n8n Code Nodes

Author: Taghi Molavi

Decision tree

1. Can an expression on the consumer node do it? → do that.
2. Can Edit Fields + arrow function cover it? → prefer that.
3. Need multi-source aggregation, npm-less algorithms, or complex branching on items? → Code.
4. Need external libs / long-running jobs? → sub-workflow, custom node, or external service — not giant Code.

Default language: JavaScript. Use Python only if user insists or ecosystem requires it.

Return contract (JS)

Code must return items in n8n shape:

return items.map(item => ({
  json: { ...item.json, score: 1 },
  pairedItem: item.pairedItem, // preserve when possible
}));

Or:

return [{ json: { ok: true, count: items.length } }];

Common breaks

• Returning raw objects/arrays without { json: ... }
• Forgetting to handle empty items
• Mutating shared state across runs
• Trying to require disallowed modules
• Hiding secrets in code

Run once for all items vs per item

• Per item: simpler expressions pairing
• All items: aggregation, dedup, sort — then output one or many

Pair with

• n8n-expressions for lighter transforms
• n8n-debugging when Code "fails silently" (often empty return)