> For the complete documentation index, see [llms.txt](https://developer.harness.io/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://developer.harness.io/feature-flags/use-feature-flags/use-ff/ff-sdks/server-sdks/node-js-sdk-reference.md).

# Node.js SDK Reference

{% hint style="info" %}
**NOTE**

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

This topic describes how to use the Harness Feature Flags Node.js SDK for your Node.js application.

For getting started quickly, you can use our [sample code from the Node.js SDK README](https://github.com/harness/ff-nodejs-server-sdk/blob/main/README.md). You can also [clone](https://docs.github.com/en/repositories/creating-and-managing-repositories/cloning-a-repository) and run a sample application from the [Node.js SDK GitHub Repository.](https://github.com/harness/ff-nodejs-server-sdk)

### Before you begin <a href="#before-you-begin" id="before-you-begin"></a>

You should read and understand the following:

* [Feature Flags Overview](/feature-flags/new-to-feature-flags/get-started/overview.md)
* [Getting Started with Feature Flags](/feature-flags/new-to-feature-flags/get-started/onboarding-guide.md)
* [Client-Side and Server-Side SDKs](/feature-flags/use-feature-flags/use-ff/ff-sdks/sdk-overview/client-side-and-server-side-sdks.md)
* [Communication Strategy Between SDKs and Harness Feature Flags](/feature-flags/use-feature-flags/use-ff/ff-sdks/sdk-overview/communication-sdks-harness-feature-flags.md)

### Version <a href="#version" id="version"></a>

Latest SDK version can be found on [GitHub Release Page](https://github.com/harness/ff-nodejs-server-sdk/releases)

### Requirements <a href="#requirements" id="requirements"></a>

To use this SDK, make sure you:

* Install Node.js version 12 or newer
* [Download the SDK from our GitHub repository](https://github.com/harness/ff-nodejs-server-sdk)
* Create a Node.js application, or [clone](https://docs.github.com/en/repositories/creating-and-managing-repositories/cloning-a-repository) our [sample application](https://github.com/harness/ff-nodejs-server-sdk).
* [Create a Feature Flag on the Harness Platform](/feature-flags/use-feature-flags/use-ff/ff-creating-flag/create-a-feature-flag.md). If you are following along with the SDK README sample code, make sure your flag is called `harnessappdemodarkmode`
* [Create an SDK key and make a copy of it](/feature-flags/use-feature-flags/use-ff/ff-creating-flag/create-a-project.md#create-an-sdk-key)

### Install the SDK <a href="#install-the-sdk" id="install-the-sdk"></a>

#### Install using npm <a href="#install-using-npm" id="install-using-npm"></a>

```
npm install @harnessio/ff-nodejs-server-sdk
```

#### Install using yarn <a href="#install-using-yarn" id="install-using-yarn"></a>

```
yarn add @harnessio/ff-nodejs-server-sdk
```

### Initialize the SDK <a href="#initialize-the-sdk" id="initialize-the-sdk"></a>

To initialize the Node.js SDK, you need to:

1. Import the package.
2. Add your Server SDK Key to connect to your Harness Environment.
3. Add a Target that you want to Evaluate against a Feature Flag.
4. (Optional) Configure the SDK options. For more details on what features you can configure for this SDK, go to [Configure the SDK](#configure-the-sdk).

#### Import the package <a href="#import-the-package" id="import-the-package"></a>

**CommonJS**

To import with CommonJS, use:

```
const { Client } = require('@harnessio/ff-nodejs-server-sdk');
```

**ES modules**

To import with ES modules use:

```
import { Client } from '@harnessio/ff-nodejs-server-sdk';
```

#### Add the Server SDK Key <a href="#add-the-server-sdk-key" id="add-the-server-sdk-key"></a>

To connect to the correct Environment that you set up on the Harness Platform, you need to add the Server SDK Key from that Environment. Input the Client SDK Key into the `API_KEY` parameter, for example:

```
const client = new Client('sdkKey');
```

#### Add a Target <a href="#add-a-target" id="add-a-target"></a>

<details>

<summary>What is a Target?</summary>

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.

</details>

For more information about Targets, go to [Targeting Users With Flags](/feature-flags/use-feature-flags/use-ff/ff-target-management/targeting-users-with-flags.md).

