JavaScript SDK Reference

info

For JavaScript SSR (Server-Side Rendered) applications we recommend using ConfigCat JS-SSR SDK.

Getting started

1. Install and import package

npm i configcat-js

import * as configcat from 'configcat-js';

<script
  type="text/javascript"
  src="https://cdn.jsdelivr.net/npm/configcat-js@latest/dist/configcat.min.js"
></script>

2. Create the ConfigCat client with your SDK Key:

const configCatClient = configcat.getClient('#YOUR-SDK-KEY#');

3. Get your setting value

The async/await way:

const value = await configCatClient.getValueAsync('isMyAwesomeFeatureEnabled', false);

if (value) {
  do_the_new_thing();
} else {
  do_the_old_thing();
}

(Please note that top-level await in modules may not be available in older browsers. If you need to target such browser versions, you will need to use Promises or wrap your code in an async function or configure your build tools to downlevel this language feature.)

The Promise way:

configCatClient
  .getValueAsync('isMyAwesomeFeatureEnabled', false)
  .then((value) => {
    if (value) {
      do_the_new_thing();
    } else {
      do_the_old_thing();
    }
  });

4. Dispose the ConfigCat client

You can safely dispose all clients at once or individually and release all associated resources on application exit.

configcat.disposeAllClients(); // disposes all clients
// -or-
configCatClient.dispose(); // disposes a specific client

Working Demo on CodePen

See the Pen ConfigCat Feature Flag Demo on CodePen.

Creating the ConfigCat Client

ConfigCat Client is responsible for:

managing the communication between your application and ConfigCat servers.
caching your setting values and feature flags.
serving values quickly in a failsafe way.

configcat.getClient('<sdkKey>') returns a client with default options.

The getClient function has optional parameters, which can be used to adjust the behavior of the client.

Parameters	Description	Default
`sdkKey`	REQUIRED. SDK Key to access your feature flags and configurations. Get it from ConfigCat Dashboard.	-
`pollingMode`	Optional. The polling mode to use to acquire the setting values from the ConfigCat servers. More about polling modes.	`PollingMode.AutoPoll`
`options`	Optional. The options object. See the table below.	-

The available options depends on the chosen polling mode. However, there are some common options which can be set in the case of every polling mode:

Option Parameter	Description	Default
`logger`	Custom `IConfigCatLogger` implementation for tracing.	`ConfigCatConsoleLogger` (with WARN level)
`requestTimeoutMs`	The amount of milliseconds the SDK waits for a response from the ConfigCat servers before returning values from the cache.	30000
`baseUrl`	Sets the CDN base url (forward proxy, dedicated subscription) from where the SDK will download the config JSON.
`dataGovernance`	Describes the location of your feature flag and setting data within the ConfigCat CDN. This parameter needs to be in sync with your Data Governance preferences. More about Data Governance. Available options: `DataGovernance.Global`, `DataGovernance.EuOnly`.	`DataGovernance.Global`
`cache`	Custom `IConfigCatCache` implementation for caching the downloaded config.	`InMemoryConfigCache`
`flagOverrides`	Local feature flag & setting overrides. More about feature flag overrides.	-
`defaultUser`	Sets the default user. More about default user.	`undefined` (none)
`offline`	Determines whether the client should be initialized to offline mode. More about offline mode.	`false`

Options also include a property named setupHook, which you can use to subscribe to the hooks (events) at the time of initialization. More about hooks.

For example:

const configCatClient = configcat.getClient(
  '#YOUR-SDK-KEY#',
  configcat.PollingMode.AutoPoll,
  {
    setupHooks: (hooks) =>
      hooks.on('clientReady', () => console.log('Client is ready!')),
  },
);

info

You can acquire singleton client instances for your SDK keys using the configcat.getClient(sdkKey: "<sdkKey>") factory function. (However, please keep in mind that subsequent calls to getClient() with the same SDK Key return a shared client instance, which was set up by the first call.)

