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console

In This Article
  1. Console Methods
    1. console.debug(...)
    2. console.error(object[, object, ...])
    3. console.exception(exception)
    4. console.info(object[, object, ...])
    5. console.log(object[, object, ...])
    6. console.time(name)
    7. console.timeEnd(name)
    8. console.trace()
    9. console.warn(object[, object, ...])
  2. Logging Levels
    1. Setting the Logging Level

Add-ons using the techniques described in this document are considered a legacy technology in Firefox. Don't use these techniques to develop new add-ons. Use WebExtensions instead. If you maintain an add-on which uses the techniques described here, consider migrating it to use WebExtensions.

From Firefox 53 onwards, no new legacy add-ons will be accepted on addons.mozilla.org (AMO).

From Firefox 57 onwards, WebExtensions will be the only supported extension type, and Firefox will not load other types.

Even before Firefox 57, changes coming up in the Firefox platform will break many legacy extensions. These changes include multiprocess Firefox (e10s), sandboxing, and multiple content processes. Legacy extensions that are affected by these changes should migrate to WebExtensions if they can. See the "Compatibility Milestones" document for more.

A wiki page containing resources, migration paths, office hours, and more, is available to help developers transition to the new technologies.

Enables your add-on to log error, warning or informational messages. If you have started Firefox for your add-on from the command line with jpm run or jpm test then these messages appear in the command shell you used. If the add-on has been installed in Firefox, then the messages appear in the Browser Console.

If you're developing your add-on using the Extension Auto-installer, then the add-on is installed in Firefox, meaning that messages will appear in the Browser Console. But see the discussion of logging levels: by default, messages logged using log(), info(), trace(), or warn() won't be logged in these situations.

Console Methods

All console methods except exception() and trace() accept one or more JavaScript objects as arguments and log them to the console. Depending on the console's underlying implementation and user interface, you may be able to examine the properties of non-primitive objects that are logged.

console.debug(...)

Deprecated, you should use console.log() instead.

console.error(object[, object, ...])

Logs the arguments to the console, preceded by "error:" and the name of your add-on:

console.error("This is an error message");
error: my-addon: This is an error message

console.exception(exception)

Logs the given exception instance as an error, outputting information about the exception's stack traceback if one is available.

try {
   doThing();
} catch (e) {
   console.exception(e);
}
function UserException(message) {
   this.message = message;
   this.name = "UserException";
}
function doThing() {
  throw new UserException("Thing could not be done!");
}
error: my-addon: An exception occurred.
UserException: Thing could not be done!

console.info(object[, object, ...])

A synonym for console.log().

console.log(object[, object, ...])

Logs the arguments to the console, preceded by "info:" and the name of your add-on:

console.log("This is an informational message");
info: my-addon: This is an informational message

console.time(name)

Starts a timer with a name specified as an input parameter. Up to 10,000 simultaneous timers can run on a given page.

console.timeEnd(name)

Stops the specified timer and logs the elapsed time in seconds since its start. See Timers.

console.trace()

Logs a stack trace at the point the function is called.

console.warn(object[, object, ...])

Logs the arguments to the console, preceded by "warn:" and the name of your add-on:

console.warn("This is a warning message");
warn: my-addon: This is a warning message

Logging Levels

Logging's useful, of course, especially during development. But the more logging there is, the more noise you see in the console output. Especially when debug logging shows up in a production environment, the noise can make it harder, not easier, to debug issues.

This is the problem that logging levels are designed to fix. The console defines a number of logging levels, from "more verbose" to "less verbose", and a number of different logging functions that correspond to these levels, which are arranged in order of "severity" from informational messages, through warnings, to errors.

At a given logging level, only calls to the corresponding functions and functions with a higher severity will have any effect.

For example, if the logging level is set to "info", then calls to info(), log(), warn(), and error() will all result in output being written. But if the logging level is "warn" then only calls to warn() and error() have any effect, and calls to info() and log() are simply discarded.

This means that the same code can be more verbose in a development environment than in a production environment - you just need to arrange for the appropriate logging level to be set.

The complete set of logging levels is given in the table below, along with the set of functions that will result in output at each level:

Level Will log calls to:
all Any console method
debug debug(), error(), exception(), info(), log(), time(), timeEnd(), trace(), warn()
info error(), exception(), info(), log(), time(), timeEnd(), trace(), warn()
warn error(), exception(), warn()
error error(), exception()
off Nothing

Setting the Logging Level

The logging level defaults to "error".

There are two system preferences that can be used to override this default:

  • extensions.sdk.console.logLevel: if set, this determines the logging level for all installed SDK-based add-ons.

  • extensions.extensionID.sdk.console.logLevel, where extensionID is an add-on's Program ID. If set, this determines the logging level for the specified add-on. This overrides the global preference if both preferences are set.

Both these preferences can be set programmatically using the preferences/service API, or manually using about:config. The value for each preference is the desired logging level, given as a string.

When you run your add-on using jpm run (without --profile set) or jpm test, the global extensions.sdk.console.logLevel preference is automatically set to "info". This means that calls to console.log() will appear in the console output.

When you install an add-on into Firefox, the logging level will be "error" by default (that is, unless you have set one of the two preferences). This means that messages written using debug(), log(), info(), trace(), and warn() will not appear in the console.

This includes add-ons being developed using the Extension Auto-installer.

