n8n-code-javascript

SKILL.md

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)

```javascript
// 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):

What runs per item
Approx. cost

Code All Items (one run for the whole set)
~0.02 ms/item

Expression in any node (IF / Set / etc.)
~0.2 ms/item

Code Each Item (a full sandbox per item)
~0.6 ms/item — ~25–30× All Items

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:

Empty code / missing return — always end with return [...], and make sure every branch returns.

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.

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.

Missing null checks — use optional chaining: item.json?.user?.email || 'fallback'.

Webhook body nesting — $json.email is undefined; use $json.body.email.

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.

Built-in Functions & Helpers

// HTTP requests (no auth — see sandbox note below)
const res = await this.helpers.httpRequest({ method: 'GET', url: 'https://api.example.com/data' });

// DateTime (Luxon): now, formatting, arithmetic
const now = DateTime.now();
const formatted = now.toFormat('yyyy-MM-dd');
const tomorrow = now.plus({ days: 1 });

// $jmespath() — query JSON structures
const adults = $jmespath($input.first().json, 'users[?age >= `18`]');

// $getWorkflowStaticData() — data that persists across executions

Sandbox (since n8n v2.0, JsTaskRunnerSandbox): the accessor is this.helpers.httpRequest() — the bare $helpers global is undefined here ($helpers.httpRequest() throws ReferenceError). Inside a nested async function where this is lost, call it as await fn.call(this, ...). this.helpers.httpRequestWithAuthentication and this.helpers.requestWithAuthenticationPaginated are deny-listed (→ UnsupportedFunctionError); for authenticated calls use an HTTP Request node with the credential (preferred), a sub-workflow, or a manual Authorization: Bearer ${token} header on this.helpers.httpRequest() only when the token already flows through the workflow as data. $env is blocked when N8N_BLOCK_ENV_ACCESS_IN_NODE=true; require() works only for allowlisted modules. Buffer, URL, and standard JS globals (Math, JSON, Object, Array) always work.

See: BUILTIN_FUNCTIONS.md for the complete reference — full httpRequest options, all DateTime/Luxon operations, JMESPath patterns, static-data use cases, and the sandbox-restriction details.

Best Practices

Validate input first — guard for empty arrays / missing .json before processing.

Try-catch risky work (HTTP calls) and return an error object instead of crashing.

Prefer array methods (filter/map/reduce) over manual loops.

Filter early, transform late — shrink the dataset before expensive work.

Descriptive names and console.log() for debugging (output goes to the browser console).

See: COMMON_PATTERNS.md → "Best Practices" for code examples of each.

Production Gotchas

Hard-won lessons from real deployments — summarized here, with code in DATA_ACCESS.md → "Production Gotchas":

SplitInBatches outputs are counterintuitive: main[0] = done (fires once, after all batches), main[1] = each batch (the loop body). Add a Limit 1 node after the done output as a safety.

Iteration count is the cost: each loop iteration re-runs the whole body through the engine (~0.8 ms overhead each). batchSize: 1 is the loop equivalent of Each Item — use the largest batch your real constraint (rate limit, page size, memory) allows, or don't loop at all.

Cross-iteration accumulation (CRITICAL): after the loop, $('Node Inside Loop').all() returns ONLY the last iteration's items. Accumulate via $getWorkflowStaticData('global') (reset before, push inside, read after).

pairedItem: when emitting items that don't map 1:1 to input, set pairedItem: { item: i } or downstream Set nodes fail with paired_item_no_info.

Node reference syntax: $('Node').first().json or $('Node').all() — never .json directly on the reference.

Float precision: compare currency at the cent level — Math.round(a*100) !== Math.round(b*100) — to avoid false positives from float noise.

When to Use Code Node

Before reaching for a Code node, walk the transform gatekeeper in the n8n Expression Syntax skill: expression → arrow-function IIFE inside an Edit Fields field → Code node, in that order. The first two paths cover most "transform this data" tasks at ~1–10ms each, versus the Code node's sandboxed ~500–1000ms — a ~100x gap on pure single-item shaping, with no functional difference. The Code node earns its place only for whole-dataset aggregation ($input.all()), allowlisted libraries, or async work. And before writing code for crypto (HMAC, hashing, signing) or XML/SOAP/RSS parsing, check for a native node — n8n has a Crypto node (nodes-base.crypto) and an XML node (nodes-base.xml) that cover those without any JavaScript. Dropping into Code for something a native node already does is one of the most common false positives.

Use Code node when:

✅ Complex transformations requiring multiple steps

✅ Custom calculations or business logic

✅ Recursive operations

✅ API response parsing with complex structure

✅ Multi-step conditionals

✅ Data aggregation across items

Consider other nodes when:

❌ Simple field mapping → Use Set node

❌ Basic filtering → Use Filter node

❌ Simple conditionals → Use IF or Switch node

❌ HTTP requests only → Use HTTP Request node

Code node excels at: Complex logic that would require chaining many simple nodes

Integration with Other Skills

Works With:

n8n Expression Syntax:

Expressions use {{ }} syntax in other nodes

Code nodes use JavaScript directly (no {{ }})

When to use expressions vs code

n8n MCP Tools Expert:

How to find Code node: search_nodes({query: "code"})

Get configuration help: get_node({nodeType: "nodes-base.code"})

Validate code: validate_node({nodeType: "nodes-base.code", config: {...}})

n8n Node Configuration:

Mode selection (All Items vs Each Item)

Language selection (JavaScript vs Python)

Understanding property dependencies

n8n Workflow Patterns:

Code nodes in transformation step

Webhook → Code → API pattern

Error handling in workflows

n8n Validation Expert:

Validate Code node configuration

Handle validation errors

Auto-fix common issues

Quick Reference Checklist

Before deploying Code nodes, verify:

 Code is not empty - Must have meaningful logic

 Return statement exists - Returns items, not a primitive/null

 Canonical return format - Each item: {json: {...}} (bare objects auto-wrap, but be explicit)

 Data access correct - Using $input.all(), $input.first(), or $input.item

 No {{ }} written as code - Use JavaScript template literals: `${value}`

 Error handling - Guard clauses for null/undefined inputs

 Webhook data - Access via .body if from webhook

 Mode selection - "All Items" for most cases

 Performance - Prefer map/filter over manual loops

 Output consistent - All code paths return same structure

Additional Resources

Related Files

DATA_ACCESS.md - Comprehensive data access patterns

COMMON_PATTERNS.md - 10 production-tested patterns

ERROR_PATTERNS.md - Top 5 errors and solutions

BUILTIN_FUNCTIONS.md - Complete built-in reference

n8n Documentation

Code Node Guide: https://docs.n8n.io/code/code-node/

Built-in Methods: https://docs.n8n.io/code-examples/methods-variables-reference/

Luxon Documentation: https://moment.github.io/luxon/

Ready to write JavaScript in n8n Code nodes! Start with simple transformations, use the error patterns guide to avoid common mistakes, and reference the pattern library for production-ready examples.