Skip to main content

Flagsmith Javascript SDK

This library can be used with pure Javascript, React (and all other popular frameworks/libraries) and React Native projects. The source code for the client is available on Github.

Example applications for a variety of Javascript frameworks such as React, Vue and Angular, as well as React Native, can be found here:



npm i flagsmith --save

Via JavaScript CDN#

<script src=""></script>

NPM for React Native#

npm i react-native-flagsmith --save

Basic Usage#

The SDK is initialised against a single environment within a project on, for example the Development or Production environment. You can find your environment key in the Environment settings page.

Example: Initialising the SDK#

import flagsmith from 'flagsmith or react-native-flagsmith'; //Add this line if you're using flagsmith via npm
environmentID: '<YOUR_ENVIRONMENT_KEY>',
// api:"http://localhost:8000/api/v1/" set this if you are self hosting, and point it to your API
cacheFlags: true, // stores flags in localStorage cache
enableAnalytics: true, // See for more info
onChange: (oldFlags, params) => {
//Occurs whenever flags are changed
const { isFromServer } = params; //determines if the update came from the server or local cached storage
//Check for a feature
if (flagsmith.hasFeature('my_cool_feature')) {
//Or, use the value of a feature
const bannerSize = flagsmith.getValue('banner_size');
//Check whether value has changed
const bannerSizeOld = oldFlags['banner_size'] && oldFlags['banner_size'].value;
if (bannerSize !== bannerSizeOld) {

Identifying users#

Identifying users allows you to target specific users from the Flagsmith dashboard and configure features and traits. You can call this before or after you initialise the project, calling it after will re-fetch features from the API.

User features can be managed by navigating to users on for your desired project.

Example: Initialising the SDK and identifying as a user#

import flagsmith from 'flagsmith';
Can be called both before or after you're done initialising the project.
Calling identify before will prevent flags being fetched twice.
flagsmith.identify('bullet_train_sample_user'); //This will create a user in the dashboard if they don't already exist
//Standard project initialisation
environmentID: 'QjgYur4LQTwe5HpvbvhpzK',
onChange: (oldFlags, params) => {
//Occurs whenever flags are changed
const { isFromServer } = params; //determines if the update came from the server or local cached storage
//Set a trait against the Identity
flagsmith.setTrait('favourite_colour', 'blue'); //This save the trait against the user, it can be queried with flagsmith.getTrait
//Check for a feature
if (flagsmith.hasFeature('my_power_user_feature')) {
//Check for a trait
if (!flagsmith.getTrait('accepted_cookie_policy')) {
//Or, use the value of a feature
const myPowerUserFeature = flagsmith.getValue('my_power_user_feature');
//Check whether value has changed
const myPowerUserFeatureOld = oldFlags['my_power_user_feature'] && oldFlags['my_power_user_feature'].value;
if (myPowerUserFeature !== myPowerUserFeatureOld) {

API Reference#

See all available types here.

Initialisation options#

PropertyDescriptionRequiredDefault Value
environmentID: stringDefines which project environment you wish to get flags for. example ACME Project - Staging.YESnull
onChange?: (flags:IFlags, params:IRetrieveInfo)=> voidYour callback function for when the flags are retrieved (flags,{isFromServer:true/false})=>{...}YESnull
onError?: (res:{message:string}) => voidCallback function on failure to retrieve flags. (error)=>{...}null
asyncStorage?:anyNeeded for cacheFlags option, used to tell the library what implementation of AsyncStorage your app uses, i.e. ReactNative.AsyncStorage vs @react-native-community/async-storage, for web this defaults to an internal implementation.null
cacheFlags?: booleanAny time flags are retrieved they will be cached, flags and identities will then be retrieved from local storage before hitting the API ```null
enableAnalytics?: booleanEnable sending flag analytics for getValue and hasFeature evaluations.false
enableLogs?: booleanEnables logging for key Flagsmith eventsnull
defaultFlags?: IFlagsAllows you define default features, these will all be overridden on first retrieval of features.null
preventFetch?: booleanIf you want to disable fetching flags and call getFlags later.false
state?: IStateSet a predefined state, useful for SSR / isomorphic applications.false
api?: stringUse this property to define where you're getting feature flags from, e.g. if you're self hosting.

