JavaScript Code Node

Expert guidance for writing JavaScript code in n8n Code nodes.

Quick Start

javascript

// 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(), DateTime (Luxon), $jmespath()

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)

javascript

// 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

javascript

// 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

javascript

// 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

javascript

// 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

javascript

// 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

javascript

// ❌ 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

javascript

// ✅ 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

javascript

// ❌ 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

javascript

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

javascript

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

javascript

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

javascript

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

javascript

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)

javascript

// ❌ 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

javascript

// ❌ 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

javascript

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

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

#4: Missing Null Checks

javascript

// ❌ 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

javascript

// ❌ 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:

javascript

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:

javascript

// 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:

javascript

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

javascript

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

javascript

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

javascript

// ✅ 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

javascript

// ✅ 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

javascript

// ✅ 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()

javascript

// 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;

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_essentials("nodes-base.code")
```
Validate code:
```
validate_node_operation()
```

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.

n8n-code-javascript

NPX Install

Tags

SKILL.md Content

JavaScript Code Node

Quick Start

Essential Rules

Mode Selection Guide

Run Once for All Items (Recommended - Default)

Run Once for Each Item

Data Access Patterns

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

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

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

Pattern 4: $node - Reference Other Nodes

Critical: Webhook Data Structure

Return Format Requirements

Correct Return Formats

Incorrect Return Formats

Common Patterns Overview

1. Multi-Source Data Aggregation

2. Filtering with Regex

3. Data Transformation & Enrichment

4. Top N Filtering & Ranking

5. Aggregation & Reporting

Error Prevention - Top 5 Mistakes

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

#2: Expression Syntax Confusion

#3: Incorrect Return Wrapper

#4: Missing Null Checks

#5: Webhook Body Nesting

Built-in Functions & Helpers

$helpers.httpRequest()

DateTime (Luxon)

$jmespath()

Best Practices

1. Always Validate Input Data

2. Use Try-Catch for Error Handling

3. Prefer Array Methods Over Loops

4. Filter Early, Process Late

5. Use Descriptive Variable Names

6. Debug with console.log()

When to Use Code Node

Integration with Other Skills

Works With:

Quick Reference Checklist

Additional Resources

Related Files

n8n Documentation