You can close all open clients at once using the configcat.disposeAllClients() function or do it individually using the configCatClient.dispose() method.

Anatomy of `getValueAsync()`

Returns a Promise with the value.

Parameters	Description
`key`	REQUIRED. The key of a specific setting or feature flag. Set on ConfigCat Dashboard for each setting.
`defaultValue`	REQUIRED. This value will be returned in case of an error.
`user`	Optional, User Object. Essential when using Targeting. Read more about Targeting.

const value = await configCatClient.getValueAsync(
  'keyOfMyFeatureFlag', // Setting Key
  false, // Default value
  new configcat.User('#UNIQUE-USER-IDENTIFIER#'), // Optional User Object
);

configCatClient
  .getValueAsync(
    'keyOfMyFeatureFlag', // Setting Key
    false, // Default value
    new configcat.User('#UNIQUE-USER-IDENTIFIER#'), // Optional User Object
  )
  .then((value) => {
    console.log(value);
  });

caution

It is important to provide an argument for the defaultValue parameter that matches the type of the feature flag or setting you are evaluating. Please refer to the following table for the corresponding types.

Setting Kind	`typeof defaultValue`
On/Off Toggle	`boolean`
Text	`string`
Whole Number	`number`
Decimal Number	`number`

In addition to the types mentioned above, you also have the option to provide null or undefined for the defaultValue parameter regardless of the setting kind. However, if you do so, the return type of the getValue method will be

boolean | string | number | null when defaultValue is null or
boolean | string | number | undefined when defaultValue is undefined.

This is because in these cases the exact return type cannot be determined at compile-time as the TypeScript compiler has no information about the setting type.

It's important to note that providing any other type for the defaultValue parameter will result in a TypeError.

If you specify an allowed type but it mismatches the setting kind, an error message will be logged and defaultValue will be returned.

Anatomy of `getValueDetailsAsync()`

getValueDetailsAsync() is similar to getValueAsync() but instead of returning the evaluated value only, it provides more detailed information about the evaluation result.

Parameters	Description
`key`	REQUIRED. The key of a specific setting or feature flag. Set on ConfigCat Dashboard for each setting.
`defaultValue`	REQUIRED. This value will be returned in case of an error.
`user`	Optional, User Object. Essential when using Targeting. Read more about Targeting.

const details = await configCatClient.getValueDetailsAsync(
  'keyOfMyFeatureFlag', // Setting Key
  false, // Default value
  new configcat.User('#UNIQUE-USER-IDENTIFIER#'), // Optional User Object
);

configCatClient
  .getValueDetailsAsync(
    'keyOfMyFeatureFlag', // Setting Key
    false, // Default value
    new configcat.User('#UNIQUE-USER-IDENTIFIER#'), // Optional User Object
  )
  .then((details) => {
    console.log(details);
  });

caution

It is important to provide an argument for the defaultValue parameter that matches the type of the feature flag or setting you are evaluating. Please refer to this table for the corresponding types.

The details result contains the following information:

Field	Type	Description
`key`	`string`	The key of the evaluated feature flag or setting.
`value`	`boolean` / `string` / `number`	The evaluated value of the feature flag or setting.
`user`	`User`	The user object used for the evaluation.
`isDefaultValue`	`boolean`	True when the default value passed to `getValueDetailsAsync()` is returned due to an error.
`errorMessage`	`string`	In case of an error, this field contains the error message.
`errorException`	`any`	In case of an error, this field contains the related exception object (if any).
`matchedEvaluationRule`	`RolloutRule`	If the evaluation was based on a targeting rule, this field contains that specific rule.
`matchedEvaluationPercentageRule`	`RolloutPercentageItem`	If the evaluation was based on a percentage rule, this field contains that specific rule.
`fetchTime`	`Date`	The last download time (UTC) of the current config.

User Object

The User Object is essential if you'd like to use ConfigCat's Targeting feature.

