Node.js Feature flags

Usage

Environment

You'll need a LAUNCHDARKLY_SDK environment variable configured, otherwise all flag queries will return false by default.

Setup

First, import and initialize the library somewhere early in the app:

const { logger } = require("./some/logging/lib");
const featureflags = require("node-featureflags");

featureflags.initialize({ logger });

The only option so far is to pass an (optional) logging tool that you'd like the library to use.

Querying a single flag

To query a single flag for a user, use isEnabled:

const { isEnabled } = require("node-featureflags");

const user = {
  key: "fancy-pants-store",
  email: "hans@fancy-pants-store.com",
  custom: {
    groups: ["shop"]
  }
};

const userGetsFeature = await isEnabled("feature-something", user);

Querying all flags

To get an object of all flags for a user and their values, use getAllFlags:

const { getAllFlags } = require("node-featureflags");

const flagsForUser = await getAllFlags(user);

/*
  {
    'feature-something': true,
    'feature-another': false,
  }
*/

The user argument in both cases should match the shape of LaumchDarkly's user object, documented here.

Both isEnabled and getAllFlags return promises, and should be called with await.

Mocking

This package includes a mocked version of the module, intended for use with Jest's node module mocking.

In the root of your project, add a file at __mocks__/node-featureflags.js. Import, and then re-export, the mock version of the module:

const mockModule = require("node-featureflags/mock");

module.exports = mockModule;

If using Jest, the feature flag library will be automatically mocked, with all flags returning false. To declare the value of a specific flag when running tests, importing the module and use the forceFlag method:

const featureFlags = require("node-featureflags");

it("has feature-a enabled", () => {
  featureFlags.forceFlag("feature-a", true);

  // Any queries for 'feature-a' in your code will return true
});

Because the feature flag library is a singleton, we'll need to reset the values of our forced flags so tests don't conflict with each other:

beforeEach(() => {
  featureFlags.reset();
});

You can also pass an object of flags to reset:

const flags = {
  featureA: "feature-a",
  featureB: "feature-b",
  featureC: "feature-c"
};

beforeEach(() => {
  featureFlags.reset(flags);
});

This will set the value of each flag to false, which may be useful when mocking getAllFlags.