How do I return a single object per item in 'Run Once for Each Item' mode in n8n?

Below is an explanation of what’s happening and how to fix it.

What's Going On

When you set the Code node to "Run Once for Each Item", n8n expects your code to return a single object for the current item (i.e. each execution run should output one object in the format below):

JAVASCRIPTCopied
{
  json: {
    // your data goes here
  }
}

If your code returns an array of objects instead—for example, if you perform some processing that results in multiple objects—this causes the error:

  "Code doesn’t return a single object [item 0]
  An array of objects was returned. If you need to output multiple items, please use the ‘Run Once for All Items’ mode instead."

What You Need to Do

Since you want to process each outputUrl separately, you should change your code to make sure that each execution returns a single object containing the processed data.

For example, suppose your input item is structured like this:

JSONCopied
{
  "outputUrl": "[\"https://api.pdfrest.com/resource/2f8d337f0-8a6b-48d8-8183-aff10704627f?format=file\",\"https://api.pdfrest.com/resource/2d95b0164-a03d-476b-86e4-70cb97968242?format=file\"]"
}

You can update your code (in "Run Once for Each Item" mode) as follows:

JAVASCRIPTCopied
// Parse the string into an array
const urls = JSON.parse(item.outputUrl);

// Map each URL to an image block
const imageBlocks = urls.map(url => {
  return {
    type: "image_url",
    image_url: {
      url: url,
    },
  };
});

// Return a single object with the imageBlocks
return {
  json: {
    imageBlocks,
  },
};

Explanation

• The code processes the individual input item (using item which is provided automatically in "Run Once for Each Item" mode).
• It parses the outputUrl property (which is a string representing an array of URLs) to an actual JavaScript array.
• It then maps each URL to your desired format.
• Finally, it returns one object (with a key json) instead of an array of objects.

With this pattern, each item processed by the Code node returns only one object, and you'll avoid the error.

Additional Note

If you intend to output multiple items from one execution, then you should switch to "Run Once for All Items" mode. But since your goal is to process each item individually, ensure that your code returns just a single object for that item.

This approach follows the conventions as outlined in the Code node common issues documentation for ensuring proper data structure when using “Run Once for Each Item” mode.