n8n Code Node JavaScript

JavaScript Code Node

Expert guidance for writing JavaScript code in n8n Code nodes.

Quick Start

// Basic template for Code nodes
const items = $input.all();

// Process data
const processed = items.map(item => ({
  json: {
    ...item.json,
    processed: true,
    timestamp: new Date().toISOString()
  }
}));

return processed;

Essential Rules

1. Choose "Run Once for All Items" mode (recommended for most use cases)
2. Access data: $input.all(), $input.first(), or $input.item
3. Return [{json: {...}}] — the canonical, mode-portable form. In Run Once for All Items mode n8n also auto-wraps a bare return {…} object, so that runs too; what genuinely fails is returning a primitive (string/number) or null.
4. CRITICAL: Webhook data is under $json.body (not $json directly)
5. Built-ins available: this.helpers.httpRequest() (no auth — the bare $helpers global is undefined in the task-runner sandbox, so $helpers.httpRequest() throws ReferenceError: $helpers is not defined), DateTime (Luxon), $jmespath(). Not available: this.helpers.httpRequestWithAuthentication (deny-listed), $env (when N8N_BLOCK_ENV_ACCESS_IN_NODE=true), require() (unless allowlisted). For anything beyond a trivial unauthenticated GET (auth, pagination, retries), prefer the HTTP Request node and keep Code nodes for pure logic.
6. Instance-allowlisted libraries: Self-hosted instances can allowlist modules via N8N_RUNNERS_ALLOWED_BUILT_IN_MODULES and N8N_RUNNERS_ALLOWED_EXTERNAL_MODULES (legacy: NODE_FUNCTION_ALLOW_BUILTIN / NODE_FUNCTION_ALLOW_EXTERNAL). If the user says their instance allows specific modules (e.g. axios, lodash, crypto), use them via require() — don't refuse. If unsure, ask or default to built-ins only.
7. Wrong skill? If you're writing code for a Custom Code Tool attached to an AI Agent (@n8n/n8n-nodes-langchain.toolCode), stop — that node has a different contract (input via query, must return a string, no $input/$helpers). Use the n8n-code-tool skill.

Mode Selection Guide

The Code node offers two execution modes. Choose based on your use case:

Run Once for All Items (Recommended - Default)

Use this mode for: 95% of use cases

• How it works: Code executes once regardless of input count
• Data access: $input.all() or items array
• Best for: Aggregation, filtering, batch processing, transformations, API calls with all data
• Performance: Faster for multiple items (single execution)

// Example: Calculate total from all items
const allItems = $input.all();
const total = allItems.reduce((sum, item) => sum + (item.json.amount || 0), 0);

return [{
  json: {
    total,
    count: allItems.length,
    average: total / allItems.length
  }
}];

When to use:

• ✅ Comparing items across the dataset
• ✅ Calculating totals, averages, or statistics
• ✅ Sorting or ranking items
• ✅ Deduplication
• ✅ Building aggregated reports
• ✅ Combining data from multiple items

Run Once for Each Item

Use this mode for: Specialized cases only

• How it works: Code executes separately for each input item
• Data access: $input.item or $item
• Best for: Item-specific logic, independent operations, per-item validation
• Performance: Slower for large datasets (multiple executions)

// Example: Add processing timestamp to each item
const item = $input.item;

return [{
  json: {
    ...item.json,
    processed: true,
    processedAt: new Date().toISOString()
  }
}];

When to use:

• ✅ Each item needs independent API call
• ✅ Per-item validation with different error handling
• ✅ Item-specific transformations based on item properties
• ✅ When items must be processed separately for business logic

Decision Shortcut:

• Need to look at multiple items? → Use "All Items" mode
• Each item completely independent? → Use "Each Item" mode
• Not sure? → Use "All Items" mode (you can always loop inside)

Why "All Items" is faster — the per-item boundary

Mode choice is the single biggest performance lever in a Code node. Each per-item execution context costs a setup tax (measured on n8n 2.x, small records):

So Run Once for Each Item over 10k items is ~6 s of pure overhead vs ~0.2 s in Run Once for All Items. Use Each Item only when an item genuinely needs isolating (independent error handling, or a per-item API call you can't batch); otherwise loop inside one All Items node. Expression complexity itself is essentially free (~90% of the cost is the per-item context, not your code) and every node→node hop re-copies all items — so reduce the number of per-item boundaries, don't micro-optimize each one. Below a few hundred items none of this matters; reach for it on the hot path (large item counts, little I/O).

See: DATA_ACCESS.md → "Mode Performance" for the corollaries, hop costs, and scale check.

Data Access Patterns

Four ways to pull data from upstream nodes. Note $node["Name"] and $('Name') need .first().json or .all() — never .json directly.

const allItems = $input.all();          // 1. All items — batch ops, aggregation (most common)
const data = $input.first().json;       // 2. First item — single objects, API responses
const item = $input.item;               // 3. Current item — "Each Item" mode ONLY (undefined otherwise)
const other = $node["Webhook"].json;    // 4. Named node — combine data across nodes

Always access fields via .json (e.g. item.json.name, not item.name), and prefer the explicit $input.first().json.field over a bare $json.field.

