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Install the latest version

Use your favorite package manager to install @prefab-cloud/prefab-cloud-js npm | github

npm install @prefab-cloud/prefab-cloud-js

TypeScript types are included with the package.


If you're using React, consider using our React Client instead.

Initialize the client

Initialize prefab with your api key:

import { prefab } from "@prefab-cloud/prefab-cloud-js";

const options = {

await prefab.init(options);

prefab.init will request the calculated feature flags for the provided context as a single HTTPS request. If you need to check for updates to feature flag values, you can learn more about polling below.

You aren't required to await the init -- it is a promise, so you can use .then, .finally, .catch, etc. instead if you prefer.


While prefab is loading, isEnabled will return false, get will return undefined, and shouldLog will use your defaultLevel.

Feature Flags

Now you can use prefab's feature flag evaluation, e.g.

if (prefab.isEnabled('cool-feature') {
// ... this code only evaluates if `cool-feature` is enabled for the current context

You can also use get to access the value of non-boolean flags

const stringValue = prefab.get("my-string-flag");


Context accepts an object with keys that are context names and key value pairs with attributes describing the context. You can use this to write targeting rules, e.g. segment your users.

import { prefab, Context } from "@prefab-cloud/prefab-cloud-js";

const options = {
context: new Context({
user: { key: "abcdef", email: "" },
device: { key: "hijklm", mobile: true },

await prefab.init(options);


After prefab.init(), you can start polling. Polling uses the context you defined in init by default. You can update the context for future polling by setting it on the prefab object.

// some time after init
prefab.poll({frequencyInMs: 300000})

// we're now polling with the context used from `init`

// later, perhaps after a visitor logs in and now you have the context of their current user
prefab.context = new Context({...prefab.context, user: { email:, key: user.trackingId })

// future polling will use the new context

Dynamic Config


JavaScript is a Client SDK and does not receive Config. Learn more about Client SDKs

Dynamic Logging

shouldLog allows you to implement dynamic logging. It takes the following properties:


If you've configured a level value for loggerName (or a parent in the dot-notation hierarchy like "my.corp.widgets") then that value will be used for comparison against the desiredLevel. If no configured level is found in the hierarchy for loggerName then the provided defaultLevel will be compared against desiredLevel.

If desiredLevel is greater than or equal to the comparison severity, then shouldLog returns true. If the desiredLevel is less than the comparison severity, then shouldLog will return false.

Example usage:

const desiredLevel = "info";
const defaultLevel = "error";
const loggerName = "my.corp.widgets.modal";

if (shouldLog({ loggerName, desiredLevel, defaultLevel })) {"...");

If no log level value is configured in Prefab for "my.corp.widgets.modal" or higher in the hierarchy, then the will not happen. If the value is configured and is INFO or more verbose, the will happen.


In your test suite, you should skip prefab.init altogether and instead use prefab.setConfig to set up your test state.

it("shows the turbo button when the feature is enabled", () => {
turbo: true,
defaultMediaCount: 3,

const rendered = new MyComponent().render();

expect(rendered).toMatch(/Enable Turbo/);
expect(rendered).toMatch(/Media Count: 3/);


prefab Properties

isEnabledprefab.isEnabled("new-logo")returns a boolean (default false) if a feature is enabled based on the current context
getprefab.get('retry-count')returns the value of a flag or config evaluated in the current context
loadedif (prefab.loaded) { ... }a boolean indicating whether prefab content has loaded
shouldLogif (prefab.shouldLog(...)) {returns a boolean indicating whether the proposed log level is valid for the current context
pollprefab.poll({frequencyInMs})starts polling every frequencyInMs ms.
stopPollingprefab.stopPolling()stops the polling process
contextprefab.context = new Context(...)get or set the current context (after init())