React.js Client SDK For Harness Feature Flags

Use this README to get started with our Feature Flags (FF) Client SDK for React. This guide outlines the basics of getting started with the SDK and provides a full code sample for you to try out.

This sample doesn't include configuration options, for in depth steps and configuring the SDK, see the React Client SDK Reference.

Requirements

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

To follow along with our test code sample, make sure you’ve:

• Created a Feature Flag on the Harness Platform called harnessappdemodarkmode
• Created a client SDK key and made a copy of it

Installing the SDK

The first step is to install the FF SDK as a dependency in your application. To install using npm, use:

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

Or to install with yarn, use:

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

Code Sample

The following is a complete code example that you can use to test the harnessappdemodarkmode Flag you created on the Harness Platform. When you run the code it will:

• Render a loading screen
• Connect to the FF service
• Retrieve all flags
• Access a flag using the useFeatureFlag hook
• Access several flags using the useFeatureFlags hook

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>
    </>
  )
}

Async mode

By default, the React Client SDK will block 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 display of loading on a component-by-component basis. The React Client SDK's asynchronous mode allows this by passing the optional asyncMode prop when connecting with the FFContextProvider.

On Flag Not Found

The onFlagNotFound option allows you to handle situations where a default variation is returned. It includes the flag, variation, and whether the SDK was still initializing (loading) when the default was served.

This can happen when:

1. Using asyncMode mode without cache or initialEvaluations and where the SDK is still initializing.
2. The flag identifier is incorrect (e.g., due to a typo).
3. The wrong API key is being used, and the expected flags are not available for that project.

<FFContextProvider
  apiKey="YOUR_API_KEY"
  target={{
    identifier: 'reactclientsdk',
    name: 'ReactClientSDK'
  }}
  onFlagNotFound={(flagNotFoundPayload, loading) => {
    if (loading) {
      console.debug(`Flag "${flagNotFound.flag}" not found because the SDK is still initializing. Returned default: ${flagNotFound.defaultVariation}`);
    } else {
      console.warn(`Flag "${flagNotFound.flag}" not found. Returned default: ${flagNotFound.defaultVariation}`);
    }
  }}
>
  <MyApp />
</FFContextProvider>

By using the onFlagNotFound prop, your application can be notified whenever a flag is missing and the default variation has been returned.

Caching evaluations

In practice flags rarely change and so it can be useful to cache the last received evaluations from the server to allow your application to get started as fast as possible. Setting the cache option as true or as an object (see interface below) will allow the SDK to store its evaluations to localStorage and retrieve at startup. This lets the SDK get started near instantly and begin serving flags, while it carries on authenticating and fetching up-to-date evaluations from the server behind the scenes.

<FFContextProvider
  apiKey="YOUR_API_KEY"
  target={{
    identifier: 'reactclientsdk',
    name: 'ReactClientSDK'
  }}
  options={{
    cache: true
  }}
>
  <MyApp />
</FFContextProvider>

The cache option can also be passed as an object with the following options.

interface CacheOptions {
  // maximum age of stored cache, in ms, before it is considered stale
  ttl?: number
  // storage mechanism to use, conforming to the Web Storage API standard, can be either synchronous or asynchronous
  // defaults to localStorage
  storage?: AsyncStorage | SyncStorage
}

interface SyncStorage {
  getItem: (key: string) => string | null
  setItem: (key: string, value: string) => void
  removeItem: (key: string) => void
}

interface AsyncStorage {
  getItem: (key: string) => Promise<string | null>
  setItem: (key: string, value: string) => Promise<void>
  removeItem: (key: string) => Promise<void>
}

Overriding the internal logger

By default, the React Client SDK will log errors and debug messages using the console object. In some cases, it can be useful to instead log to a service or silently fail without logging errors.

const myLogger = {
  debug: (...data) => {
    // do something with the logged debug message
  },
  info: (...data) => {
    // do something with the logged info message
  },
  error: (...data) => {
    // do something with the logged error message
  },
  warn: (...data) => {
    // do something with the logged warning message
  }
}

return (
  <FFContextProvider
    apiKey="YOUR_API_KEY"
    target={{
      identifier: 'reactclientsdk',
      name: 'ReactClientSDK'
    }}
    options={{
      logger: myLogger
    }}
  >
    <MyApp />
  </FFContextProvider>
)

API

FFContextProvider

The FFContextProvider component is used to set up the React context to allow your application to access Feature Flags using the useFeatureFlag and useFeatureFlags hooks and withFeatureFlags HOC. At minimum, it requires the apiKey you have set up in your Harness Feature Flags account, and the target. You can think of a target as a user.

The FFContextProvider component also accepts an options object, a fallback component, an array of initialEvaluations, an onError handler, and can be placed in Async mode using the asyncMode prop. The fallback component will be displayed while the SDK is connecting and fetching your flags. The initialEvaluations prop allows you pass an array of evaluations to use immediately as the SDK is authenticating and fetching flags. The onError prop allows you to pass an event handler which will be called whenever a network error occurs.

