Mozilla ESLint Plugin


These environments are available by specifying a comment at the top of the file, e.g.

/* eslint-env mozilla/chrome-worker */

There are also built-in ESLint environments available as well:


Defines the environment for scripts that are in the main browser.xhtml scope.


Defines the environment for chrome workers. This differs from normal workers by the fact that ctypes can be accessed as well.


Defines the environment for frame scripts.


Defines the environment for jsm files (javascript modules).



Rejects grabbing the current time via or new Date() for timing purposes when the less problematic can be used instead.

The function returns milliseconds since page load. To convert that to milliseconds since the epoch, use:

performance.timing.navigationStart +

Often timing relative to the page load is adequate and that conversion may not be necessary.


Rejects using element.parentNode.removeChild(element) when element.remove() can be used instead.


Checks that for every occurrence of ‘addEventListener’ or ‘on’ there is an occurrence of ‘removeEventListener’ or ‘off’ with the same event name.


Checks that if/elseif/else bodies are braced consistently, so either all bodies are braced or unbraced. Doesn’t enforce either of those styles though.


For scripts included in browser-window, this will automatically inject the browser-window global scopes into the file.


For files containing ContentTask.spawn calls, this will automatically declare the frame script variables in the global scope. ContentTask is only available to test files, so by default the configs only specify it for the mochitest based configurations.

This saves setting the file as a mozilla/frame-script environment.

Note: due to the way ESLint works, it appears it is only easy to declare these variables on a file global scope, rather than function global. This may mean that they are incorrectly allowed, but given they are test files, this should be detected during testing.


Checks the filename of imported files e.g. Cu.import("some/path/Blah.jsm") adds Blah to the global scope.

Note: uses modules.json for a list of globals listed in each file.


Parses a file for globals defined in various unique Mozilla ways.

When a “import-globals-from <path>” comment is found in a file, then all globals from the file at <path> will be imported in the current scope. This will also operate recursively.

This is useful for scripts that are loaded as <script> tag in a window and rely on each other’s globals.

If <path> is a relative path, then it must be relative to the file being checked by the rule.


Import globals from head.js and from any files that were imported by head.js (as far as we can correctly resolve the path).

The following file import patterns are supported:

  • Services.scriptloader.loadSubScript(path)

  • loader.loadSubScript(path)

  • loadSubScript(path)

  • loadHelperScript(path)

  • import-globals-from path

If path does not exist because it is generated e.g. testdir + "/somefile.js" we do our best to resolve it.

The following patterns are supported:

  • Cu.import("resource://devtools/client/shared/widgets/ViewHelpers.jsm");

  • loader.lazyImporter(this, "name1");

  • loader.lazyRequireGetter(this, "name2"

  • loader.lazyServiceGetter(this, "name3"

  • XPCOMUtils.defineLazyModuleGetter(this, "setNamedTimeout", ...)

  • loader.lazyGetter(this, "toolboxStrings"

  • XPCOMUtils.defineLazyGetter(this, "clipboardHelper"


Simply marks test (the test method) or run_test as used when in mochitests or xpcshell tests respectively. This avoids ESLint telling us that the function is never called.


Checks that function argument names don’t start with lowercase ‘a’ followed by a capital letter. This is to prevent the use of Hungarian notation whereby the first letter is a prefix that indicates the type or intended use of a variable.


Checks that boolean expressions do not compare against literal values of true or false. This is to prevent overly verbose code such as if (isEnabled == true) when if (isEnabled) would suffice.


This disallows statements such as:

var Cc = Components.classes;
var Ci = Components.interfaces;
var {Ci: interfaces, Cc: classes, Cu: utils} = Components;

These used to be necessary but have now been defined globally for all chrome contexts.


This is similar to the ESLint built-in rule no-throw-literal. It disallows statements such as:

throw Components.results.NS_ERROR_ABORT;
throw new Error(Cr.NS_ERROR_NO_INTERFACE);

Throwing bare literals is inferior to throwing Exception objects, which provide stack information. Cr.ERRORs should be be passed as the second argument to Components.Exception() to create an Exception object with stack info, and the correct result property corresponding to the NS_ERROR that other code expects. Using a regular new Error() to wrap just turns it into a string and doesn’t set the result property, so the errors can’t be recognised.

This option can be autofixed (--fix).


Reject common XPCOM methods called with useless optional parameters (eg., null, null), or non-existent parameters (eg. Services.obs.removeObserver(name, observer, false)).

This option can be autofixed (--fix).


Reject calls to removeEventListener where {once: true} could be used instead.


Designed for xpcshell-tests. Rejects definitions of run_test() where the function only contains a single call to run_next_test(). xpcshell’s head.js already defines a utility function so there is no need for duplication.


Rejects calls to Cu.importGlobalProperties. Use of this function is undesirable in some parts of the tree.


This takes an option, a regular expression. Invocations of require with a string literal argument are matched against this regexp; and if it matches, the require use is flagged.


Treats top-level assignments like this.mumble = value as declaring a global.

Note: These are string matches so we will miss situations where the parent object is assigned to another variable e.g.:

var b = gBrowser;
b.content // Would not be detected as a CPOW.


This requires using Cc rather than Components.classes, and the same for Components.interfaces, Components.results and Components.utils. This has a slight performance advantage by avoiding the use of the dot.


Require use of ChromeUtils.import and ChromeUtils.defineModuleGetter rather than Components.utils.import and XPCOMUtils.defineLazyModuleGetter.


Require providing a second parameter to get*Pref methods instead of using a try/catch block.


Require .ownerGlobal instead of .ownerDocument.defaultView.


Use .includes instead of .indexOf to check if something is in an array or string.


Warn when idempotent methods are called and their return value is unused.


Requires the use of Services.jsm rather than Cc[].getService() where a service is already defined in Services.jsm.


Marks all var declarations that are not at the top level invalid.


Possible values for all rules









Example configuration:

"rules": {
  "mozilla/balanced-listeners": 2,
  "mozilla/mark-test-function-used": 1,
  "mozilla/var-only-at-top-level": 1,


The tests for eslint-plugin-mozilla are run via mochajs on top of node. Most of the tests use the ESLint Rule Unit Test framework.

Running Tests

The rules have some self tests, these can be run via:

$ cd tools/lint/eslint/eslint-plugin-mozilla
$ npm install
$ npm run test

Disabling tests

In the unlikely event of needing to disable a test, currently the only way is by commenting-out. Please file a bug if you have to do this.

Filing Bugs

Bugs should be filed in the Testing product under Lint.