To add a Target, build it and pass in arguments for the following:

|               |                                                                                                                                                                                                                                                                            |                                                                                                                                                      |                            |
| ------------- | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | ---------------------------------------------------------------------------------------------------------------------------------------------------- | -------------------------- |
| **Parameter** | **Description**                                                                                                                                                                                                                                                            | **Required?**                                                                                                                                        | **Example**                |
| `identifier`  | Unique ID for the Target.Read **Regex requirements for Target names and identifiers** below for accepted characters.                                                                                                                                                       | Required                                                                                                                                             | `identifier: 'HT_1'`       |
| `name`        | <p>Name for this Target. This does not have to be unique.<br><strong>Note</strong>: If you don’t provide a value, the name will be the same as the identifier.Read <strong>Regex requirements for Target names and identifiers</strong> below for accepted characters.</p> | <p>Optional<br><strong>Note</strong>: If you don't want to send a name, don't send the parameter. Sending an empty argument will cause an error.</p> | `name: 'Harness_Target_1'` |
| `attributes`  | Additional data you can store for a Target, such as email addresses or location.                                                                                                                                                                                           | Optional                                                                                                                                             | `attributes: {`            |

<details>

<summary>Regex requirements for Target names and identifiers</summary>

**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`.

</details>

For example:

```
const target = {  
    identifier: 'HT_1',  
    name: 'Harness_Target_1',  
    attributes: {  
       'email': 'demo@harness.io'  
    }  
  };
```

#### Configure the SDK <a href="#configure-the-sdk" id="configure-the-sdk"></a>

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, change this to: `http://localhost:7000`                      | `https://config.ff.harness.io/api/1.0` |
| eventsUrl       | The URL for posting metrics data to the Feature Flag service. When using the Relay Proxy, change this to: `http://localhost:7000`        | `https://events.ff.harness.io/api/1.0` |
| pollInterval    | The interval **in milliseconds** that we poll for changes when you are using stream mode.                                                | `60` (seconds)                         |
| enableStream    | Set to `true` to enable streaming mode.Set to `false` to disable streaming mode.                                                         | `true`                                 |
| enableAnalytics | Set to `true` to enable analytics.Set to `false` to disable analytics.**Note**: When enabled, analytics data is posted every 60 seconds. | `true`                                 |

For example:

```
// Create Options  
const options = {  
  baseUrl: "http://localhost:7000",  
  eventsUrl: "http://localhost:7000"  
}
```

#### Complete the initialization <a href="#complete-the-initialization" id="complete-the-initialization"></a>

To complete the initialization:

1. Create an instance of the Feature Flag client and pass in the Server SDK Key and configuration options:

```
// Create client with options  
const client = new Client(apiKey, options);
```

1. Wait for the SDK to complete initialization and to fetch the Flag data:

```
await client.waitForInitialization();
```

### Evaluate a Flag <a href="#evaluate-a-flag" id="evaluate-a-flag"></a>

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 Target object you want to evaluate against
* The default Variation

For example:

#### Evaluate a boolean Variation <a href="#evaluate-a-boolean-variation" id="evaluate-a-boolean-variation"></a>

```
function boolVariation(  
  identifier: string,  
  target: Target,  
  defaultValue: boolean = true,  
): Promise<boolean>;
```

#### Evaluate a string Variation <a href="#evaluate-a-string-variation" id="evaluate-a-string-variation"></a>

```
function stringVariation(  
  identifier: string,  
  target: Target,  
  defaultValue: boolean = '',  
): Promise<string>;
```

#### Evaluate a number Variation <a href="#evaluate-a-number-variation" id="evaluate-a-number-variation"></a>

```
function numberVariation(  
  identifier: string,  
  target: Target,  
  defaultValue: boolean = 1.0,  
): Promise<number>;
```

#### Evaluate a JSON Variation <a href="#evaluate-a-json-variation" id="evaluate-a-json-variation"></a>

```
function jsonVariation(  
  identifier: string,  
  target: Target,  
  defaultValue: boolean = {},  
): Promise<Record<string, unknown>>;
```

### Listen for events <a href="#listen-for-events" id="listen-for-events"></a>

#### Register the event listener <a href="#register-the-event-listener" id="register-the-event-listener"></a>

You can listen for the following events:

* Event.READY - Indicates the SDK was successfully initialized.
* Event.FAILED - Indicates the SDK had thrown an error.
* Event.CHANGED - Indicates a Flag or Segment has been updated.

