JavaScript SDK Reference

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

ConfigCat JavaScript SDK on GitHub

Getting started

1. Install and import package

via NPM:

npm i configcat-js

import * as configcat from "configcat-js";

via CDN:

<script type="text/javascript" src="https://cdn.jsdelivr.net/npm/[email protected]/dist/configcat.min.js"></script>

2. Create the ConfigCat client with your SDK Key:

var configCatClient = configcat.createClient("#YOUR-SDK-KEY#");

3. Get your setting value

The Promise (async/await) way:

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

or the Callback way:

configCatClient.getValue("isMyAwesomeFeatureEnabled", false, (value) => {
    if(value) {
        do_the_new_thing();
    } else {
        do_the_old_thing();
    }
});

4. Dispose ConfigCat client

You can safely dispose() the client instance and release all associated resources on application exit.

configCatClient.dispose();

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.

createClient(sdkKey, options) returns a client with default options.

Properties	Description	Default
sdkKey	REQUIRED. SDK Key to access your feature flags and configurations. Get it from ConfigCat Dashboard.	-
options	Optional. More about the options parameter.	-

createClientWithAutoPoll(), createClientWithLazyLoad(), createClientWithManualPoll()
Creating the client is different for each polling mode. See below for details.

caution

We strongly recommend you to use the ConfigCat Client as a Singleton object in your application. If you want to use multiple SDK Keys in the same application, create only one ConfigCat Client per SDK Key.

Anatomy of getValue()

Parameters	Description
key	REQUIRED. Setting-specific key. Set on ConfigCat Dashboard for each setting.
defaultValue	REQUIRED. This value will be returned in case of an error.
callback	REQUIRED. Called with the actual setting value.
user	Optional, User Object. Essential when using Targeting. Read more about Targeting.

configCatClient.getValue(
    "keyOfMySetting", // Setting Key
    false, // Default value
    function(value) { console.log(value) }, // Callback function
    { identifier : "435170f4-8a8b-4b67-a723-505ac7cdea92" } // Optional User Object
);

Anatomy of getValueAsync()

Returns a Promise with the value.

Parameters	Description
key	REQUIRED. Setting-specific key. 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(
    "keyOfMySetting", // Setting Key
    false, // Default value
    { identifier : "435170f4-8a8b-4b67-a723-505ac7cdea92" } // Optional User Object
);

or

configCatClient.getValueAsync(
    "keyOfMySetting", // Setting Key
    false, // Default value
    { identifier : "435170f4-8a8b-4b67-a723-505ac7cdea92" }) // Optional User Object
.then((value) => { console.log(value) });

User Object

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

var userObject = {
    identifier : "435170f4-8a8b-4b67-a723-505ac7cdea92"
};   

or

var userObject = {
    identifier : "[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 for custom attributes of a user for advanced targeting rule definitions. e.g. User role, Subscription type.

For advanced targeting:

var userObject = {
    identifier : "435170f4-8a8b-4b67-a723-505ac7cdea92",
    email : "[email protected]",
    country : "United Kingdom",
    custom : {
        "SubscriptionType": "Pro",
        "UserRole": "Admin"
    }
};

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 internal cache then all getValue() 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.

createClientWithAutoPoll(sdkKey, options)

Option Parameter	Description	Default
pollIntervalSeconds	Polling interval. Range: 1 - Number.MAX_SAFE_INTEGER	60
configChanged	Callback to get notified about changes.	-
logger	Custom logger. See below for details.	Console logger
requestTimeoutMs	The amount of milliseconds the SDK waits for a response from the ConfigCat servers before returning values from the cache.	30000
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
maxInitWaitTimeSeconds	Maximum waiting time between the client initialization and the first config acquisition in seconds.	5
flagOverrides	Local feature flag & setting overrides. More about feature flag overrides.	-

Use the pollIntervalSeconds option parameter to change the polling interval.

configcat.createClientWithAutoPoll("#YOUR-SDK-KEY#", { pollIntervalSeconds: 95 });

Adding a callback to configChanged option parameter will get you notified about changes.

configcat.createClientWithAutoPoll("#YOUR-SDK-KEY#", { configChanged: function() {
    console.log("Your config has been changed!");
}});

Lazy loading

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

createClientWithLazyLoad(sdkKey, options)

Option Parameter	Description	Default
cacheTimeToLiveSeconds	Cache TTL. Range: 1 - Number.MAX_SAFE_INTEGER	60
logger	Custom logger. See below for details.	Console logger
requestTimeoutMs	The amount of milliseconds the SDK waits for a response from the ConfigCat servers before returning values from the cache.	30000
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
flagOverrides	Local feature flag & setting overrides. More about feature flag overrides.	-

