Work with data

The item structure#

Data between nodes is an array of items, each holding a json object and optionally binary data:

[
  { "json": { "id": 1, "email": "ada@example.com" } },
  { "json": { "id": 2, "email": "grace@example.com" } }
]

A node receiving this runs twice, once per item. Understanding that single fact explains most surprising behaviour in n8n — including why a node sometimes appears to run more times than you expected.

Read values with expressions#

An expression reaches data from anywhere earlier in the run:

{{ $json.email }}                        // field on the current item
{{ $node["HTTP Request"].json.id }}      // output of a named node
{{ $json.items[0].price }}               // nested value
{{ $now.minus(1, 'day').toISO() }}       // a timestamp, via Luxon
{{ $json.name?.toUpperCase() ?? 'N/A' }} // optional chaining and a fallback

That last line is worth copying as a habit. Data from real APIs has missing fields, and ?. with ?? turns a failed run into a sensible default.

Reshape with the Set node#

Edit Fields (Set) builds a clean output object. Use it right after any node whose response is larger than what you need — a 60-field API response passed downstream becomes 60 fields you have to read past every time you debug.

Enable Include Other Input Fields only when you actually want the rest carried along.

Drop into code#

When an expression stops fitting on one line, use the Code node. It runs JavaScript or Python over the items.

// Run Once for All Items
const byDomain = {};
 
for (const item of $input.all()) {
  const domain = item.json.email.split('@')[1];
  byDomain[domain] ??= [];
  byDomain[domain].push(item.json.email);
}
 
return Object.entries(byDomain).map(([domain, emails]) => ({
  json: { domain, count: emails.length, emails },
}));

Two modes, and the choice matters:

Run Once for All Items — the code runs once and sees the whole set through $input.all(). Required for grouping, sorting, or aggregating.
Run Once for Each Item — the code runs per item and sees $json. Simpler when each item is independent.

Whatever you return must be a list of objects with a json key. Returning a bare array of values is the most common Code node error.

Binary data#

Files — downloads, uploads, generated PDFs — travel on the item's binary property, separate from json. Nodes that produce files write there, nodes that consume them read from there, and the field name (data by default) has to match on both sides.

Binary data is held in memory during a run. Moving large files through many nodes is the usual cause of memory pressure on small self-hosted instances.

Flow logic — routing the data you just reshaped
Core concepts — items and expressions, from the top
Handle errors — when a transformation hits a missing field

Updated Aug 15, 2026