See: DATA_ACCESS.md for the full guide — every pattern with examples, a decision tree, and the common mistakes (mutating originals, missing length checks, $input.item in the wrong mode).

Critical: Webhook Data Structure

MOST COMMON MISTAKE: Webhook data is nested under .body

// ❌ WRONG - Will return undefined
const name = $json.name;
const email = $json.email;

// ✅ CORRECT - Webhook data is under .body
const name = $json.body.name;
const email = $json.body.email;

// Or with $input
const webhookData = $input.first().json.body;
const name = webhookData.name;

Why: Webhook node wraps all request data under body property. This includes POST data, query parameters, and JSON payloads.

See: DATA_ACCESS.md for full webhook structure details

Return Format Requirements

Canonical form: [{json: {...}}] — an array of objects each with a json property. It is unambiguous and works identically in both execution modes, so make it your default.

In Run Once for All Items mode n8n auto-normalizes looser shapes on the way out: a single bare object, or an array of bare objects, gets wrapped under json for you. So return {foo: 1} runs. What has nothing to wrap — and therefore genuinely fails at runtime with "Code doesn't return items properly" — is a primitive (string/number/boolean) or null/undefined. (n8n-mcp ≥ 2.63.0 no longer flags a bare-object return as an error; it reflects this auto-wrap behavior.)

Correct Return Formats

// ✅ Single result
return [{
  json: {
    field1: value1,
    field2: value2
  }
}];

// ✅ Multiple results
return [
  {json: {id: 1, data: 'first'}},
  {json: {id: 2, data: 'second'}}
];

// ✅ Transformed array
const transformed = $input.all()
  .filter(item => item.json.valid)
  .map(item => ({
    json: {
      id: item.json.id,
      processed: true
    }
  }));
return transformed;

// ✅ Empty result (when no data to return)
return [];

// ✅ Conditional return
if (shouldProcess) {
  return [{json: processedData}];
} else {
  return [];
}

Non-Canonical Returns (auto-wrapped — prefer the canonical form)

// ⚠️ Auto-wrapped in All Items mode → [{json: {field: value}}]. Runs, but prefer the array form.
return {
  json: {field: value}
};

// ⚠️ Auto-wrapped → [{json: {field: value}}]. Runs, but add the json wrapper for clarity.
return [{field: value}];

// ✅ Fine — input items already carry a json property, so returning them unchanged is a valid passthrough
return $input.all();

Genuinely Broken Returns

// ❌ FAILS: primitive — n8n errors "Code doesn't return items properly"
return "processed";

// ❌ FAILS: null / undefined — nothing to pass to the next node
return null;

Why it matters: The canonical [{json: {...}}] is unambiguous and behaves the same in both modes. n8n auto-normalizes bare objects and arrays-of-objects in All Items mode, but a primitive or null return has nothing to wrap and stops execution.

See: ERROR_PATTERNS.md #3 for detailed error solutions

Common Patterns Overview

The most useful Code node shapes from production workflows. One quick example — sum/aggregate across all items:

const items = $input.all();
const total = items.reduce((sum, item) => sum + (item.json.amount || 0), 0);
return [{ json: { total, count: items.length, average: total / items.length } }];

The full library covers 10 patterns: multi-source aggregation, regex filtering, markdown/structured-text parsing, JSON comparison, CRM/form transformation, release processing, array transformation with computed fields, Slack Block Kit formatting, top-N ranking, and string-aggregation reporting — each with variations.

See: COMMON_PATTERNS.md for the 10 detailed production patterns (and the Best Practices section: validate input, try-catch, filter-early, array methods over loops, console.log debugging).

Error Prevention - Top Mistakes

The recurring Code node failures, in rough frequency order:

1. Empty code / missing return — always end with return [...], and make sure every branch returns.
2. Expression syntax as code — don't write {{ }} where JavaScript belongs (return {{ $json.x }} is a syntax error). Use `${$json.field}` or $input.first().json.field. {{ }} inside a string literal is fine — it's just literal text n8n won't evaluate.
3. Return shape — prefer return [{json:{...}}]. A bare return {…} auto-wraps in All Items mode, but returning a primitive (string/number) or null is what actually fails.
4. Missing null checks — use optional chaining: item.json?.user?.email || 'fallback'.
5. Webhook body nesting — $json.email is undefined; use $json.body.email.
6. Auth helpers blocked (httpRequestWithAuthentication) and $env blocked — route secrets through credentials/HTTP Request node, not the Code node sandbox.

See: ERROR_PATTERNS.md for the comprehensive guide — each error with wrong/right code, escaping rules, the sandbox restrictions (Errors #6–#7), a prevention checklist, and a quick error-message lookup table.