Use cacheTimeToLiveSeconds option parameter to set cache lifetime.

configcat.createClientWithLazyLoad("#YOUR-SDK-KEY#", { cacheTimeToLiveSeconds: 600 });

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 forceRefresh() or forceRefreshAsync() is your application's responsibility.

createClientWithManualPoll(sdkKey, options)

Option Parameter	Description	Default
logger	Custom logger. See below for details.	Console logger
requestTimeoutMs	The amount of milliseconds the SDK waits for a response from the ConfigCat servers before returning values from the cache.	30000
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
flagOverrides	Local feature flag & setting overrides. More about feature flag overrides.	-

const configCatClient = configcat.createClientWithManualPoll("#YOUR-SDK-KEY#");

configCatClient.forceRefresh(() =>{

    configCatClient.getValue("key", "my default value", (value)=>{
        
        console.log(value);
    });
});

getValue() returns defaultValue if the cache is empty. Call forceRefresh() or forceRefreshAsync() to update the cache.

const configCatClient = configcat.createClientWithManualPoll("#YOUR-SDK-KEY#");

configCatClient.getValue("key", "my default value", (value)=>{

    console.log(value) // console: "my default value"
});

configCatClient.forceRefresh(() =>{

    configCatClient.getValue("key", "my default value", (value)=>{

        console.log(value); // console: "value from server"
    });
});

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/Offline mode (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.createClientWithAutoPoll('#YOUR-SDK-KEY#', { 
    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(3); // Setting log level to 3 (= Info)

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

Available log levels:

Level	Name	Description
-1	Off	Nothing gets logged.
1	Error	Only error level events are logged.
2	Warn	Errors and Warnings are logged.
3	Info	Errors, Warnings and feature flag evaluation is logged.
4	Debug	All of the above plus debug info is logged.

You can use LogLevel enum type from configcat-common package:

import { LogLevel } from 'configcat-common';

const logger = configcat.createConsoleLogger(LogLevel.Info);

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

ConfigCat - INFO - 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

getAllKeys(), getAllKeysAsync()

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

const configCatClient = configcat.createClient("#YOUR-SDK-KEY#");
configCatClient.getAllKeys(function(keys) {
    console.log(keys);
});

const configCatClient = configcat.createClient("#YOUR-SDK-KEY#");
const keys = await configCatClient.getAllKeysAsync();
console.log(keys);

getAllValues(), getAllValuesAsync()

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

const configCatClient = configcat.createClient("#YOUR-SDK-KEY#");

configCatClient.getAllValues(function(settingValues) {
    settingValues.forEach(i => console.log(i.settingKey + ' -> ' + i.settingValue));
});

// invoke with user object
const userObject = { identifier : "[email protected]" }; 
configCatClient.getAllValues(function(settingValues) {
    settingValues.forEach(i => console.log(i.settingKey + ' -> ' + i.settingValue));
}, userObject);

const configCatClient = configcat.createClient("#YOUR-SDK-KEY#");

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

// invoke with user object
const userObject = { identifier : "[email protected]" }; 
const settingValuesTargeting = await configCatClient.getAllValuesAsync(userObject);
settingValuesTargeting.forEach(i => console.log(i.settingKey + ' -> ' + i.settingValue));

Using custom cache implementation

Config's data stored in a cache, it is efficiency increases in retrieval of data and performance of the client. If you would like to use your cache solution (for example your system uses external or distributed cache) you can implement those function and set to cache parameters in the setting.

function myCustomCache() { }

myCustomCache.prototype.get = function(key) {
    // `key` [string] - a unique cachekey

    // insert here your cache read logic

}

myCustomCache.prototype.set = function(key, item) {
    // `key` [string] - a unique cachekey
    // `item` [object] - configcat's cache config item

    // insert here your cache write logic
}

// set the `myCustomCache` when create a client instance

const configCatClient = configcat.createClientWithAutoPoll('PKDVCLf-Hq-h-kCzMp-L7Q/HhOWfwVtZ0mb30i9wi17GQ',
{
     cache: new myCustomCache()
});

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.
Suppose you don't want your SDK key or the content of your config.json file visible to your users. In that case, we recommend you use the SDK only in your backend applications and call a backend endpoint in your frontend/mobile application to evaluate the feature flags for a specific application customer.
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

• Angular 2+
• React
• Pure HTML + JS

Look under the hood

• ConfigCat JavaScript SDK on GitHub
• ConfigCat JavaScript SDK in NPM