React Client 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 SDK for your React Client application.

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

Before you begin

Version

Latest SDK version can be found on GitHub Release Page

Prerequisites

To use this SDK, make sure you've:

Installed Node.js v12 or a newer version.
Installed React.js v16.7 or a newer version.
Created a Feature Flag on the Harness Platform. If you are following along with the SDK README sample code, make sure your flag is called harnessappdemodarkmode.
Created an SDK key and made a copy of it

Install the SDK

First, install the SDK as a dependency in your application. You can install using npm or Yarn:

Install using npm

npm install @harnessio/ff-react-client-sdk

Install using Yarn

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

Initialize the SDK

To initialize the React Client SDK, you need to:

Import the relevant components.
Add your Client SDK key to connect to your Harness Environment.
Add a Target that you want to Evaluate against a Feature Flag.
(Optional) Configure the SDK options.
Complete the initialization.

Import the components

Import FFContextProvider from @harnessio/ff-react-client-sdk to wrap your application. This allows you to access the Feature Flags service. For example:

import { FFContextProvider } from '@harnessio/ff-react-client-sdk'

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 apiKey parameter, for example:

apiKey="YOUR_API_KEY"

Add a target

Details

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 add a target that you want to evaluate, build it using cfTarget and pass in arguments for the following:


Parameter	Description	Required?	Example
identifier	Unique ID for the target.	Required	`identifier: 'HT_1',`
name	Name for this target. This does not have to be unique. Note: If you don’t provide a value, the name will be the same as the identifier. Read Regex requirements for target names and identifiers below for accepted characters.	Required	`name: 'Harness_Target_1'`
attributes	Additional data you can store for a target, such as email addresses or location. These are key/pair values.	Optional	`email: 'demo@harness.io’ location: 'EMEA'`

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.

For example:

<FFContextProvider
      apiKey="YOUR_API_KEY"
      target={{
    identifier: 'HT_1',
    name: 'Harness_Target_1'   attributes: { // OPTIONAL: key/value pairs of attributes
           email: 'demo@harness.io',
           location: 'EMEA'
        }
      }}
    >

Configure the SDK

You can configure the following features of the SDK:


Name	Description	Default Value
baseUrl	The URL used to fetch feature flag evaluations. When using the relay proxy, make sure the URL you provide is publicly available.	`https://config.ff.harness.io/api/1.0`
eventUrl	The URL for posting metrics data to the feature flag service. When using the relay proxy, make sure the URL you provide is publicly available.	`https://events.ff.harness.io/api/1.0`
streamEnabled	Set to `true` to enable streaming mode. Set to `false` to disable streaming mode.	`true`

options={{ // OPTIONAL: advanced options
        baseUrl: 'https://url-to-access-flags.com',
        eventUrl: 'https://url-for-events.com',
        streamEnabled: true,
        debug: true
      }}

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. The default value is undefined, however you can optionally set the default value to something else, which is particularly useful if you plan to use async mode on startup. For more information about async mode, go to Use async mode.

With this SDK you can evaluate a single flag or a multiple flags. To do this, import and use one of the following hooks and pass in:

Identifier of the flag you want to evaluate
(Optional) The default variation

Evaluating a single flag

Import and use useFeatureFlag, for example:

import { useFeatureFlag } from '@harnessio/ff-react-client-sdk'
 
// ...

function MyComponent() {
  const myFlagValue = useFeatureFlag('flagIdentifier', 'default value')
 
  return <p>My flag value is: {myFlagValue}</p>
}

note

If the flag can’t be found, undefined is returned unless you passed in a different default value.

Evaluating multiple flags

Import and use useFeatureFlags, for example:

import { useFeatureFlags } from '@harnessio/ff-react-client-sdk'

// ...
 
function MyComponent() {
  const myFlagValues = useFeatureFlags()
 
  return (
    <>
      <p>My flag values are:</p>
      <pre>{JSON.stringify(myFlagValues, null, 2)}</pre>
    </>
  )
}

You can also return a subset of flags by using the following:

Returning a subset of flags

const myFlagValues = useFeatureFlags(['flag1', 'flag2'])

Returning a subset of flags with customized default values

const myFlagValues = useFeatureFlags({
  flag1: 'defaultForFlag1',
  flag2: 'defaultForFlag2'
})

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.

Mock the SDK when testing with Jest

When testing your React application with Jest, you may want to mock the SDK to avoid making network requests. You can do this by using the TestWrapper component included in the SDK. This component accepts a listing of flags and their values, and mocks the SDK to return those values.

note

To use the TestWrapper component, you must import it from the dist/cjs/test-utils directory (not from the main SDK package), as shown in the example below.

Example

In the example below, we use Testing Library to render the component <MyComponent /> that internally uses the useFeatureFlag hook.

import { render, screen } from '@testing-library/react'
import { TestWrapper } from '@harnessio/ff-react-client-sdk/dist/cjs/test-utils'

// ...

test('it should render the flag value', () => {
  render(
    <TestWrapper flags={{ flag1: 'value1', flag2: 'value2' }}>
      <MyComponent />
    </TestWrapper>
  )

  expect(screen.getByText('value1')).toBeInTheDocument()
})

