n8n-code-javascript

# JavaScript Code Node Expert guidance for writing JavaScript code in n8n Code nodes. --- ## Quick Start ```javascript // Basic template for Code nodes const items = $input.all(); // Process data const processed = items.map(item => ({ json: { ...item.json, processed: true, timestamp: new Date().toISOString() } })); return processed; ``` ### Essential Rules 1. **Choose "Run Once for All Items" mode** (recommended for most use cases) 2. **Access data**: `$input.all()`, `$input.first()`, or `$input.item` 3. **Return `[{json: {...}}]`** — the canonical, mode-portable form. In *Run Once for All Items* mode n8n also auto-wraps a bare `return {…}` object, so that runs too; what genuinely fails is returning a primitive (string/number) or `null`. 4. **CRITICAL**: Webhook data is under `$json.body` (not `$json` directly) 5. **Built-ins available**: `this.helpers.httpRequest()` (no auth — the bare `$helpers` global is **undefined** in the task-runner sandbox, so `$helpers.httpRequest()` throws `ReferenceError: $helpers is not defined`), DateTime (Luxon), $jmespath(). **Not available**: `this.helpers.httpRequestWithAuthentication` (deny-listed), $env (when N8N_BLOCK_ENV_ACCESS_IN_NODE=true), require() (unless allowlisted). For anything beyond a trivial unauthenticated GET (auth, pagination, retries), prefer the **HTTP Request node** and keep Code nodes for pure logic. 6. **Instance-allowlisted libraries**: Self-hosted instances can allowlist modules via `N8N_RUNNERS_ALLOWED_BUILT_IN_MODULES` and `N8N_RUNNERS_ALLOWED_EXTERNAL_MODULES` (legacy: `NODE_FUNCTION_ALLOW_BUILTIN` / `NODE_FUNCTION_ALLOW_EXTERNAL`). If the user says their instance allows specific modules (e.g. `axios`, `lodash`, `crypto`), use them via `require()` — don't refuse. If unsure, ask or default to built-ins only. 7. **Wrong skill?** If you're writing code for a **Custom Code Tool** attached to an AI Agent (`@n8n/n8n-nodes-langchain.toolCode`), stop — that node has a different contract (input via `query`, must return a string, no `$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) ```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 = $input.item; return [{ json: { ...item.json, processed: true, processedAt: new Date().toISOString() } }]; ``` **When to use:** - ✅ Each item needs independent API call - ✅ Per-item validation with different error handling - ✅ Item-specific transformations based on item properties - ✅ When items must be processed separately for business logic **Decision Shortcut:** - **Need to look at multiple items?** → Use "All Items" mode - **Each item completely independent?** → Use "Each Item" mode - **Not sure?** → Use "All Items" mode (you can always loop inside) ### Why "All Items" is faster — the per-item boundary Mode choice is the single biggest performance lever in a Code node. Each *per-item* execution context costs a setup tax (measured on n8n 2.x, small records): | What runs per item | Approx. cost | |---|---| | Code **All Items** (one run for the whole set) | ~0.02 ms/item | | Expression in any node (IF / Set / etc.) | ~0.2 ms/item | | Code **Each Item** (a full sandbox per item) | ~0.6 ms/item — ~25–30× All Items | So `Run Once for Each Item` over 10k items is ~6 s of pure overhead vs ~0.2 s in `Run Once for All Items`. Use Each Item only when an item genuinely needs isolating (independent error handling, or a per-item API call you can't batch); otherwise loop *inside* one All Items node. Expression complexity itself is essentially free (~90% of the cost is the per-item context, not your code) and every node→node hop re-copies all items — so reduce the *number* of per-item boundaries, don't micro-optimize each one. Below a few hundred items none of this matters; reach for it on the hot path (large item counts, little I/O). **See**: [DATA_ACCESS.md](DATA_ACCESS.md) → "Mode Performance" for the corollaries, hop costs, and scale check. --- ## Data Access Patterns Four ways to pull data from upstream nodes. Note `$node["Name"]` and `$('Name')` need `.first().json` or `.all()` — never `.json` directly. ```javascript 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 `$json.field`. **See**: [DATA_ACCESS.md](DATA_ACCESS.md) for the full guide — every pattern with examples, a decision tree, and the common mistakes (mutating originals, missing length checks, `$input.item` in the wrong mode). --- ## Critical: Webhook Data Structure **MOST COMMON MISTAKE**: Webhook data is nested under `.body` ```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](DATA_ACCESS.md) for full webhook structure details --- ## Return Format Requirements **Canonical form**: `[{json: {...}}]` — an array of objects each with a `json` property. It is unambiguous and works identically in both execution modes, so make it your default. In *Run Once for All Items* mode n8n auto-normalizes looser shapes on the way out: a single bare object, or an array of bare objects, gets wrapped under `json` for you. So `return {foo: 1}` runs. What has nothing to wrap — and therefore genuinely fails at runtime with "Code doesn't return items properly" — is a primitive (string/number/boolean) or `null`/`undefined`. (n8n-mcp ≥ 2.63.0 no longer flags a bare-object return as an error; it reflects this auto-wrap behavior.) ### 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 []; } ``` ### Non-Canonical Returns (auto-wrapped — prefer the canonical form) ```javascript // ⚠️ Auto-wrapped in All Items mode → [{json: {field: value}}]. Runs, but prefer the array form. return { json: {field: value} }; // ⚠️ Auto-wrapped → [{json: {field: value}}]. Runs, but add the json wrapper for clarity. return [{field: value}]; // ✅ Fine — input items already carry a json property, so returning them unchanged is a valid passthrough return $input.all(); ``` ### Genuinely Broken Returns ```javascript // ❌ FAILS: primitive — n8n errors "Code doesn't return items properly" return "processed"; // ❌ FAILS: null / undefined — nothing to pass to the next node return null; ``` **Why it matters**: The canonical `[{json: {...}}]` is unambiguous and behaves the same in both modes. n8n auto-normalizes bare objects and arrays-of-objects in All Items mode, but a primitive or `null` return has nothing to wrap and stops execution. **See**: [ERROR_PATTERNS.md](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: ```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 } }]; ``` 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](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 as code** — don't write `{{ }}` where JavaScript belongs (`return {{ $json.x }}` is a syntax error). Use `` `${$json.field}` `` or `$input.first().json.field`. `{{ }}` *inside a string literal* is fine — it's just literal text n8n won't evaluate. 3. **Return shape** — prefer `return [{json:{...}}]`. A bare `return {…}` auto-wraps in All Items mode, but returning a primitive (string/number) or `null` is what actually fails. 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](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 ```javascript // HTTP requests (no auth — see sandbox note below) const res = await this.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`]'); // $getWorkflowStaticData() — data that persists across executions ``` **Sandbox (since n8n v2.0, JsTaskRunnerSandbox):** the accessor is `this.helpers.httpRequest()` — the bare `$helpers` global is **undefined** here (`$helpers.httpRequest()` throws `ReferenceError`). Inside a nested async function where `this` is lost, call it as `await fn.call(this, ...)`. `this.helpers.httpRequestWithAuthentication` and `this.helpers.requestWithAuthenticationPaginated` are deny-listed (→ `UnsupportedFunctionError`); for authenticated calls use an **HTTP Request node** with the credential (preferred), a sub-workflow, or a manual `Authorization: Bearer ${token}` header on `this.helpers.httpRequest()` only when the token already flows through the workflow as data. `$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](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