Feature Flags: Overview

What is a Feature Flag?

A feature flag (also called a feature toggle) is a decision point in your code that lets you turn behaviour on or off without deploying new code. Instead of shipping a risky feature to everyone at once, you wrap it in a flag and control it from a dashboard.

In GrowthBook, a feature flag is called a Feature. Every Feature has:

A key — the stable string your code uses to look it up (e.g. new-checkout).
A value type — boolean, string, number, or JSON.
A default value — what the SDK returns when no rule matches.
Rules per environment — who sees which value, and when.

🚀 Run GrowthBook with Docker first

Save this as compose.yaml, then start the stack:

# compose.yaml
services:
  mongo:
    image: mongo:latest
    environment:
      - MONGO_INITDB_ROOT_USERNAME=root
      - MONGO_INITDB_ROOT_PASSWORD=password
  growthbook:
    image: growthbook/growthbook:latest
    ports: ["3000:3000", "3100:3100"]
    depends_on: [mongo]
    environment:
      - MONGO_URL=mongodb://root:password@mongo:27017/
    volumes: ["uploads:/usr/local/src/app/packages/back-end/uploads"]
volumes:
  uploads:

Then run docker compose up -d and open http://localhost:3000.

No Docker? Use GrowthBook Cloud (free tier).

What this module covers

Lesson	What you will learn
Create a Flag	How to create a Feature in the GrowthBook UI and read it via the SDK
Flag Types	Boolean, string, number, and JSON value types — when to use each
Environments	Dev / staging / production — independent flag states and SDK keys
Default Values	What the SDK returns when no rule matches, and why a safe fallback matters

A first taste: reading a flag in code

GrowthBook ships SDKs for every major language and framework. Here is the minimal Node.js pattern:

import { GrowthBook } from '@growthbook/growthbook';

const gb = new GrowthBook({
  apiHost: 'https://cdn.growthbook.io',
  clientKey: 'sdk-abc123',
});

await gb.init({ timeout: 2000 });

if (gb.isOn('new-checkout')) {
  // show the new checkout flow
} else {
  // show the current checkout flow
}

The call gb.isOn('new-checkout') asks GrowthBook: “is the new-checkout flag on for the current user context?” The answer comes from the rules you configure in the GrowthBook UI — your code never hard-codes it.

import { GrowthBook } from '@growthbook/growthbook';

const gb = new GrowthBook({
  apiHost: 'https://cdn.growthbook.io',
  clientKey: 'sdk-abc123',
});

await gb.init({ timeout: 2000 });

if (gb.isOn('new-checkout')) {
  // new checkout flow
} else {
  // current checkout flow
}