For example:

```
on(Event.READY, () => {  
  console.log('READY');  
});  
  
on(Event.FAILED, () => {  
  console.log('FAILED');  
});  
  
on(Event.CHANGED, (identifier) => {  
  console.log('Changed', identifier);  
});
```

#### Close the event listener <a href="#close-the-event-listener" id="close-the-event-listener"></a>

To avoid unexpected behavior, when the listener isn't needed, turn it off.

To remove the `functionReference` listener for `Event.READY,` use:

```
off(Event.READY, functionReference);
```

To remove all listeners, use:

```
off(Event.READY);
```

If you call `off()` without parameters it will close the client.

#### Test your app is connected to Harness <a href="#test-your-app-is-connected-to-harness" id="test-your-app-is-connected-to-harness"></a>

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.

{% hint style="info" %}
**NOTE**

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

### Close the SDK client <a href="#close-the-sdk-client" id="close-the-sdk-client"></a>

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.

{% hint style="info" %}
**IMPORTANT**

The SDK does not evaluate flags after the client is closed.
{% endhint %}

To close the SDK, call the following function:

```
function close(): void;
```

### Additional options <a href="#additional-options" id="additional-options"></a>

#### Configure your logger <a href="#configure-your-logger" id="configure-your-logger"></a>

You can provide your own logger to the SDK by passing it in as a config option. The following example creates an instance of the winston logger, sets the level to DEBUG, and passes it to the client.

```
const winston = require('winston')  
  
// Create client with logger  
const client = new Client(apiKey, {  
  logger: new winston.createLogger({  
    level: 'debug',  
    transports: [new winston.transports.Console()]  
  })  
});
```

### Sample code for a Node.js application <a href="#sample-code-for-a-nodejs-application" id="sample-code-for-a-nodejs-application"></a>

Here is a sample code for using the SDK with a Node.js application:

```
  
  const pkg = require('ff-nodejs-server-sdk');  
const { Client } = pkg;  
  
const client = new Client('1c100d25-4c3f-487b-b198-3b3d01df5794', {  
  enableStream: true,  
  pollInterval: 2 * 60 * 1000 // two min pollInterval  
});  
  
setInterval(() => {  
  const target = {  
    identifier: 'harness',  
  };  
  const value = client.boolVariation('test', target, false);  
  console.log('Evaluation for flag test and target: ', value, target);  
}, 10000);
```

### Troubleshooting <a href="#troubleshooting" id="troubleshooting"></a>

The SDK logs the following codes for certain lifecycle events, for example authentication, which can aid troubleshooting.

| **Code** | **Description**                                                                                               | **Log Level** |
| -------- | ------------------------------------------------------------------------------------------------------------- | ------------- |
| **1000** | Successfully initialized                                                                                      | Info          |
| **1001** | Failed to initialize due to authentication error                                                              | Error         |
| **1002** | Failed to initialize due to a missing or empty API key                                                        | Error         |
| **1003** | `WaitForInitialization` configuration option was provided and the SDK is waiting for initialization to finish | Info          |
| **2000** | Successfully authenticated                                                                                    | Info          |
| **2001** | Authentication failed with a non-recoverable error                                                            | Error         |
| **2002** | Authentication failed and is retrying                                                                         | Warn          |
| **2003** | Authentication failed and max retries have been exceeded                                                      | Error         |
| **3000** | SDK closing                                                                                                   | Info          |
| **3001** | SDK closed successfully                                                                                       | Info          |
| **4000** | Polling service started                                                                                       | Info          |
| **4001** | Polling service stopped                                                                                       | Info          |
| **5000** | Streaming connected                                                                                           | Info          |
| **5001** | Streaming disconnected                                                                                        | Warn          |
| **5002** | Streaming event received                                                                                      | Debug         |
| **5003** | Streaming disconnected and is retrying to connect                                                             | Info          |
| **5004** | Streaming service stopped                                                                                     | Info          |
| **5005** | Stream is still retrying to connect after 4 attempts                                                          | Warn          |
| **6000** | Evaluation was successful                                                                                     | Debug         |
| **6001** | Evaluation failed and the default value was returned                                                          | Info          |
| **7000** | Metrics service has started                                                                                   | Info          |
| **7001** | Metrics service has stopped                                                                                   | Info          |
| **7002** | Metrics posting failed                                                                                        | Warn          |
| **7003** | Metrics posting success                                                                                       | Debug         |
| **7004** | Metrics max target size exceeded                                                                              | Warn          |
| **7007** | Metrics max evaluation size reached                                                                           | Warn          |