For simple targeting:

let userObject = new configcat.User('#UNIQUE-USER-IDENTIFIER#');

let userObject = new configcat.User('[email protected]');

Parameters	Description
`identifier`	REQUIRED. Unique identifier of a user in your application. Can be any `string` value, even an email address.
`email`	Optional parameter for easier targeting rule definitions.
`country`	Optional parameter for easier targeting rule definitions.
`custom`	Optional `dictionary of strings` representing the custom attributes of a user for advanced targeting rule definitions. e.g. User role, Subscription type.

For advanced targeting:

let userObject = new configcat.User(
  /* identifier: */ '#UNIQUE-USER-IDENTIFIER#',
  /*      email: */ '[email protected]',
  /*    country: */ 'United Kingdom',
  /*     custom: */ {
    SubscriptionType: 'Pro',
    UserRole: 'Admin',
  },
);

Default user

It's possible to set a default user object that will be used on feature flag and setting evaluation. It can be useful when your application has a single user only or rarely switches users.

You can set the default user object either on SDK initialization:

const configCatClient = configcat.getClient(
  '#YOUR-SDK-KEY#',
  configcat.PollingMode.AutoPoll,
  {
    defaultUser: new configcat.User('[email protected]'),
  },
);

...or using the setDefaultUser() method of the configCatClient object:

configCatClient.setDefaultUser(new configcat.User('[email protected]'));

Whenever the evaluation methods like getValueAsync(), getValueDetailsAsync(), etc. are called without an explicit user object parameter, the SDK will automatically use the default user as a user object.

const user = new configcat.User('[email protected]');
configCatClient.setDefaultUser(user);

// The default user will be used in the evaluation process.
const value = await configCatClient.getValueAsync('keyOfMyFeatureFlag', false);

When a user object parameter is passed to the evaluation methods, it takes precedence over the default user.

const user = new configcat.User('[email protected]');
configCatClient.setDefaultUser(user);

const otherUser = new configcat.User('[email protected]');

// otherUser will be used in the evaluation process.
const value = await configCatClient.getValueAsync('keyOfMyFeatureFlag', false, otherUser);

You can also remove the default user by doing the following:

configCatClient.clearDefaultUser();

Polling Modes

The ConfigCat SDK supports 3 different polling mechanisms to acquire the setting values from ConfigCat. After latest setting values are downloaded, they are stored in the local cache then all getValueAsync() calls are served from there. With the following polling modes, you can customize the SDK to best fit to your application's lifecycle. More about polling modes.

Auto polling (default)

The ConfigCat SDK downloads the latest values and stores them automatically every 60 seconds.

Use the pollIntervalSeconds option parameter to change the polling interval.

const configCatClient = configcat.getClient(
  '#YOUR-SDK-KEY#',
  configcat.PollingMode.AutoPoll,
  {
    pollIntervalSeconds: 95,
  },
);

Available options (in addition to the common ones):

Option Parameter	Description	Default
`pollIntervalSeconds`	Polling interval in seconds.	60s
`maxInitWaitTimeSeconds`	Maximum waiting time between the client initialization and the first config acquisition in seconds.	5s

Lazy loading

When calling getValueAsync() the ConfigCat SDK downloads the latest setting values if they are not present or expired in the cache. In this case getValueAsync() will return the setting value after the cache is updated.

Use cacheTimeToLiveSeconds option parameter to set cache lifetime.

const configCatClient = configcat.getClient(
  '#YOUR-SDK-KEY#',
  configcat.PollingMode.LazyLoad,
  {
    cacheTimeToLiveSeconds: 600,
  },
);

Available options (in addition to the common ones):

Option Parameter	Description	Default
`cacheTimeToLiveSeconds`	Cache TTL in seconds.	60s

Manual polling

Manual polling gives you full control over when the config JSON (with the setting values) is downloaded. ConfigCat SDK will not update them automatically. Calling forceRefreshAsync() is your application's responsibility.

