chrome window. This means you can interact with all the browser's tabs using the <a href="/en-US/docs/Code_snippets/Tabbed_browser" title="/en-US/docs/Code_snippets/Tabbed_browser"><code>gBrowser</code></a> global, and even with the XUL used to specify the browser's user interface.</p>
<p>To open the Browser Console, select "Browser Console" from the Web Developer submenu in the Firefox Menu (or Tools menu if you display the menu bar or are on Mac OS X).</p>
<p>From Firefox 27 onwards, you can also start the Browser Console by launching Firefox from the command line and passing the <code>-jsconsole</code> argument:</p>
<pre>/Applications/FirefoxAurora.app/Contents/MacOS/firefox-bin -jsconsole</pre>
<p>The Browser Console looks like this:</p>
<p><img alt="" src="https://mdn.mozillademos.org/files/5847/browser-console-window.png" style="display: block; margin-left: auto; margin-right: auto;"></p>
<p>You can see that the Browser Console looks and behaves very much like the <a href="/en-US/docs/Tools/Web_Console" title="/en-US/docs/Tools/Web_Console">Web Console</a>:</p>
<ul>
<li>most of the window is occupied by a <a href="/en-US/docs/Tools/Web_Console#Message_Display_Pane" title="/en-US/docs/Tools/Web_Console#Message_Display_Pane">pane that display messages</a></li>
<li>at the top, a <a href="/en-US/docs/Tools/Web_Console#Filtering_and_searching" title="/en-US/docs/docs/Tools/Web_Console#Filtering_and_searching">toolbar</a> enables you to filter the messages that appear</li>
<li>at the bottom, a <a href="/en-US/docs/Tools/Web_Console#The_command_line_interpreter" title="/en-US/docs/docs/Tools/Web_Console#The_command_line_interpreter">command line interpreter</a> enables you to evaluate JavaScript expressions.</li>
</ul>
<h2 id="Browser_Console_logging">Browser Console logging</h2>
<p>The Browser console logs the same sorts of messages as the Web Console:</p>
<ul>
<li><a href="/en-US/docs/Tools/Web_Console#HTTP_requests" title="/en-US/docs/Tools/Web_Console#HTTP_requests">HTTP requests</a></li>
<li><a href="/en-US/docs/Tools/Web_Console#Warnings_and_errors" title="/en-US/docs/Tools/Web_Console#Warnings_and_errors">Warnings and errors</a> (including JavaScript, CSS, security warnings and errors, and messages explicitly logged by JavaScript code using the <a href="https://developer.mozilla.org/en-US/docs/Web/API/console" title="/en-US/docs/Web/API/console">console</a> API)</li>
<li><a href="/en-US/docs/Tools/Web_Console#Input.2Foutput_messages" title="/en-US/docs/Tools/Web_Console#Input.2Foutput_messages">Input/output messages</a>: commands send to the browser via the command line, and the result of executing them.</li>
</ul>
<p>However, it displays such messages from:</p>
<ul>
<li>web content hosted by all browser tabs</li>
<li>the browser's own code</li>
<li>add-ons.</li>
</ul>
<h3 id="Messages_from_add-ons">Messages from add-ons</h3>
<p>The Browser Console displays messages logged by all Firefox add-ons.</p>
<h4 id="Console.jsm">Console.jsm</h4>
<p>To use the console API from a traditional or bootstrapped add-on, get it from the Console module.</p>
<p>One exported symbol from Console.jsm is "console". Below is an example of how to acess it, which adds a message to the Browser Console.</p>
<pre class="brush: js">const Cu = Components.utils;
const consoleJSM =
Cu.import("resource://gre/modules/devtools/Console.jsm", {});
let console = consoleJSM.console; //access exported symbol of "console" from the Console.jsm
<code>console.log("Hello from Firefox code"); //output messages to the console</code></pre>
<p> </p>
<p>The full module with other functionality is found here on <a href="http://mxr.mozilla.org/mozilla-release/source/toolkit/devtools/Console.jsm">Mozilla Cross-Reference</a>.</p>
