Need custom logic?
├── Is it a transformation of one or two fields?
│   └── Expression: {{ $json.foo.toUpperCase() }} or {{ $json.items.map(item => item.name).join(', ') }}
│       Most "just transform this" cases land here.
│
├── Is it multi-line, but pure data shaping (map, filter, reduce, conditional)?
│   └── Edit Fields with arrow function expression. See references/ARROW_FUNCTIONS_IN_EDIT_FIELDS.md
│
├── Does it need full statements, multiple data sources, or external libs?
│   ├── Are you SURE the above two don't work?
│   │   └── Re-read the parent. The bar is high.
│   └── Yes, genuinely needs it
│       └── Code node. See references/JAVASCRIPT_PATTERNS.md
│
└── Is it actually two separate transformations stitched together?
    └── Use two nodes (Edit Fields → Edit Fields, or Edit Fields → IF). Composability beats one big Code block.

// ❌ Code node
return { name: $input.first().json.name.toUpperCase() }

// ✅ Expression in Edit Fields, "name" field
{{ $json.name.toUpperCase() }}

// ❌ Code node
const items = $input.first().json.items
return { tags: items.map(item => item.tag).filter(tag => tag).join(', ') }

// ✅ Expression
{{ $json.items.map(item => item.tag).filter(tag => tag).join(', ') }}

// ❌ Code node
const date = new Date($input.first().json.created_at)
return { formatted: date.toISOString().slice(0, 10) }

// ✅ Expression with n8n's date extension
{{ $json.created_at.toDateTime().format('yyyy-MM-dd') }}

// In Edit Fields, "summary" field:
{{ (() => {
    const items = $json.items
    const total = items.reduce((sum, item) => sum + item.price, 0)
    const tax = total * 0.08
    return `Total: $${(total + tax).toFixed(2)}`
})() }}

// Real-world shape: aggregate test results from one source,
// model metadata from another, category mapping from a third,
// and produce per-model-per-category averages.

const testResults = $('Get Test Results').all().map(item => item.json)
const models = $('Get Models').all().map(item => item.json)
const categoryMap = $('Get Category Map').first().json.testCategoryMap

const categoryByTestId = Object.fromEntries(
    categoryMap.map(mapping => [mapping.testId, mapping.category])
)

const result = models.map(model => {
    const modelTests = testResults.filter(test => test.modelId === model.id)
    const stats = modelTests.reduce((acc, test) => {
        const cat = categoryByTestId[test.testId]
        if (!cat) return acc
        acc[cat] ??= { scored: 0, available: 0, count: 0 }
        acc[cat].scored += test.pointsScored ?? 0
        acc[cat].available += test.pointsAvailable ?? 0
        acc[cat].count += 1
        return acc
    }, {})

    const averages = Object.fromEntries(
        Object.entries(stats).map(([category, stat]) => [
            category,
            { avg: stat.available > 0 ? stat.scored / stat.available : 0, n: stat.count }
        ])
    )

    return { modelId: model.id, modelName: model.modelName, ...averages }
})

return result.map(json => ({ json }))

// WRONG (recurring AI slip):
const crypto = require('crypto')
const hash = crypto.createHash('sha256').update(buf).digest('hex').slice(0, 12)

// RIGHT: configure the Crypto node with operation: 'hash', type: 'SHA256'
//   then read $('Hash').item.json.<output> downstream.

// WRONG (recurring AI slip with binary):
const crypto = require('crypto')
const buf = await this.helpers.getBinaryDataBuffer($itemIndex, 'data')
const hash = crypto.createHash('sha256').update(buf).digest('hex')

// RIGHT: Crypto node configured to hash binaryPropertyName='data', SHA256.
//   Then $('Crypto').item.json.<output> has the hash; chain a Set node for any field shaping.

// WRONG (recurring AI slip):
// XML node already parsed → another Code node to extract a few fields:
const entry = $('Parse XML').item.json.feed.entry
const firstEntry = Array.isArray(entry) ? entry[0] : entry
return { json: { title: firstEntry.title, url: firstEntry.link.find(link => link.type === 'pdf').href } }

// RIGHT: Edit Fields with arrow function expressions:
//   title:  ={{ (() => { const entry = $('Parse XML').item.json.feed.entry; return Array.isArray(entry) ? entry[0].title : entry.title; })() }}
//   pdfUrl: ={{ $('Parse XML').item.json.feed.entry.link.find(link => link.type === 'pdf')?.href }}

// Run Once for All Items
const items = $input.all()
const totals = items.map(item => ({
  ...item.json,
  total: item.json.qty * item.json.price,
}))
return totals.map(json => ({ json }))