Document Tags and Contributors

Tags: 
  • Add-on SDK
 Contributors to this page: wbamberg, nearwood, evold, pasqLisena, maybe
 Last updated by: wbamberg, Dec 1, 2016, 10:47:59 AM
See also
  1. WebExtensions
  2. Getting started
    1. What are WebExtensions?
    2. Your first WebExtension
    3. Your second WebExtension
    4. Anatomy of a WebExtension
    5. Example WebExtensions
  3. How to
    1. Intercept HTTP requests
    2. Modify a web page
    3. Add a button to the toolbar
    4. Implement a settings page
  4. Concepts
    1. Using the JavaScript APIs
    2. User interface components
    3. Content scripts
    4. Match patterns
    5. Internationalization
    6. Content Security Policy
    7. Native messaging
  5. Porting
    1. Porting a Google Chrome extension
    2. Porting a legacy Firefox add-on
    3. Embedded WebExtensions
    4. Comparison with the Add-on SDK
    5. Comparison with XUL/XPCOM extensions
    6. Chrome incompatibilities
  6. Firefox workflow
    1. Temporary Installation in Firefox
    2. Debugging
    3. Getting started with web-ext
    4. web-ext command reference
    5. WebExtensions and the Add-on ID
    6. Publishing your WebExtension
  7. JavaScript APIs
    1. Browser support for JavaScript APIs
    2. alarms
    3. bookmarks
    4. browserAction
    5. browsingData
    6. commands
    7. contextMenus
    8. contextualIdentities
    9. cookies
    10. downloads
    11. events
    12. extension
    13. extensionTypes
    14. history
    15. i18n
    16. identity
    17. idle
    18. management
    19. notifications
    20. omnibox
    21. pageAction
    22. runtime
    23. sessions
    24. sidebarAction
    25. storage
    26. tabs
    27. topSites
    28. webNavigation
    29. webRequest
    30. windows
  8. Manifest keys
    1. applications
    2. author
    3. background
    4. browser_action
    5. chrome_url_overrides
    6. commands
    7. content_scripts
    8. content_security_policy
    9. default_locale
    10. description
    11. developer
    12. homepage_url
    13. icons
    14. manifest_version
    15. name
    16. omnibox
    17. options_ui
    18. page_action
    19. permissions
    20. short_name
    21. sidebar_action
    22. version
    23. web_accessible_resources
  9. Add-on SDK
  10. Getting started
    1. Installation
    2. Getting started
    3. Troubleshooting
  11. High-Level APIs
    1. addon-page
    2. base64
    3. clipboard
    4. context-menu
    5. hotkeys
    6. indexed-db
    7. l10n
    8. notifications
    9. page-mod
    10. page-worker
    11. panel
    12. passwords
    13. private-browsing
    14. querystring
    15. request
    16. selection
    17. self
    18. simple-prefs
    19. simple-storage
    20. system
    21. tabs
    22. timers
    23. ui
    24. url
    25. webextension
    26. widget
    27. windows
  12. Low-Level APIs
    1. /loader
    2. chrome
    3. console/plain-text
    4. console/traceback
    5. content/content
    6. content/loader
    7. content/mod
    8. content/symbiont
    9. content/worker
    10. core/heritage
    11. core/namespace
    12. core/promise
    13. dev/panel
    14. event/core
    15. event/target
    16. frame/hidden-frame
    17. frame/utils
    18. fs/path
    19. io/byte-streams
    20. io/file
    21. io/text-streams
    22. lang/functional
    23. lang/type
    24. loader/cuddlefish
    25. loader/sandbox
    26. net/url
    27. net/xhr
    28. places/bookmarks
    29. places/favicon
    30. places/history
    31. platform/xpcom
    32. preferences/event-target
    33. preferences/service
    34. remote/child
    35. remote/parent
    36. stylesheet/style
    37. stylesheet/utils
    38. system/child_process
    39. system/environment
    40. system/events
    41. system/runtime
    42. system/unload
    43. system/xul-app
    44. tabs/utils
    45. test/assert
    46. test/harness
    47. test/httpd
    48. test/runner
    49. test/utils
    50. ui/button/action
    51. ui/button/toggle
    52. ui/frame
    53. ui/id
    54. ui/sidebar
    55. ui/toolbar
    56. util/array
    57. util/collection
    58. util/deprecate
    59. util/list
    60. util/match-pattern
    61. util/object
    62. util/uuid
    63. window/utils
  13. Firefox for Android
  14. Getting started
    1. Walkthrough
    2. Debugging
    3. Code snippets
  15. APIs
    1. Accounts.jsm
    2. BrowserApp
    3. HelperApps.jsm
    4. Home.jsm
    5. HomeProvider.jsm
    6. JavaAddonManager.jsm
    7. NativeWindow
    8. Notifications.jsm
    9. PageActions.jsm
    10. Prompt.jsm
    11. RuntimePermissions.jsm
    12. Snackbars.jsm
    13. Sound.jsm
    14. Tab
  16. Legacy
  17. Restartless extensions
    1. Overview
  18. Overlay extensions
    1. Overview
  19. Themes
  20. Lightweight themes
    1. Overview
  21. Complete themes
    1. Overview
  22. Publishing add-ons
  23. Guides
    1. Signing and distribution overview
    2. Submit an add-on
    3. Review policies
    4. Developer agreement
    5. Featured add-ons
    6. Contact addons.mozilla.org
  24. Community and support
  25. Channels
    1. Add-ons blog
    2. Add-on forums
    3. Stack Overflow
    4. Development newsgroup
    5. IRC Channel