const configCatClient = configcat.getClient(
  '#YOUR-SDK-KEY#',
  configcat.PollingMode.ManualPoll,
);

await configCatClient.forceRefreshAsync();
let value = await configCatClient.getValueAsync('keyOfMyTextSetting', 'my default value');
console.log(value);

getValueAsync() returns defaultValue if the cache is empty. Call forceRefreshAsync() to update the cache.

const configCatClient = configcat.getClient(
  '#YOUR-SDK-KEY#',
  configcat.PollingMode.ManualPoll,
);

let value = await configCatClient.getValueAsync('keyOfMyTextSetting', 'my default value');
console.log(value); // console: "my default value"

await configCatClient.forceRefreshAsync();
value = await configCatClient.getValueAsync('keyOfMyTextSetting', 'my default value');
console.log(value);

Hooks

The SDK provides several hooks (events), by means of which you can get notified of its actions. You can subscribe to the following events emitted by the client:

clientReady: This event is emitted when the SDK reaches the ready state. If the SDK is set up to use lazy load or manual polling, it's considered ready right after instantiation. If auto polling is used, the ready state is reached when the SDK has a valid config JSON loaded into memory either from cache or from HTTP. If the config couldn't be loaded neither from cache nor from HTTP, the clientReady event fires when the auto polling's MaxInitWaitTime has passed.
configChanged: This event is emitted first when the SDK loads a valid config JSON into memory from cache, then each time afterwards when a config JSON with changed content is downloaded via HTTP.
flagEvaluated: This event is emitted each time when the SDK evaluates a feature flag or setting. The event provides the same evaluation details that you would get from getValueDetailsAsync().
clientError: This event is emitted when an error occurs within the ConfigCat SDK.

You can subscribe to these events either on initialization:

const configCatClient = configcat.getClient(
  '#YOUR-SDK-KEY#',
  configcat.PollingMode.ManualPoll,
  {
    setupHooks: (hooks) =>
      hooks.on('flagEvaluated', () => {
        /* handle the event */
      }),
  },
);

...or directly on the ConfigCatClient instance:

configCatClient.on('flagEvaluated', () => {
  /* handle the event */
});

Online / Offline mode

In cases where you want to prevent the SDK from making HTTP calls, you can switch it to offline mode:

configCatClient.setOffline();

In offline mode, the SDK won't initiate HTTP requests and will work only from its cache.

To switch the SDK back to online mode, do the following:

configCatClient.setOnline();

Using the configCatClient.isOffline property you can check whether the SDK is in offline mode.

Flag Overrides

With flag overrides you can overwrite the feature flags & settings downloaded from the ConfigCat CDN with local values. Moreover, you can specify how the overrides should apply over the downloaded values. The following 3 behaviours are supported:

Local only (OverrideBehaviour.LocalOnly): When evaluating values, the SDK will not use feature flags & settings from the ConfigCat CDN, but it will use all feature flags & settings that are loaded from local-override sources.
Local over remote (OverrideBehaviour.LocalOverRemote): When evaluating values, the SDK will use all feature flags & settings that are downloaded from the ConfigCat CDN, plus all feature flags & settings that are loaded from local-override sources. If a feature flag or a setting is defined both in the downloaded and the local-override source then the local-override version will take precedence.
Remote over local (OverrideBehaviour.RemoteOverLocal): When evaluating values, the SDK will use all feature flags & settings that are downloaded from the ConfigCat CDN, plus all feature flags & settings that are loaded from local-override sources. If a feature flag or a setting is defined both in the downloaded and the local-override source then the downloaded version will take precedence.

