n8n-code-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. CRITICAL: Must return [{json: {...}}] format
4. CRITICAL: Webhook data is under $json.body (not $json directly)
5. Built-ins available: $helpers.httpRequest() (no auth), DateTime (Luxon), $jmespath(). Not available: $helpers.httpRequestWithAuthentication, $env (when N8N_BLOCK_ENV_ACCESS_IN_NODE=true), require() (unless allowlisted)
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

CRITICAL RULE: Always return array of objects with json property

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 [];
}

Incorrect Return Formats

// ❌ WRONG: Object without array wrapper
return {
  json: {field: value}
};

// ❌ WRONG: Array without json wrapper
return [{field: value}];

// ❌ WRONG: Plain string
return "processed";

// ❌ WRONG: Raw data without mapping
return $input.all();  // Missing .map()

// ❌ WRONG: Incomplete structure
return [{data: value}];  // Should be {json: value}

Why it matters: Next nodes expect array format. Incorrect format causes workflow execution to fail.

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 in code — no {{ }}. Use JavaScript: ` ${$json.field} or $input.first().json.field`.
3. Wrong return wrapper — return {json:{...}} fails; must be return [{json:{...}}].
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.

---

Built-in Functions & Helpers

// HTTP requests (no auth — see sandbox note below)
const res = await $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): $helpers.httpRequestWithAuthentication is blocked; $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(a100) !== Math.round(b100) — 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 - Must return array of objects
• [ ] Proper return format - Each item: {json: {...}}
• [ ] Data access correct - Using $input.all(), $input.first(), or $input.item
• [ ] No n8n expressions - 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.