JavaScript Code Node Skill

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

Choose "Run Once for All Items" mode (recommended for most use cases)
Access data: $input.all(), $input.first(), or $input.item
CRITICAL: Must return [{json: {...}}] format
CRITICAL: Webhook data is under $json.body (not $json directly)
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)

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)

Data Access Patterns

Pattern 1: $input.all() - Most Common

Use when: Processing arrays, batch operations, aggregations

// Get all items from previous node
const allItems = $input.all();

// Filter, map, reduce as needed
const valid = allItems.filter(item => item.json.status === 'active');
const mapped = valid.map(item => ({
  json: {
    id: item.json.id,
    name: item.json.name
  }
}));

return mapped;

Pattern 2: $input.first() - Very Common

Use when: Working with single objects, API responses, first-in-first-out

// Get first item only
const firstItem = $input.first();
const data = firstItem.json;

return [{
  json: {
    result: processData(data),
    processedAt: new Date().toISOString()
  }
}];

Pattern 3: $input.item - Each Item Mode Only

Use when: In "Run Once for Each Item" mode

// Current item in loop (Each Item mode only)
const currentItem = $input.item;

return [{
  json: {
    ...currentItem.json,
    itemProcessed: true
  }
}];

Pattern 4: $node - Reference Other Nodes

Use when: Need data from specific nodes in workflow

// Get output from specific node
const webhookData = $node["Webhook"].json;
const httpData = $node["HTTP Request"].json;

return [{
  json: {
    combined: {
      webhook: webhookData,
      api: httpData
    }
  }
}];

See: DATA_ACCESS.md for comprehensive guide

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

Based on production workflows, here are the most useful patterns:

1. Multi-Source Data Aggregation

Combine data from multiple APIs, webhooks, or nodes

const allItems = $input.all();
const results = [];

for (const item of allItems) {
  const sourceName = item.json.name || 'Unknown';
  // Parse source-specific structure
  if (sourceName === 'API1' && item.json.data) {
    results.push({
      json: {
        title: item.json.data.title,
        source: 'API1'
      }
    });
  }
}

return results;

2. Filtering with Regex

Extract patterns, mentions, or keywords from text

const pattern = /\b([A-Z]{2,5})\b/g;
const matches = {};

for (const item of $input.all()) {
  const text = item.json.text;
  const found = text.match(pattern);

  if (found) {
    found.forEach(match => {
      matches[match] = (matches[match] || 0) + 1;
    });
  }
}

return [{json: {matches}}];

3. Data Transformation & Enrichment

Map fields, normalize formats, add computed fields

const items = $input.all();

return items.map(item => {
  const data = item.json;
  const nameParts = data.name.split(' ');

  return {
    json: {
      first_name: nameParts[0],
      last_name: nameParts.slice(1).join(' '),
      email: data.email,
      created_at: new Date().toISOString()
    }
  };
});

4. Top N Filtering & Ranking

Sort and limit results

const items = $input.all();

const topItems = items
  .sort((a, b) => (b.json.score || 0) - (a.json.score || 0))
  .slice(0, 10);

return topItems.map(item => ({json: item.json}));

5. Aggregation & Reporting

Sum, count, group data

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,
    timestamp: new Date().toISOString()
  }
}];

See: COMMON_PATTERNS.md for 10 detailed production patterns

Error Prevention - Top 5 Mistakes

#1: Empty Code or Missing Return (Most Common)

// ❌ WRONG: No return statement
const items = $input.all();
// ... processing code ...
// Forgot to return!

// ✅ CORRECT: Always return data
const items = $input.all();
// ... processing ...
return items.map(item => ({json: item.json}));

#2: Expression Syntax Confusion

// ❌ WRONG: Using n8n expression syntax in code
const value = "{{ $json.field }}";

// ✅ CORRECT: Use JavaScript template literals
const value = `${$json.field}`;

// ✅ CORRECT: Direct access
const value = $input.first().json.field;

#3: Incorrect Return Wrapper

// ❌ WRONG: Returning object instead of array
return {json: {result: 'success'}};

// ✅ CORRECT: Array wrapper required
return [{json: {result: 'success'}}];

#4: Missing Null Checks

// ❌ WRONG: Crashes if field doesn't exist
const value = item.json.user.email;

// ✅ CORRECT: Safe access with optional chaining
const value = item.json?.user?.email || 'no-email@example.com';

// ✅ CORRECT: Guard clause
if (!item.json.user) {
  return [];
}
const value = item.json.user.email;

#5: Webhook Body Nesting

// ❌ WRONG: Direct access to webhook data
const email = $json.email;

// ✅ CORRECT: Webhook data under .body
const email = $json.body.email;

See: ERROR_PATTERNS.md for comprehensive error guide

Built-in Functions & Helpers

$helpers.httpRequest()

Make HTTP requests from within code:

const response = await $helpers.httpRequest({
  method: 'GET',
  url: 'https://api.example.com/data',
  headers: {
    'Authorization': 'Bearer token',
    'Content-Type': 'application/json'
  }
});

return [{json: {data: response}}];