You can set up the SDK to load your feature flag & setting overrides from a { [name: string]: any } map.

const configCatClient = configcat.getClient(
  '#YOUR-SDK-KEY#',
  configcat.PollingMode.AutoPoll,
  {
    flagOverrides: configcat.createFlagOverridesFromMap(
      {
        enabledFeature: true,
        disabledFeature: false,
        intSetting: 5,
        doubleSetting: 3.14,
        stringSetting: 'test',
      },
      configcat.OverrideBehaviour.LocalOnly,
    ),
  },
);

Logging

Setting log levels

const logger = configcat.createConsoleLogger(configcat.LogLevel.Info); // Setting log level to Info

const configCatClient = configcat.getClient(
  '#YOUR-SDK-KEY#',
  configcat.PollingMode.AutoPoll,
  {
    logger: logger,
  },
);

Available log levels:

Level	Description
Off	Nothing gets logged.
Error	Only error level events are logged.
Warn	Default. Errors and Warnings are logged.
Info	Errors, Warnings and feature flag evaluation is logged.
Debug	All of the above plus debug info is logged.

Info level logging helps to inspect the feature flag evaluation process:

ConfigCat - INFO - [5000] Evaluate 'isPOCFeatureEnabled'
 User : {"identifier":"#SOME-USER-ID#","email":"[email protected]"}
 Evaluating rule: '[email protected]' CONTAINS '@something.com' => no match
 Evaluating rule: '[email protected]' CONTAINS '@example.com' => MATCH
 Returning value : true

`getAllKeysAsync()`

You can query the keys from your configuration in the SDK with the getAllKeysAsync() method.

const configCatClient = configcat.getClient('#YOUR-SDK-KEY#');

const keys = await configCatClient.getAllKeysAsync();
console.log(keys);

`getAllValuesAsync()`

Evaluates and returns the values of all feature flags and settings. Passing a User Object is optional.

const configCatClient = configcat.getClient('#YOUR-SDK-KEY#');

let settingValues = await configCatClient.getAllValuesAsync();
settingValues.forEach((i) =>
  console.log(i.settingKey + ' -> ' + i.settingValue),
);

// invoke with user object
const userObject = new configcat.User('[email protected]');

settingValues = await configCatClient.getAllValuesAsync(userObject);
settingValues.forEach((i) =>
  console.log(i.settingKey + ' -> ' + i.settingValue),
);

`getAllValueDetailsAsync()`

Evaluates and returns the values along with evaluation details of all feature flags and settings. Passing a User Object is optional.

const configCatClient = configcat.getClient('#YOUR-SDK-KEY#');

let settingValues = await configCatClient.getAllValueDetailsAsync();
settingValues.forEach((details) => console.log(details));

// invoke with user object
const userObject = new configcat.User('[email protected]');

settingValues = await configCatClient.getAllValueDetailsAsync(userObject);
settingValues.forEach((details) => console.log(details));

Snapshots and synchronous feature flag evaluation

On JavaScript platforms, the ConfigCat client provides only asynchronous methods for evaluating feature flags and settings because these operations may involve network communication (downloading config data from the ConfigCat CDN servers), which is necessarily an asynchronous operation in JavaScript.

However, there may be use cases where synchronous evaluation is preferable, thus, since v8.1.0, the JavaScript SDK provides a way to synchronously evaluate feature flags and settings via snapshots.

Using the snapshot() method, you can capture the current state of the ConfigCat client (including the latest downloaded config data) and you can use the resulting snapshot object to synchronously evaluate feature flags and settings based on the captured state:

const configCatClient = configcat.getClient("#YOUR-SDK-KEY#", configcat.PollingMode.ManualPoll);

// Make sure that the latest config data is available locally.
await configCatClient.forceRefreshAsync();

const snapshot = configCatClient.snapshot();

const user = new configcat.User('#UNIQUE-USER-IDENTIFIER#');
for (const key of snapshot.getAllKeys()) {
  const value = snapshot.getValue(key, null, user);
  console.log(`${key}: ${value}`);
}

In Auto Poll mode, you can use the waitForReady method to wait for that latest config data to become available locally:

const configCatClient = configcat.getClient("#YOUR-SDK-KEY#", configcat.PollingMode.AutoPoll);

// Make sure that the latest config data is available locally.
await configCatClient.waitForReady();

const snapshot = configCatClient.snapshot();

