n8n Function Node Returns Object Instead of Expected String

Below is some guidance to help troubleshoot and resolve the issue.

Understanding n8n Output Format

n8n’s Function (or Code) node expects that data is returned as an array of items, where each item is an object with a key named json. For example, if you want to return a string for further processing, you need to embed it within that structure. For instance, if you’re expecting a plain string for a GET request response, it’s common to assign that string to a property (such as response) within the returned object:

JAVASCRIPTCopied
// Correct way if you want to pass a string as the "response" property
return [
  {
    json: {
      response: 'your string here'
    }
  }
];

If your Function node code is returning something like:

JAVASCRIPTCopied
return [
  { json: { response: { some: 'object' } } }
];

then subsequent nodes expecting a string for the response property will fail with an error like:
"The response property should be a string, but it is an object".

How to Resolve

1. Ensure That You’re Returning a String
   If your code is currently producing an object within the response property, convert the object to a string. For example, if you need to send a JSON object as a string, use JSON.stringify:

   JAVASCRIPTCopied
   const data = { key: 'value' }; // computed object from your AI agent, for example
   return [
     {
       json: {
         response: JSON.stringify(data)
       }
     }
   ];

2. Return the String Directly in the Expected Structure
   If your logic is already creating a string, ensure it’s inserted as a string value rather than wrapping it in another object:

   JAVASCRIPTCopied
   const resultString = "This is my GET response string";
   return [
     {
       json: {
         response: resultString
       }
     }
   ];

3. Double-check Subnode-Specific Behavior
   Since you mentioned you’re using a Subnode Code Tool to process data from an AI agent, be aware that subnodes behave similarly but make sure that any expression or data mapping does not inadvertently wrap your string result in an object.

Additional Considerations

• Data Structure Documentation
  Review the data structure documentation to ensure that you’re returning data in a format that conforms to n8n’s internal requirements.

• Verify Expression Resolution
  If you’re using expressions to transform your data within the node, confirm that these expressions resolve as expected and do not modify the data type unexpectedly.

By ensuring that your Function node returns an array with an object whose json property contains the string (or a property like response that is explicitly a string), you should be able to resolve the “response property should be a string, but it is an object” error.