Preview Expressions in Real-Time with the Expression Editor

n8n's expression editor (accessed by clicking the formula icon next to any text field) shows the evaluated result of your expression in real-time using data from the most recent execution. This is faster than running the entire workflow to test an expression and catches syntax errors, typos, and incorrect property paths immediately.

Real-world example: You need to build a complex expression that formats a date, concatenates fields, and handles null values. Instead of guessing and running the workflow repeatedly, use the expression editor to iterate instantly.

1. Click the formula icon (fx) next to any text input field.
2. The expression editor opens in a panel.
3. Type your expression -- the result preview updates live.
4. If the previous node has execution data (from a prior run
   or pinned data), the preview shows the actual computed value.
```text
```javascript title="Building an Expression Iteratively"
// Step 1: Start simple, verify the base property exists
{{ $json.customer.name }}
// Preview: "Jane Martinez"

// Step 2: Add formatting
{{ $json.customer.name.toUpperCase() }}
// Preview: "JANE MARTINEZ"

// Step 3: Add conditional logic
{{ $json.customer.name ?? "Unknown Customer" }}
// Preview: "Jane Martinez" (or "Unknown Customer" if null)

// Step 4: Build the full expression
{{ `Order #${$json.order_id} for ${$json.customer.name ?? "Unknown"} — $${($json.total / 100).toFixed(2)} ${$json.currency.toUpperCase()}` }}
// Preview: "Order #ORD-4521 for Jane Martinez — $49.99 USD"
```text
> **Note: Data Must Exist**
>
> The expression editor needs data from a previous execution or pinned data to show a preview. If the node has never executed, the preview will show `[no data]`. Run the workflow once or pin test data on the preceding node to enable live previews.

The expression editor eliminates the guess-and-check cycle for building expressions. A complex expression that might take five full-workflow test runs to get right can be built in under a minute using the live preview.

**Related:** [Always Set an Error Workflow on Every Production Workflow](../error-handling-and-reliability/01-always-set-an-error-workflow-on-every-production-workflow.md) | [Break Large Workflows into Sub-Workflows](../workflow-architecture/01-break-large-workflows-into-sub-workflows.md)