<p>For more methods of the console exported attribute of the Console.jsm see this article: <a href="/en-US/docs/Web/API/console?redirectlocale=en-US&redirectslug=DOM%2Fconsole">Console</a></p>
<h4 id="HUDService">HUDService</h4>
<p>There is also the HUDService which allows access to the Browse Console. The module is available at <a href="http://mxr.mozilla.org/mozilla-release/source/browser/devtools/webconsole/hudservice.js">Mozilla Cross-Reference</a>. We see we can not only access the Browser Console but also Web Console.</p>
<p>Here is an example on how to clear the contents of the Browser console:</p>
<pre class="brush: js">var devtools = Cu.import("resource://gre/modules/devtools/Loader.jsm", {}).devtools;
var HUDService = devtools.require("devtools/webconsole/hudservice");
var hud = HUDService.getBrowserConsole();
hud.jsterm.clearOutput(true);</pre>
<p>If you would like to access the content document of the Browser Console this can be done with the HUDService. This example here makes it so that when you mouse over the "Clear" button it will clear the Browser Console:</p>
<pre class="brush: js">var devtools = Cu.import("resource://gre/modules/devtools/Loader.jsm", {}).devtools;
var HUDService = devtools.require("devtools/webconsole/hudservice");
var hud = HUDService.getBrowserConsole();
var clearBtn = hud.chromeWindow.document.querySelector('.webconsole-clear-console-button');
clearBtn.addEventListener('mouseover', function() {
hud.jsterm.clearOutput(true);
}, false);</pre>
<h4 id="Bonus_Features_Available">Bonus Features Available</h4>
<p>For <a href="https://addons.mozilla.org/en-US/developers/docs/sdk/latest/dev-guide/index.html" title="https://addons.mozilla.org/en-US/developers/docs/sdk/latest/dev-guide/index.html">Add-on SDK</a> add-ons, the console API is available automatically. Here's an example add-on that just logs an error when the user clicks a widget:</p>
<pre class="brush: js">widget = require("sdk/widget").Widget({
id: "an-error-happened",
label: "Error!",
width: 40,
content: "Error!",
onClick: logError
});
function logError() {
console.error("something went wrong!");
}</pre>
<p>If you <a href="https://addons.mozilla.org/en-US/developers/docs/sdk/latest/dev-guide/tutorials/installation.html" title="https://addons.mozilla.org/en-US/developers/docs/sdk/latest/dev-guide/tutorials/installation.html">build this as an XPI file</a>, then open the Browser Console, then open the XPI file in Firefox and install it, you'll see a widget labeled "Error!" in the Add-on bar:</p>
<p><img alt="" src="https://mdn.mozillademos.org/files/5937/browser-console-addon.png" style="display: block; margin-left: auto; margin-right: auto;">Click the icon. You'll see output like this in the Browser Console:</p>
<p><img alt="" src="https://mdn.mozillademos.org/files/5851/browser-console-addon-output.png" style="display: block; margin-left: auto; margin-right: auto;"></p>
<p>For Add-on SDK-based add-ons only, the message is prefixed with the name of the add-on ("log-error"), making it easy to find all messages from this add-on using the <a href="/en-US/docs/Tools/Web_Console#Filtering_and_searching" title="/en-US/docs/Tools/Web_Console#Filtering_and_searching">"Filter output"</a> search box. By default, only error messages are logged to the console, although <a href="https://addons.mozilla.org/en-US/developers/docs/sdk/latest/dev-guide/console.html#Logging%20Levels" title="https://addons.mozilla.org/en-US/developers/docs/sdk/latest/dev-guide/console.html#Logging%20Levels">you can change this in the browser's preferences</a>.</p>
<h2 id="Browser_Console_command_line">Browser Console command line</h2>
