Understand `$json` vs `$input.first()` vs `$('NodeName').first()`

n8n provides multiple ways to reference data, and using the wrong one causes subtle bugs. Understanding when to use each reference pattern is critical for building reliable workflows.

Real-world example: A workflow fetches a configuration record from a database (node: "Get Config"), then processes a batch of customer items. During batch processing, you need the config value alongside each customer item.

$json
  → The current item's JSON data in the current node.
  → Use in expressions that process each item: {{ $json.email }}

$input.first().json
  → The first item from the immediately preceding node.
  → Use when only one item is expected: {{ $input.first().json.api_key }}

$input.all()
  → All items from the immediately preceding node (Code node only).
  → Use for aggregation or cross-item logic.

$('NodeName').first().json
  → The first item output by a specific named node, anywhere upstream.
  → Use to reach back to a node that is not directly preceding.

$('NodeName').all()
  → All items from a specific named node (Code node only).
```text
```javascript
// Code node — accessing data from multiple upstream nodes

// Current batch of items from the previous node
const customers = $input.all();

// Config value from a node earlier in the workflow
const apiEndpoint = $('Get Config').first().json.api_endpoint;
const batchSize = $('Get Config').first().json.batch_size;

const results = [];
for (const customer of customers) {
  results.push({
    json: {
      name: customer.json.name,
      endpoint: apiEndpoint,
      processedAt: new Date().toISOString()
    }
  });
}

return results;
```text
> **Warning: Common Pitfall**
>
> Using `$json` inside a Code node that runs once for all items only gives you the first item. Use `$input.all()` and iterate. Conversely, `$('NodeName').all()` in an expression (outside Code) does not work -- expressions evaluate per item.

Choosing the right reference pattern prevents the "missing data" bugs that are hardest to debug in n8n.

**Related:** [Flatten Deeply Nested API Responses](../code-node-mastery/01-flatten-deeply-nested-api-responses.md) | [Use the HTTP Request Node as a Universal Connector](../integration-patterns/01-use-the-http-request-node-as-a-universal-connector.md)