How can I aggregate file IDs into a single string after a loop in n8n?

You can’t simply “accumulate” a value across loop iterations by running code in each iteration—each iteration is processed separately. Instead, you need to let the loop finish and then combine the file IDs from all items (which are output of the loop) in one final node (typically a Function or Code node running once for all items).

For example, add a Function/Code node after the loop (make sure it’s set to “Run Once for All Items”) and use code like this:

JAVASCRIPTCopied
// Get all file IDs from the items array (adjust the property name accordingly)
const fileIDs = items.map(item => item.json.fileID);

// Join them into a single concatenated string (comma-separated, for example)
const concatenatedIDs = fileIDs.join(',');

// Return a single item with the result available for later nodes
return [{ json: { concatenatedIDs } }];

Here’s a breakdown of the approach:

1. Output from the Loop:
   Make sure the loop node passes on every file ID (each being on a separate item in its output).

2. Final Aggregation Node:
   Place a Function (or Code) node after the loop. Configure it to “Run Once for All Items” so that it receives an array of items from the loop.
   In the node, map all the file IDs and combine them (e.g., using join).

3. Using the Concatenated String:
   Now your aggregated value (in this example in the field concatenatedIDs) is available for the next node.

This method avoids trying to combine values during each loop iteration (where the state isn’t preserved between iterations) and instead performs the accumulation once the loop has processed all items.