<p>Like the Web Console, the command line interpreter enables you to evaluate JavaScript expressions in real time:<img alt="" src="https://mdn.mozillademos.org/files/5855/browser-console-commandline.png" style="display: block; margin-left: auto; margin-right: auto;">Also like the Web Console's command line interpreter, this command line supports <a href="/en-US/docs/Tools/Web_Console#Autocomplete" title="/en-US/docs/Tools/Web_Console#Autocomplete">autocomplete</a>, <a href="/en-US/docs/Tools/Web_Console#Command_history" title="/en-US/docs/Tools/Web_Console#Command_history">history</a>, and various <a href="/en-US/docs/Tools/Web_Console#Keyboard_shortcuts" title="/en-US/docs/docs/Tools/Web_Console#Keyboard_shortcuts">keyboard shortcuts </a>and <a href="/en-US/docs/Tools/Web_Console#Helper_commands" title="/en-US/docs/Tools/Web_Console#Helper_commands">helper commands</a>. If the result of a command is an object, you can <a href="/en-US/docs/Tools/Web_Console#Inspecting_objects" title="/en-US/docs/Tools/Web_Console#Inspecting_objects">click on the object to see its details</a>.</p>
<p>But while the Web Console executes code in the scope of the content window it's attached to, the browser console executes code in the scope of the chrome window of the browser. You can confirm this by evaluating <code>window</code>:</p>
<p><img alt="" src="https://mdn.mozillademos.org/files/5867/browser-console-chromewindow.png" style="display: block; margin-left: auto; margin-right: auto;"></p>
<p>This means you can control the browser: opening, closing tabs and windows and changing the content that they host, and modify the browser's UI by creating, changing and removing XUL elements.</p>
<h3 id="Controlling_the_browser">Controlling the browser</h3>
<p>The command line interpreter gets access to the <a href="/en-US/docs/XUL/tabbrowser" title="/en-US/docs/XUL/tabbrowser"><code>tabbrowser</code></a> object, through the <code>gBrowser</code> global, and that enables you to control the browser through the command line. Try running this code in the Browser Console's command line (remember that to send multiple lines to the Browser Console, use Shift+Enter):</p>
<pre class="brush: js">var newTabBrowser = gBrowser.getBrowserForTab(gBrowser.selectedTab);
newTabBrowser.addEventListener("load", function() {
newTabBrowser.contentDocument.body.innerHTML = "<h1>this page has been eaten</h1>";
}, true);
newTabBrowser.contentDocument.location.href = "https://mozilla.org/";</pre>
<p>It adds a listener to the currently selected tab's <code>load</code> event that will eat the new page, then loads a new page.</p>
<h3 id="Modifying_the_browser_UI">Modifying the browser UI</h3>
<p>Since the global <code>window</code> object is the browser's chrome window, you can also modify the browser's user interface. On Windows, the following code will add a new item to the browser's main menu:</p>
<pre class="brush: js">var parent = window.document.getElementById("appmenuPrimaryPane");
var makeTheTea = gBrowser.ownerDocument.defaultView.document.createElementNS("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul", "menuitem");
makeTheTea.setAttribute("label", "A nice cup of tea?");
parent.appendChild(makeTheTea);</pre>
<p><img alt="" src="https://mdn.mozillademos.org/files/5859/browser-console-modify-ui-windows.png" style="display: block; margin-left: auto; margin-right: auto;">On OS X, this similar code will add a new item to the "Tools" menu:</p>
<pre class="brush: js">var parent = window.document.getElementById("menu_ToolsPopup");
var makeTheTea = gBrowser.ownerDocument.defaultView.document.createElementNS("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul", "menuitem");
makeTheTea.setAttribute("label", "A nice cup of tea?");
parent.appendChild(makeTheTea);</pre>
<p><img alt="" src="https://mdn.mozillademos.org/files/5861/browser-console-modify-ui-osx.png" style="display: block; margin-left: auto; margin-right: auto;"></p>
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