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Migration Guide (JS)

This guide will help you integrate ExperimentAPI.jsm in your Desktop front-end code to run Nimbus experiments, while still being able to use preferences for default values and local development.

In order to use ExperimentAPI.jsm your code must be able to import jsms in the parent process or a privileged child process.

Add a new feature to the manifest#

Take a look a the Desktop Feature API docs to register a new feature in the manifest. If you are already using Firefox preferences to experiment using Normandy, you can add those as fallbackPref

// If these are your preferences in firefox.js
pref("browser.aboutwelcome.enabled", true);
pref("browser.aboutwelcome.skipFocus", false);
// This is what your manifest entry would look like
const MANIFEST = {
aboutwelcome: {
description: "The about:welcome page",
variables: {
enabled: {
type: "boolean",
fallbackPref: "browser.aboutwelcome.enabled",
skipFocus: {
type: "boolean",
fallbackPref: "browser.aboutwelcome.skipFocus",

Switch Services.prefs usages to NimbusFeatures#

Anywhere in the code your are using Services.prefs to get values, use NimbusFeatures instead:





If you've configured fallback preferences your tests should pass, but we recommend also using the the Testing Guide to add tests for your feature that enroll experiment configurations.