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GrowthBook — From Zero to Hero

Default Values

Default value vs override rules

Section titled “Default value vs override rules”

Every GrowthBook Feature has two layers of value resolution:

  1. Override rules — conditions, percentage rollouts, or experiment assignments you configure per environment. When a rule matches the current user context, it wins.
  2. Default value — the value returned when no rule matches. This is your safe floor.

When GrowthBook is completely unreachable (network error, CDN outage), the SDK falls back to the fallback argument you pass directly in your code — it never returns null or throws.

The three fallback layers

Section titled “The three fallback layers”
flowchart TD
  call["SDK call"] --> loaded{"GrowthBook loaded?"}
  loaded -- "YES" --> rules["Evaluate rules"]
  loaded -- "NO" --> fb["Return the fallback argument from your code"]
  rules --> match{"Match found?"}
  match -- "Yes" --> rv["Return rule value"]
  match -- "No match" --> dv["Return Feature default value"]
The three fallback layers

Always pass a sensible fallback

Section titled “Always pass a sensible fallback”
// BAD — if GrowthBook is unreachable, getFeatureValue returns undefined
const text = gb.getFeatureValue('banner-text');


// GOOD — safe fallback keeps the app working
const text = gb.getFeatureValue('banner-text', 'Welcome');

The fallback argument is the last line of defence. It should represent the experience you want users to have if GrowthBook is down — usually the current, stable behaviour.

Setting the default value in GrowthBook

Section titled “Setting the default value in GrowthBook”
  1. Open a Feature in the GrowthBook UI.
  2. Click Edit next to the Default Value field.
  3. Enter the value, e.g. Welcome for a string flag.
  4. Click Save.

The default value takes effect immediately for all environments where no override rule produces a match.

Changing the default value

Section titled “Changing the default value”

To update a default value without adding a rule:

  1. Open the Feature.
  2. Click Edit on the Default Value.
  3. Change the value and click Save.

The change is applied globally across every environment that does not have an overriding rule. No deploy required.

Worked example — banner text

Section titled “Worked example — banner text”
import { GrowthBook } from '@growthbook/growthbook';


const gb = new GrowthBook({
  apiHost: 'https://cdn.growthbook.io',
  clientKey: process.env.GROWTHBOOK_CLIENT_KEY,
  attributes: {
    id: 'user-123',
    country: 'TH',
  },
});


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


// Returns the GrowthBook default value when no rule matches,
// or 'Welcome' when GrowthBook cannot be reached.
const bannerText = gb.getFeatureValue('banner-text', 'Welcome');


document.getElementById('banner').textContent = bannerText;
import { GrowthBook } from '@growthbook/growthbook';

const gb = new GrowthBook({
  apiHost: 'https://cdn.growthbook.io',
  clientKey: process.env.GROWTHBOOK_CLIENT_KEY,
  attributes: {
    id: 'user-123',
    country: 'TH',
  },
});

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

// Safe fallback: 'Welcome' is used only if GrowthBook
// cannot be reached at all.
const bannerText = gb.getFeatureValue('banner-text', 'Welcome');

document.getElementById('banner').textContent = bannerText;
What value does the SDK return when a Feature exists in GrowthBook but no override rule matches?
What value does the SDK return when GrowthBook cannot be reached at all?
Which SDK call is MISSING a safe fallback and could return undefined?
What should the fallback argument represent?