DateTime (Luxon)

Date and time operations:

// Current time
const now = DateTime.now();

// Format dates
const formatted = now.toFormat('yyyy-MM-dd');
const iso = now.toISO();

// Date arithmetic
const tomorrow = now.plus({days: 1});
const lastWeek = now.minus({weeks: 1});

return [{
  json: {
    today: formatted,
    tomorrow: tomorrow.toFormat('yyyy-MM-dd')
  }
}];

$jmespath()

Query JSON structures:

const data = $input.first().json;

// Filter array
const adults = $jmespath(data, 'users[?age >= `18`]');

// Extract fields
const names = $jmespath(data, 'users[*].name');

return [{json: {adults, names}}];

See: BUILTIN_FUNCTIONS.md for complete reference

Best Practices

1. Always Validate Input Data

const items = $input.all();

// Check if data exists
if (!items || items.length === 0) {
  return [];
}

// Validate structure
if (!items[0].json) {
  return [{json: {error: 'Invalid input format'}}];
}

// Continue processing...

2. Use Try-Catch for Error Handling

try {
  const response = await $helpers.httpRequest({
    url: 'https://api.example.com/data'
  });

  return [{json: {success: true, data: response}}];
} catch (error) {
  return [{
    json: {
      success: false,
      error: error.message
    }
  }];
}

3. Prefer Array Methods Over Loops

// ✅ GOOD: Functional approach
const processed = $input.all()
  .filter(item => item.json.valid)
  .map(item => ({json: {id: item.json.id}}));

// ❌ SLOWER: Manual loop
const processed = [];
for (const item of $input.all()) {
  if (item.json.valid) {
    processed.push({json: {id: item.json.id}});
  }
}

4. Filter Early, Process Late

// ✅ GOOD: Filter first to reduce processing
const processed = $input.all()
  .filter(item => item.json.status === 'active')  // Reduce dataset first
  .map(item => expensiveTransformation(item));  // Then transform

// ❌ WASTEFUL: Transform everything, then filter
const processed = $input.all()
  .map(item => expensiveTransformation(item))  // Wastes CPU
  .filter(item => item.json.status === 'active');

5. Use Descriptive Variable Names

// ✅ GOOD: Clear intent
const activeUsers = $input.all().filter(item => item.json.active);
const totalRevenue = activeUsers.reduce((sum, user) => sum + user.json.revenue, 0);

// ❌ BAD: Unclear purpose
const a = $input.all().filter(item => item.json.active);
const t = a.reduce((s, u) => s + u.json.revenue, 0);

6. Debug with console.log()

// Debug statements appear in browser console
const items = $input.all();
console.log(`Processing ${items.length} items`);

for (const item of items) {
  console.log('Item data:', item.json);
  // Process...
}

return result;

Production Gotchas

Hard-won lessons from real-world n8n workflow deployments:

SplitInBatches Loop Semantics

The SplitInBatches node has two outputs — and the naming is counterintuitive:

main[0] = done — fires ONCE after all batches are processed
main[1] = each batch — fires for every batch (this is the loop body)

Always add a Limit 1 node after the done output before downstream processing, as a safety against edge cases where done fires with extra items.

Cross-Iteration Data Accumulation (CRITICAL)

After a SplitInBatches loop, $('Node Inside Loop').all() returns ONLY the last iteration's items, not cumulative results. This silently drops data from all but the final batch.

Fix: Use workflow static data to accumulate across iterations:

// BEFORE the loop (reset accumulator):
const staticData = $getWorkflowStaticData('global');
staticData.results = [];
return $input.all();

// INSIDE the loop body (accumulate):
const staticData = $getWorkflowStaticData('global');
const results = [];
for (const item of $input.all()) {
  const processed = { /* ... */ };
  results.push({ json: processed });
  staticData.results.push(processed);
}
return results;

// AFTER the loop (read accumulated data):
const staticData = $getWorkflowStaticData('global');
const allResults = staticData.results || [];
// Now aggregate across ALL iterations

pairedItem for New Output Items

When creating new items that don't map 1:1 to input items, include pairedItem — otherwise downstream Set nodes fail with paired_item_no_info:

const results = [];
for (let i = 0; i < $input.all().length; i++) {
  const item = $input.all()[i];
  results.push({
    json: { /* new data */ },
    pairedItem: { item: i }
  });
}
return results;

Correct Node Reference Syntax

// ❌ WRONG - .json directly on node reference
const data = $('HTTP Request').json;

// ✅ CORRECT - call .first() then access .json
const data = $('HTTP Request').first().json;

// ✅ Also correct - get all items
const allData = $('HTTP Request').all();

Float Precision for Price/Currency Comparison

When comparing prices or currency values, floating point noise can cause false positives. Round to cents:

// ❌ Unreliable - float comparison
if (newPrice !== oldPrice) { /* triggers on noise */ }

// ✅ Reliable - compare at cent level
if (Math.round(newPrice * 100) !== Math.round(oldPrice * 100)) {
  // Real price change detected
}

When to Use Code Node

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.

Agent Skills: JavaScript Code Node

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