Modify API Response

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Why Use Modify API Response Rule?

• Frontend Development: Work seamlessly even if the backend is unavailable or incomplete.
• Custom Testing: Test application behaviour with modified or custom data.
• Error Simulation: Trigger specific errors by modifying response status codes.
• Backend Independence: Make changes to API responses without requiring backend changes.

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Steps to Configure a Modify API Response Rule

1

Create a Response Rule

Navigate to the HTTP Rules section in the Requestly app. Click + New Rule and choose the Modify API Response option.

2

Select Resource Type

Choose the resource type for modification:

• REST API: Modify XHR/Fetch requests initiated by web pages.
• GraphQL: Target and modify GraphQL APIs by operation.
• HTML/JS/CSS (Desktop App Only): Modify static files for desktop applications.

3

Define Matching Criteria

Define the criteria for the rule to be applied, such as URL, Host, or Path. Choose from Regex, Contains, Wildcard, or Equals, or use Advance Filters for more precise conditions.For more detailed guidance, refer to the Source Conditions Guide.

4

Filter GraphQL Queries (Optional)

If targeting GraphQL APIs, filter queries by their operation name. Add the Payload JSON Key (e.g., operationName) and specify the corresponding Value (e.g., getUsers) to target specific GraphQL operations.

5

Change Response Status Code

Modify the status code of the response using the dropdown menu. Choose from:

• 2XX for successful responses
• 4XX for client errors
• 5XX for server errors

If left empty, the original status code remains unchanged.

6

Modify Response Data

Choose one of the following methods to modify the response data:

• Static Response Modification: Input a static value as the response body.
• Dynamic Modification Script (JavaScript): Use JavaScript to modify responses dynamically. The script provides access to parameters like:
  • method: HTTP Method (GET, POST, etc.)
  • url: Full URL of the request
  • response: Original response body (string)
  • responseType: Content type of the original response
  • requestHeaders: Headers sent with the request
  • responseJSON: Original response parsed as a JSON object

Example:

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function modifyResponse({ responseJSON }) {
    responseJSON.modifiedKey = "modifiedValue";
    return responseJSON;
}

Desktop App feature

You can store and retrieve data within and across Modify Request and Modify Response rules using shared state in programmatic JavaScript mode.
→Learn more about shared state here.

• 
  You can check the “Serve this response body without making call to the server” checkbox as the label suggests it will ensure you get the response locally without hitting your API server.

7

Save and Activate the Rule

Click Save to store the rule and toggle it ON. This will apply the rule to the defined source conditions.

8

Test the Rule

This feature is available on the browser extension only.

To ensure the rule is working correctly, click the Test button and enter the URL of the page where you want to apply this rule. This will open the webpage in a new tab, with a widget showing the status of applied rules.

There are multiple ways to validate if a rule is applied or not. Learn more about how to validate rule execution.

Changes applied by this rule won’t appear in browser devtools due to technical constraints, but are sent with the actual requests.