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JavaScript SDK reference

note

This SDK is not currently supported on Harness Self-Managed Enterprise Edition (on premises).

This topic describes how to use the Harness Feature Flags Javascript SDK for your JavaScript application.

For getting started quickly, you can use our sample code from the JavaScript SDK README. You can also clone and run a sample application from the JavaScript SDK GitHub Repository.

Before You Begin​

Make sure you read and understand:

Version​

The current version of this SDK is 1.14.0.

Requirements​

To use this SDK, make sure you:

Install the SDK​

You can install the SDK using npm, Yarn, or by importing it directly from unpkg.

Install using npm​

Run the following command:

npm i @harnessio/ff-javascript-client-sdk

Then, import the SDK using the following command:

import { initialize, Event } from '@harnessio/ff-javascript-client-sdk'

Install using Yarn​

Run the following command:

yarn add @harnessio/ff-javascript-client-sdk

Then, import the SDK using the following command:

import { initialize, Event } from '@harnessio/ff-javascript-client-sdk'

Install directly from unpkg​

Run the following script:

<script type="module">  
  import { initialize, Event } from 'https://unpkg.com/@harnessio/ff-javascript-client-sdk@1.8.0/dist/sdk.client.js'  
</script>

Make sure you install the latest version of the SDK, which you can view in GitHub and in Version.If you need to support browsers that no longer support ES modules, run the following script instead:

<script src="https://unpkg.com/@harnessio/ff-javascript-client-sdk@1.8.0/dist/sdk.client.js"></script>  
<script>  
  var initialize = HarnessFFSDK.initialize  
  var Event = HarnessFFSDK.Event  
</script>

Initialize the SDK​

To initialize the JavaScript SDK, you need to:

  1. Add your Client SDK key to connect to your Harness Environment.
  2. Add a target that you want to evaluate against a Feature Flag.
  3. (Optional) Configure the SDK.
  4. Complete the initialization with the SDK using the client SDK Key, target, and Configuration parameters you set.

Add your Client SDK Key​

To connect to the correct Environment that you set up on the Harness Platform, you need to add the Client SDK Key from that Environment. Input the Client SDK Key into the FeatureFlagSDKKey parameter.

Add a target​

What is a target?
Targets are used to control which users see which variation of a Feature Flag, for example, if you want to do internal testing, you can enable the flag for some users and not others. When creating a target, you give it a name and a unique identifier. Often targets are users but you can create a target from anything that can be uniquely identified, such as an app or a machine.

For more information about targets, go to Targeting users with flags.

To create a target, you must enter an identifier but you can optionally add a name and additional attributes.

The below shows the data type for each parameter:

interface Target {  
  identifier: string  
  name?: string  
  anonymous?: boolean  
  attributes?: object  
}
Regex requirements for target names and identifiers

Identifier

Regex: ^[A-Za-z0-9.@_-]*$
Must consist of only alphabetical characters, numbers, and the following symbols:
. (period)
@ (at sign)
-(dash)
_ (underscore)

The characters can be lowercase or uppercase but cannot include accented letters, for example Cafe_789.

Name Regex: ^[\\p{L}\\d .@_-]*$

Must consist of only alphabetical characters, numbers, and the following symbols:
. (period)
@ (at sign)
-(dash)
_ (underscore)
(space)

The characters can be lowercase or uppercase and can include accented letters, for example Café_123.

Configure the SDK​

To configure the SDK, you can add Options, for example:

interface Options {  
  baseUrl?: string  
  debug?: boolean  
}

Complete the initialization​

Complete the initialization using the FeatureFlagSDKKey, target, and Options variables:

initialize(FeatureFlagSDKKey: string, target: Target, options?: Options)

For example:

const cf = initialize('00000000-1111-2222-3333-444444444444', {  
    identifier: YOUR-TARGET-IDENTIFIER,      // Target identifier  
    name: YOUR-TARGET-NAME,                  // Optional target name  
    attributes: {                            // Optional target attributes  
      email: 'sample@sample.com'  
    }  
  });

Evaluate a flag​

Evaluating a flag is when the SDK processes all flag rules and returns the correct variation of that flag for the target you provide. If a matching flag can’t be found, or the SDK can’t remotely fetch flags, the default value is returned.