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

// ...

function MyComponent() {
  return (
    <FFContextProvider
      asyncMode={false} // OPTIONAL: whether or not to use async mode
      apiKey="YOUR_API_KEY" // your SDK API key
      target={{
        identifier: 'targetId', // unique ID of the Target
        name: 'Target Name',  // name of the Target
        attributes: { // OPTIONAL: key/value pairs of attributes of the Target
          customAttribute: 'this is a custom attribute',
          anotherCustomAttribute: 'this is something else'
        }
      }}
      fallback={<p>Loading ...</p>} // OPTIONAL: component to display when the SDK is connecting
      options={{ // OPTIONAL: advanced configuration options
        cache: false,
        baseUrl: 'https://url-to-access-flags.com',
        eventUrl: 'https://url-for-events.com',
        streamEnabled: true,
        debug: true,
        eventsSyncInterval: 60000
      }}
      initialEvaluations={evals} // OPTIONAL: array of evaluations to use while fetching
      onError={handler} // OPTIONAL: event handler to be called on network error
    >
      <CompontToDisplayAfterLoad /> <!-- component to display when Flags are available -->
    </FFContextProvider>
  )
}

useFeatureFlag

The useFeatureFlag hook returns a single named flag value. An optional second argument allows you to set what value will be returned if the flag does not have a value. By default useFeatureFlag will return undefined if the flag cannot be found.

N.B. when rendered in Async mode, the default value will be returned until the Flags are retrieved. Consider using the useFeatureFlagsLoading hook to determine when the SDK has finished loading.

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

// ...

function MyComponent() {
  const myFlagValue = useFeatureFlag('flagIdentifier', 'default value')

  return <p>My flag value is: {myFlagValue}</p>
}

useFeatureFlags

The useFeatureFlags hook returns an object of Flag identifier/Flag value pairs. You can pass an array of Flag identifiers or an object of Flag identifier/default value pairs. If an array is used and a Flag cannot be found, the returned value for the flag will be undefined. If no arguments are passed, all Flags will be returned.

N.B. when rendered in Async mode, the default value will be returned until the Flags are retrieved. Consider using the useFeatureFlagsLoading hook to determine when the SDK has finished loading.

import { useFeatureFlag } 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>
    </>
  )
}

Get a subset of Flags

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

Get a subset of Flags with custom default values

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

useFeatureFlagsLoading

The useFeatureFlagsLoading hook returns a boolean value indicating whether the SDK is currently loading Flags from the server.

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

// ...

function MyComponent() {
  const isLoading = useFeatureFlagsLoading()
  const flags = useFeatureFlags()

  if (isLoading) {
    return <p>Loading ...</p>
  }

  return (
    <>
      <p>My flag values are:</p>
      <pre>{JSON.stringify(flags, null, 2)}</pre>
    </>
  )
}

useFeatureFlagsClient

The React Client SDK internally uses the Javascript Client SDK to communicate with Harness. Sometimes it might be useful to be able to access the instance of the Javascript Client SDK rather than use the existing hooks or higher-order components (HOCs). The useFeatureFlagsClient hook returns the current Javascript Client SDK instance that the React Client SDK is using. This instance will be configured, initialized and have been hooked up to the various events the Javascript Client SDK provides.

import {
  useFeatureFlagsClient,
  useFeatureFlagsLoading
} from '@harnessio/ff-react-client-sdk'

// ...

function MyComponent() {
  const client = useFeatureFlagsClient()
  const loading = useFeatureFlagsLoading()

  if (loading || !client) {
    return <p>Loading...</p>
  }

  return (
    <p>
      My flag value is: {client.variation('flagIdentifier', 'default value')}
    </p>
  )
}

ifFeatureFlag

The ifFeatureFlag higher-order component (HOC) wraps your component and conditionally renders only when the named flag is enabled or matches a specific value.

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')(MyComponent)

You can then use MyConditionalComponent as a normal component, and only render if flag1's value is truthy.

Conditionally with a specific value

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
)

You can then use MyConditionalComponent as a normal component, only render if flag1's value matches the passed condition.

Loading fallback when in async mode

If Async mode is used, by default the component will wait for Flags to be retrieved before showing. This behaviour can be overridden by passing an element as loadingFallback; when loading the loadingFallback will be displayed until the Flags are retrieved, at which point the component will either show or hide as normal.

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)

withFeatureFlags

The withFeatureFlags higher-order component (HOC) wraps your component and adds flags and loading as additional props. flags contains the evaluations for all known flags and loading indicates whether the SDK is actively fetching Flags.

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

// ...

function MyComponent({ flags }) {
  return <p>Flag1's value is {flags.flag1}</p>
}

const MyComponentWithFlags = withFeatureFlags(MyComponent)

Loading in async mode

If Async mode is used, the loading prop will indicate whether the SDK has completed loading the Flags. When loading completes, the loading prop will be false and the flags prop will contain all known Flags.

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)

withFeatureFlagsClient

The React Client SDK internally uses the Javascript Client SDK to communicate with Harness. Sometimes it might be useful to be able to access the instance of the Javascript Client SDK rather than use the existing hooks or higher-order components (HOCs). The withFeatureFlagsClient HOC wraps your component and adds featureFlagsClient as additional prop. featureFlagsClient is the current Javascript Client SDK instance that the React Client SDK is using. This instance will be configured, initialized and have been hooked up to the various events the Javascript Client SDK provides.

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

// ...

function MyComponent({ featureFlagsClient }) {
  if (featureFlagsClient) {
    return (
      <p>
        Flag1's value is {featureFlagsClient.variation('flag1', 'no value')}
      </p>
    )
  }

  return <p>The Feature Flags client is not currently available</p>
}

const MyComponentWithClient = withFeatureFlagsClient(MyComponent)

Testing with Jest

When running tests with Jest, you may want to mock the SDK to avoid making network requests. You can do this by using the included TestWrapper component. This component accepts a listing of flags and their values, and will mock the SDK to return those values. In the example below, we use Testing Library to render the component <MyComponent /> that internally uses the useFeatureFlag hook.

N.B. to use the TestWrapper component, you must import it from the dist/cjs/test-utils directory, not from the main package.

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 Reading

For further examples and config options, see the React.js Client SDK Reference For more information about Feature Flags, see our Feature Flags documentation.