Target filters

By default all records in a collection are made available. However, there are some use cases where it is more practical to use a single server side collection and filter record visibility in the browser. Target filters allows for these use cases.

Filters are conditional expressions evaluated in the client’s browser and, if they pass, the corresponding record is available. Filters have access to information about the user, such as their locale, addons, and Firefox version.


All records are downloaded to Firefox. Filters only control what is returned by .get() or the sync event.


The ideal use case for target filters is to use a single collection to service a small number (dozens) of different groups of users. Filtering by channel, browser version or locale are good use cases.


When a record has a string in the filter_expression field, it is evaluated and the record is potentially filtered for some users.

In order to restrict a setting to a particular audience, just write the proper filter string in the Admin UI and save the record.

    id: "68b19efa-1067-401b-b1c1-8d7b4263bb86",
    last_modified: 1531762863373,
    title: "Only for users of 57",
    filter_expression: "env.version == 57"

When calling RemoteSettings("key").get() or listening to sync events, you will only see the settings entries whose filter_expression resolved to a truthy value (and those who don’t have any as by default).

See the dedicated section about testing and debugging.

Filter Expressions

Filter expressions are written using a language called JEXL. JEXL is an open-source expression language that is given a context (in this case, information about the user’s browser) and evaluates a statement using that context. JEXL stands for “JavaScript Expression Language” and uses JavaScript syntax for several (but not all) of its features.


The rest of this document includes examples of JEXL syntax that has comments inline with the expressions. JEXL does not have any support for comments in statements, but we’re using them to make understanding our examples easier.

JEXL Basics

The JEXL Readme describes the syntax of the language in detail; the following section covers the basics of writing valid JEXL expressions.


Normally, JEXL doesn’t allow newlines or other whitespace besides spaces in expressions, but filter expressions in Remote Settings allow arbitrary whitespace.

A JEXL expression evaluates down to a single value. JEXL supports several basic types, such as numbers, strings (single or double quoted), and booleans. JEXL also supports several operators for combining values, such as arithmetic, boolean operators, comparisons, and string concatenation.

// Arithmetic
2 + 2 - 3 // == 1

// Numerical comparisons
5 > 7 // == false

// Boolean operators
false || 5 > 4 // == true

// String concatenation
"Mozilla" + " " + "Firefox" // == "Mozilla Firefox"

Expressions can be grouped using parenthesis:

((2 + 3) * 3) - 3 // == 7
JEXL also supports lists and objects (known as dictionaries in other languages)

as attribute access:

[1, 2, 1].length // == 3
{foo: 1, bar: 2}.foo // == 1

Unlike JavaScript, JEXL supports an in operator for checking if a substring is in a string or if an element is in an array:

"bar" in "foobarbaz" // == true
3 in [1, 2, 3, 4] // == true

The context passed to JEXL can be expressed using identifiers, which also support attribute access:

env.locale == 'en-US' // == true if the client's locale is en-US

Another unique feature of JEXL is transforms, which modify the value given to them. Transforms are applied to a value using the | operator, and may take additional arguments passed in the expression:

'1980-01-07'|date // == a date object


This section defines the context passed to filter expressions when they are evaluated. In other words, this is the client information available within filter expressions.


The env object contains general information about the client.


Example: '47.0.1'

String containing the user’s Firefox version.

String containing the update channel. Valid values include, but are not limited to:

  • 'release'

  • 'aurora'

  • 'beta'

  • 'nightly'

  • 'default' (self-built or automated testing builds)


Boolean specifying whether Firefox is set as the user’s default browser.


Object containing application details:

  • ID: String containing the XUL application ID, eg. Firefox is "{ec8030f7-c20a-464f-9b0e-13a3a9e97384}".

  • platformVersion: The version of the XULRunner platform

  • platformBuildID: The build ID/date of gecko and the XULRunner platform

  • version: The version of the XUL application. It is different than the version of the XULRunner platform. Be careful about which one you want.

  • more…


Example: 'google'

String containing the user’s default search engine identifier. Identifiers are lowercase, and may be locale-specific (Wikipedia, for example, often has locale-specific codes like 'wikipedia-es').

The default identifiers included in Firefox are:

  • 'google'

  • 'yahoo'

  • 'amazondotcom'

  • 'bing'

  • 'ddg'

  • 'twitter'

  • 'wikipedia'


Boolean containing whether the user has set up Firefox Sync.


Integer specifying the number of desktop clients the user has added to their Firefox Sync account.


Integer specifying the number of mobile clients the user has added to their Firefox Sync account.


Integer specifying the total number of clients the user has added to their Firefox Sync account.


An object mapping of plugin names to plugin objects describing the plugins installed on the client.


Example: 'en-US'

String containing the user’s locale.


String set to the user’s distribution ID. This is commonly used to target funnelcake builds of Firefox.

On Firefox versions prior to 48.0, this value is set to undefined.


Object containing data for the most recent Telemetry packet of each type. This allows you to target recipes at users based on their Telemetry data.

The object is keyed off the ping type, as documented in the Telemetry data documentation (see the type field in the packet example). The value is the contents of the ping.

// Target clients that are running Firefox on a tablet

// Target clients whose last crash had a BuildID of "201403021422"
env.telemetry.crash.payload.metadata.BuildID == '201403021422'

Boolean specifying whether the user has enabled Do Not Track.


Object containing information about installed add-ons. The keys on this object are add-on IDs. The values contain the following attributes:

String ID of the add-on.


