# sourceConfigurationOverride Parameter Reference

This guide provides a detailed reference for the `sourceConfigurationOverride` parameter used for dynamically configuring device mode destinations in the RudderStack JavaScript SDK.

## Overview

The `sourceConfigurationOverride` option of the JavaScript SDK's [`load` API]({{< ref "sources/event-streams/sdks/rudderstack-javascript-sdk/load-js-sdk.md" >}}) lets you dynamically modify destination configurations when initializing the RudderStack JavaScript SDK. This parameter provides programmatic control over device mode destinations without requiring dashboard configuration changes.

See [How to Dynamically Configure Device Mode Destinations]({{< ref "sources/event-streams/sdks/rudderstack-javascript-sdk/dynamic-configuration/" >}}) for more information on using this parameter.

## `sourceConfigurationOverride` structure

```javascript
{
  sourceConfigurationOverride: {
    destinations: [
      {
        id: string,           // Required
        config: object        // Optional
      }
    ]
  }
}
```

### The `destinations` array

The `destinations` array contains objects that specify which destinations to override and how to modify them. Each object in the array contains the following properties:

| Parameter | Type | Description |
| :----------- | :-------------- | :-------- |
| `id` <br/> <span style="color: #4D4DFF;font-size:12px;">Required</span> | String | The unique identifier of the destination configured in your RudderStack dashboard |
| `config` | Object | Configuration properties to override for the destination. |

### The `config` object

The `config` object contains destination-specific configuration properties that override the corresponding values from the RudderStack dashboard configuration.

Note that different destinations support different configuration properties. Some examples are listed in the below table:

| Destination | Configuration properties | Example values |
| :----------- | :-------------- | :----| 
| Google Analytics 4 | `measurement_id`  | `G-XXXXXXXXXX` |
| Facebook Pixel | `pixel_id`  |  `1234567890123456` |

{{< info >}}
The configuration fields may change over time as destinations are updated. [Verify the current field names]({{< ref "sources/event-streams/sdks/rudderstack-javascript-sdk/dynamic-configuration/_index.md#determine-the-configuration-fields-to-override" >}}) in the source configuration response before implementing the overrides.
{{< /info >}}

## Configuration merging strategy

The SDK merges the configuration overrides specified in the `sourceConfigurationOverride` parameter with the source configuration from RudderStack using the following rules:

- Only explicitly overridden fields are changed — all other configuration values are inherited from the dashboard configuration.
- Override values can change the data type of a property (for example, string to object).
- Setting a field to `null` or `undefined` removes it from the final configuration. RudderStack does not perform any further validation.

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