Additional options

Use async mode

By default, the React Client SDK blocks the rendering of children until the initial load of feature flags has completed. This ensures that children have immediate access to all flags when they are rendered. However, in some circumstances it may be beneficial to immediately render the application and handle the display of loading on a component-by-component basis. The React Client SDK's asynchronous mode allows this by passing the optional async prop when connecting with the FFContextProvider. For example:

import { FFContextProvider } from '@harnessio/ff-react-client-sdk'
 
function MyComponent() {
  return (
    <FFContextProvider
      async // OPTIONAL: whether or not to use async mode
      apiKey="YOUR_API_KEY" // your SDK API key
      target={{
        identifier: 'HT_1', // unique ID of the Target
        name: 'Harness_Target_1',  // name of the Target
        attributes: { // OPTIONAL: key/value pairs of attributes
          email: 'demo@harness.io',
          location: 'EMEA'
        }

When using async mode, the loading prop indicates whether the SDK has finished loading the flags. When loading completes, the loading prop will be false, and the flags prop will contain all known flags. To use this, import the withFeatureFlags higher-order component and pass in your component, for example:

import { withFeatureFlags } from '@harnessio/ff-react-client-sdk'

function MyComponent({ flags, loading }) {
  if (loading) {
    return <p>Loading...</p>
  }
  return <p>Flag1's value is {flags.flag1}</p>
}
const MyComponentWithFlags = withFeatureFlags(MyComponent)

warning

When using async mode, the default value is returned until the flags are retrieved. If you use the useFeatureFlag or useFeatureFlags hooks, the default value is undefined unless you change it when evaluating the flag. If you are using withFeatureFlags, you will receive an empty object.

Display a loading component until flags are returned in async mode

When using async mode and the ifFeatureFlag HOC, by default the component won’t display until the flags are retrieved. This behavior can be overridden by passing a loadingFallback element in the options object. This will display the loadingFallback until the flags are retrieved, then the component will either show or hide as normal.

For example:

import { ifFeatureFlag } from '@harnessio/ff-react-client-sdk'
 
// ...
 
function MyComponent() {
  return <p>This should render if the flag is on</p>
}
 
const MyConditionalComponent = ifFeatureFlag('flag1', {
  loadingFallback: <p>Loading...</p>
})(MyComponent)

Conditionally render a component

You can conditionally render a component by using ifFeatureFlag, which is a higher-order component (HOC) that wraps your component and returns it only when the named Flag is enabled or matches a specific value.

Render if the flag is enabled

By default ifFeatureFlag will render the component if the named flag evaluates as truthy. In the below example, you can use MyConditionalComponent as a normal component and it will render the MyComponent component if the flag is enabled.

import { ifFeatureFlag } from '@harnessio/ff-react-client-sdk'
// ...
function MyComponent() {
  return <p>This renders if the flag is enabled</p>
}
const MyConditionalComponent = ifFeatureFlag('flag1')(MyComponent)

Render if the condition is matched

By default the component will display if the flag value is evaluated as truthy. This behavior can be overridden by passing the matchValue option in the options object. The flag value will then be compared with matchValue to determine whether it should display. For example:

import { ifFeatureFlag } from '@harnessio/ff-react-client-sdk'
 
// ... 

function MyComponent() {
  return <p>This should render if the flag evaluates to 'ABC123'</p>
}
 
const MyConditionalComponent = ifFeatureFlag('flag1', { matchValue: 'ABC123' })(
  MyComponent
)

Sample code for a React Client application

import React from 'react'
import ReactDOM from 'react-dom'
 
import {
  FFContextProvider,
  useFeatureFlag,
  useFeatureFlags
} from '@harnessio/ff-react-client-sdk'
 
ReactDOM.render(<App />, document.querySelector('#react-root'))
 
function App() {
  return (
    <FFContextProvider
      apiKey="YOUR_API_KEY"
      target={{
        identifier: 'reactclientsdk',
        name: 'ReactClientSDK'
      }}
    >
      <SingleFeatureFlag />
      <MultipleFeatureFlags />
    </FFContextProvider>
  )
}
 
function SingleFeatureFlag() {
  const flagValue = useFeatureFlag('harnessappdemodarkmode')
 
  return (
    <p>The value of "harnessappdemodarkmode" is {JSON.stringify(flagValue)}</p>
  )
}

function MultipleFeatureFlags() {
  const flags = useFeatureFlags()
 
  return (
    <>
      <p>Here are all our flags:</p>
      <pre>{JSON.stringify(flags, null, 2)}</pre>
    </>
  )
}

Before you begin​

Version​

Prerequisites​

Install the SDK​

Install using npm​

Install using Yarn​

Initialize the SDK​

Import the components​

Add your Client SDK key​

Add a target​

Configure the SDK​

Evaluate a flag​

Evaluating a single flag​

Evaluating multiple flags​

Returning a subset of flags​

Returning a subset of flags with customized default values​

Test your app is connected to Harness​

Close the SDK client​

Mock the SDK when testing with Jest​

Example​

Additional options​

Use async mode​

Display a loading component until flags are returned in async mode​

Conditionally render a component​

Render if the flag is enabled​

Render if the condition is matched​

Sample code for a React Client application​

Before you begin

Version

Prerequisites

Install the SDK

Install using npm

Install using Yarn

Initialize the SDK

Import the components

Add your Client SDK key

Add a target

Configure the SDK

Evaluate a flag

Evaluating a single flag

Evaluating multiple flags

Returning a subset of flags

Returning a subset of flags with customized default values

Test your app is connected to Harness

Close the SDK client

Mock the SDK when testing with Jest

Example

Additional options

Use async mode

Display a loading component until flags are returned in async mode

Conditionally render a component

Render if the flag is enabled

Render if the condition is matched

Sample code for a React Client application