Available Functions#

init(initialisationOptions)=> Promise<void>Initialise the sdk against a particular environment
hasFeature(key:string)=> booleanGet the value of a particular feature e.g. flagsmith.hasFeature("powerUserFeature") // true
getValue(key:string)=> string|number|booleanGet the value of a particular feature e.g. flagsmith.getValue("font_size") // 10
getTrait(key:string)=> string|number|booleanOnce used with an identified user you can get the value of any trait that is set for them e.g. flagsmith.getTrait("accepted_cookie_policy")
getState()=>IStateRetrieves the current state of flagsmith, useful in NextJS / isomorphic applications. flagsmith.getState()
setTrait(key:string, value:string|number|boolean)=> Promise<IFlags>Once used with an identified user you can set the value of any trait relevant to them e.g. flagsmith.setTrait("accepted_cookie_policy", true)
setTraits(values:Record<string, string|number|boolean>)=> Promise<IFlags>Set multiple traits e.g. flagsmith.setTraits({foo:"bar",numericProp:1,boolProp:true}). Setting a value of null for a trait will remove that trait.
incrementTrait(key:string, value:number)=> Promise<IFlags>You can also increment/decrement a particular trait them e.g. flagsmith.incrementTrait("click_count", 1)
startListening(ticks=1000:number)=>voidPoll the api for changes every x milliseconds
stopListening()=>voidStop polling the api
getFlags()=> Promise<IFlags>Trigger a manual fetch of the environment features, if a user is identified it will fetch their features. Resolves a promise when the flags are updated.
identify(userId:string, traits?:Record<string, string|number|boolean>)=> Promise<IFlags>Identify as a user, optionally with traits e.g. {foo:"bar",numericProp:1,boolProp:true}. This will create a user for your environment in the dashboard if they don't exist, it will also trigger a call to getFlags(), resolves a promise when the flags are updated.
logout()=>Promise<IFlags>Stop identifying as a user, this will trigger a call to getFlags()

Multiple SDK Instances#

Version 1.5 and above allows you to create multiple instances of the Flagsmith SDK. This may be used when you wish to identify multiple users simultaneously within your app and retain access to getValue, hasFeature etc for each user.


export function createFlagsmithInstance (): IFlagsmith<br class="Apple-interchange-newline">


import { createFlagsmithInstance } from 'flagsmith';
const flagsmith = createFlagsmithInstance();
const flagsmithB = createFlagsmithInstance();
// now you can use flagsmith as before but in its own instance


How do I call identify, setTraits etc alongside init?

  • init should be called once in your application, we recommend you call init before any other flagsmith call.
  • init retrieves flags by default, you can turn this off with the preventFetch option to init. This is useful for when you know you're identifying a user straight after.

When does onChange callback?

onChange calls when flags are fetched this can be a result of:

  • init
  • setTrait
  • incrementTrait
  • getFlags
  • identify
  • flags evaluated by local storage

Using onChange is best used in combination with your application's state management e.g. onChange triggering an action to re-evaluate flags with hasFeature and getValue.

However, if this does not fit in with your development pattern, all the above flagsmith functions return a promise that resolves when fresh flags have been retrieved.

For example by doing the following:

await flagsmith.setTrait('age', 21);
const hasFeature = flagsmith.hasFeature('my_feature');

On change calls back with information telling you what has changed, you can use this to prevent any unnecessary re-renders.

onChange(this.oldFlags, {
isFromServer: true, // flags have come from the server or local storage
flagsChanged: deepEqualsCheck(oldFlags, newFlags),
traitsChanged: deepEqualsCheck(oldFlags, newFlags),

How does caching flags work?

If the cacheFlags is set to true on init, the SDK will cache flag evaluations in local async storage. Upon reloading the browser, an onChange event will be fired immediately with the local storage flags. The flow for this is as follows

  1. init is called

  2. if cacheFlags is enabled, local storage checks for any stored flags and traits.

  3. if flags have been found in local storage, onChange is triggered with the stored flags.

  4. at the same time, fresh flags will be retrieved which will result in another onChange callback.

  5. whenever flags have been retrieved local storage will be updated.

By default, these flags will be persisted indefinitely, you can clear this by removing "BULLET_TRAIN_DB" from localStorage.