There are different methods for the different variation types and for each method you need to pass in:

  • Identifier of the flag you want to evaluate
  • The default variation

The flag is evaluated against the target you pass in when initializing the SDK.

const value = cf.variation('Dark_Theme', false) // second argument is the default value when variation does not exist

(Optional) Provide a set of evaluations​

In some cases it might be useful to provide the SDK with a set of evaluations that it can serve instantly. You might consider this when you need to:

  • Reduce application startup time by providing default values or a snapshot of evaluations. For example, if your application is server-side generated, then it might make sense to retrieve evaluations on the server and provide them in the HTML of the page to be injected into the SDK.
  • Provide network redundancy by allowing your app to detect network connectivity issues when accessing the service and loading evaluations from another source.

To provide a set of evaluations:

  • Call the setEvaluations method at any time after initializing the client. The setEvaluations method takes an array of Evaluation objects as an argument.

    client.setEvaluations(evals);

    Where Evaluation is defined as:

    export interface Evaluation {
      flag: string // Feature flag identifier
      identifier: string // variation identifier
      value: boolean | string | number | object | undefined // variation value
      kind: string // boolean | json | string | int
      deleted?: boolean // mark that feature flag is deleted
    }

Listen for events​

Register the event listener​

The cf method allows you to listen for the different events triggered by SDK and deal with them as needed, for example:

cf.on(Event.READY, flags => {  
  // Event happens when connection to server is established  
  // flags contains all evaluations against SDK key  
})  
  
cf.on(Event.CHANGED, flagInfo => {  
  // Event happens when a changed event is pushed  
  // flagInfo contains information about the updated feature flag  
})  
  
cf.on(Event.DISCONNECTED, () => {  
  // Event happens when connection is disconnected  
})  
  
cf.on(Event.ERROR, () => {  
  // Event happens when a connection error has occurred  
})

Close the event listener​

To turn off the listeners, use cf.off. You can turn off the listener when there is an error, or turn off all listeners on command.

Turn off listener when there is an error:

cf.off(Event.ERROR, () => {  
  // Do something when an error occurs  
})

Turn off all listeners:

cf.off()

Test your app is connected to Harness​

When you receive a response showing the current status of your Feature Flag, go to the Harness Platform and toggle the flag on and off. Then, check your app to verify if the flag variation displayed is updated with the variation you toggled.

note

The SDK must run for at least 60 seconds before it sends metrics. Please ensure metrics have not been disabled in the SDK.

Close the SDK client​

In most applications, you won't need to close the SDK client.

However, you should close the SDK client if:

  • Your application is about to terminate. Closing the client ensures that all associated resources are released.
  • You have determined that you do not need to evaluate flags again in your application lifecycle.
important

The SDK does not evaluate flags after the client is closed.

To close the SDK client, call the following function:

cf.close();

Sample code for a JavaScript application​

Here is a sample code for using Harness Feature Flag SDKs with a JavaScript application:

var initialize = HarnessFFSDK.initialize  
      var Event = HarnessFFSDK.Event  
  
      var log = msg => {  
        document.querySelector('#log').innerHTML += `${msg}\n`  
      }  
  
      var cf = initialize(  
        'ed56f3ec-1aa6-4bc2-a519-6ebcc3f0541f',  
        {  
          identifier: 'Harness1',  
          attributes: {  
            lastUpdated: Date(),  
            host: location.href  
          }  
        }  
      )  
  
      cf.on(Event.READY, flags => {  
        log(JSON.stringify(flags, null, 2))  
      })  
  
      cf.on(Event.CHANGED, flagInfo => {  
        if (flagInfo.deleted) {  
          log('Flag is deleted')  
          log(JSON.stringify(flagInfo, null, 2))  
        } else {  
          log('Flag is changed')  
          log(JSON.stringify(flagInfo, null, 2))  
        }  
      })