n8n-code-javascript

JavaScript Code Node

Expert guidance for writing JavaScript code in n8n Code nodes.

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

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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)

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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

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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

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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

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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

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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 || '[email protected]';

// ✅ 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

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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

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