Using custom cache implementation

The ConfigCat SDK stores the downloaded config data in a local cache to minimize network traffic and enhance client performance. If you prefer to use your own cache solution, such as an external or distributed cache in your system, you can implement the IConfigCatCache interface and set the cache property in the options passed to getClient. This allows you to seamlessly integrate ConfigCat with your existing caching infrastructure.

class MyCustomCache implements IConfigCatCache {
  set(key: string, value: string): Promise<void> | void {
    // insert your cache write logic here
  }

  get(key: string): Promise<string | null | undefined> | string | null | undefined {
    // insert your cache read logic here
  }
}

function MyCustomCache() { }

MyCustomCache.prototype.set = function (key, value) {
  // insert your cache write logic here
};
MyCustomCache.prototype.get = function (key) {
  // insert your cache read logic here
};

then

// set the `MyCustomCache` implementation on creating the client instance

const configCatClient = configcat.getClient(
  '#YOUR-SDK-KEY#',
  configcat.PollingMode.AutoPoll,
  {
    cache: new MyCustomCache(),
  },
);

info

The JavaScript SDK supports shared caching. You can read more about this feature and the required minimum SDK versions here.

Sensitive information handling

The frontend/mobile SDKs are running in your users' browsers/devices. The SDK is downloading a config JSON file from ConfigCat's CDN servers. The URL path for this config JSON file contains your SDK key, so the SDK key and the content of your config JSON file (feature flag keys, feature flag values, targeting rules, % rules) can be visible to your users. This SDK key is read-only, it only allows downloading your config JSON file, but nobody can make any changes with it in your ConfigCat account.

If you do not want to expose the SDK key or the content of the config JSON file, we recommend using the SDK in your backend components only. You can always create a backend endpoint using the ConfigCat SDK that can evaluate feature flags for a specific user, and call that backend endpoint from your frontend/mobile applications.

Also, we recommend using confidential targeting comparators in the targeting rules of those feature flags that are used in the frontend/mobile SDKs.

Browser compatibility

This SDK should be compatible with all modern browsers.

The SDK is tested against the following browsers:

Chrome (stable, latest, beta)
Chromium (64.0.3282.0, 72.0.3626.0, 80.0.3987.0)
Firefox (latest, latest-beta, 84.0).

These tests are running on each pull request, before each deploy, and on a daily basis. You can view a sample run here.

Sample Applications

Guides

See the guides on how to use ConfigCat's JavaScript SDK with the following libraries and frameworks:

Getting started​

1. Install and import package​

2. Create the ConfigCat client with your SDK Key:​

3. Get your setting value​

4. Dispose the ConfigCat client​

Working Demo on CodePen​

Creating the ConfigCat Client​

Anatomy of getValueAsync()​

Anatomy of getValueDetailsAsync()​

User Object​

Default user​

Polling Modes​

Auto polling (default)​

Lazy loading​

Manual polling​

Hooks​

Online / Offline mode​

Flag Overrides​

Logging​

Setting log levels​

getAllKeysAsync()​

getAllValuesAsync()​

getAllValueDetailsAsync()​

Snapshots and synchronous feature flag evaluation​

Using custom cache implementation​

Sensitive information handling​

Browser compatibility​

Sample Applications​

Guides​

Look under the hood​

Getting started

1. Install and import package

2. Create the ConfigCat client with your SDK Key:

3. Get your setting value

4. Dispose the ConfigCat client

Working Demo on CodePen

Creating the ConfigCat Client

Anatomy of `getValueAsync()`

Anatomy of `getValueDetailsAsync()`

User Object

Default user

Polling Modes

Auto polling (default)

Lazy loading

Manual polling

Hooks

Online / Offline mode

Flag Overrides

Logging

Setting log levels

`getAllKeysAsync()`

`getAllValuesAsync()`

`getAllValueDetailsAsync()`

Snapshots and synchronous feature flag evaluation

Using custom cache implementation

Sensitive information handling

Browser compatibility

Sample Applications

Guides

Look under the hood