Date object indicating when the add-on was installed.


Boolean indicating whether the add-on is active (disabling an add-on but not uninstalling it will set this to false).

String containing the user-visible name of the add-on.


String indicating the add-on type. Common values are extension, theme, and plugin.


String containing the add-on’s version number.

// Target users with a specific add-on installed

// Target users who have at least one of a group of add-ons installed
env.addons|keys intersect [


This section describes the special operators available to filter expressions on top of the standard operators in JEXL. They’re documented as functions, and the parameters correspond to the operands.

intersect(list1, list2)

Returns an array of all values in list1 that are also present in list2. Values are compared using strict equality. If list1 or list2 are not arrays, the returned value is undefined.

  • list1 – The array to the left of the operator.

  • list2 – The array to the right of the operator

// Evaluates to [2, 3]
[1, 2, 3, 4] intersect [5, 6, 2, 7, 3]


This section describes the transforms available to filter expressions, and what they do. They’re documented as functions, and the first parameter to each function is the value being transformed.

stableSample(input, rate)

Randomly returns true or false based on the given sample rate. Used to sample over the set of matched users.

Sampling with this transform is stable over the input, meaning that the same input and sample rate will always result in the same return value.

  • input – A value for the sample to be stable over.

  • rate – A number between 0 and 1 with the sample rate. For example, 0.5 would be a 50% sample rate.

// True 50% of the time, stable per-version per-locale.
[env.locale, env.version]|stableSample(0.5)
bucketSample(input, start, count, total)

Returns true or false if the current user falls within a “bucket” in the given range.

Bucket sampling randomly groups users into a list of “buckets”, in this case based on the input parameter. Then, you specify which range of available buckets you want your sampling to match, and users who fall into a bucket in that range will be matched by this transform. Buckets are stable over the input, meaning that the same input will always result in the same bucket assignment.

Importantly, this means that you can use an independent input across several settings to ensure they do not get delivered to the same users. For example, if you have two settings that are variants of each other, you can ensure they are not shown to the same cohort:

// Half of users will match the first filter and not the
// second one, while the other half will match the second and not
// the first, even across multiple settings.
[env.locale]|bucketSample(0, 5000, 10000)
[env.locale]|bucketSample(5000, 5000, 10000)

The range to check wraps around the total bucket range. This means that if you have 100 buckets, and specify a range starting at bucket 70 that is 50 buckets long, this function will check buckets 70-99, and buckets 0-19.

  • input – A value for the bucket sampling to be stable over.

  • start – The bucket at the start of the range to check. Bucket indexes larger than the total bucket count wrap to the start of the range, e.g. bucket 110 and bucket 10 are the same bucket if the total bucket count is 100.

  • count – The number of buckets to check, starting at the start bucket. If this is large enough to cause the range to exceed the total number of buckets, the search will wrap to the start of the range again.

  • total – The number of buckets you want to group users into.


Parses a string as a date and returns a Date object. Date strings should be in ISO 8601 format.

  • dateString – String to parse as a date.

'2011-10-10T14:48:00'|date // == Date object matching the given date

Return an array of the given object’s own keys (specifically, its enumerable properties). Similar to Object.keys, except that if given a non-object, keys will return undefined.

  • obj – Object to get the keys for.

// Evaluates to ['foo', 'bar']
{foo: 1, bar:2}|keys

Preference Filters

preferenceValue(prefKey, defaultValue)
  • prefKey – Full dotted-path name of the preference to read.

  • defaultValue – The value to return if the preference does not have a value. Defaults to undefined.


The value of the preference.

// Match users with more than 2 content processes
'dom.ipc.processCount'|preferenceValue > 2
  • prefKey – Full dotted-path name of the preference to read.


true if the preference has a value that is different than its default value, or false if it does not.

// Match users who have modified add-on signature checks
  • prefKey – Full dotted-path name of the preference to read.


true if the preference has any value (whether it is the default value or a user-set value), or false if it does not.

// Match users with an HTTP proxy


This section lists some examples of commonly-used filter expressions.

// Match users using the en-US locale
env.locale == 'en-US'

// Match users in any English locale using Firefox Beta
   env.locale in ['en-US', 'en-AU', 'en-CA', 'en-GB', 'en-NZ', 'en-ZA']
   && == 'beta'

// Match users located in the US who have Firefox as their default browser == 'US' && env.isDefaultBrowser

// Match users with the Flash plugin installed. If Flash is missing, the
// plugin list returns `undefined`, which is a falsy value in JavaScript and
// fails the match. Otherwise, it returns a plugin object, which is truthy.
env.plugins['Shockwave Flash']

Advanced: Testing Filter Expressions in the Browser Console

  1. Open the browser console

    • Tools > Web Developer > Browser Console

    • Cmd + Shift + J

  2. Run the following in the console:

    const { RemoteSettings } = ChromeUtils.import("resource://services-settings/remote-settings.js", {});
    const client = RemoteSettings("a-key");

    The following lines create a local record with a filter expression field and fetch the current settings list.

    let FILTER_TO_TEST = `
        env.locale == "fr-FR"
      async function () {
        await client.db.clear();
        await client.db.importChanges({}, 42);
        const record = await client.db.create({
          id: "68b19efa-1067-401b-b1c1-8d7b4263bb86",  // random uuidgen
          filter_expression: FILTER_TO_TEST
        const filtered = await client.get();
        console.log(filtered.length == 1);
  3. The console will log true or false depending on whether the expression passed for your client or not.