# Expression Engine
## Overview
In **iX Hello Customer v2**, the **Expression Engine** is the **rules and logic layer** that allows teams to define **dynamic behavior** using expressions instead of hard‑coded logic.
It is used to:
* Evaluate conditions
* Compute values at runtime
* Control visibility, routing, validation, and automation
* Personalize customer and agent experiences
Expression Engine V2 offers a secure, JavaScript-native solution for evaluating expressions in customer v2. It operates within isolated Worker threads, ensuring robust hard timeout protection.
### Where Expressions Are Used
| Location | How expressions are used |
| ----------------------- | ----------------------------------- |
| **Say Node** | Dynamic text using `{{expression}}` |
| **Agent System Prompt** | Personalization & context |
| **Menu Prompts** | Dynamic options and text |
### Key Principles:
* Use standard JavaScript syntax — no custom $helper functions for things JavaScript can do\
natively
* Expressions are read-only — they can read variables and compute results but cannot mutate\
session state
* Only concise arrow functions are supported (x => x.active) — no block-body arrows\
(x => { return x.active })
### Template Interpolation
Text fields (Say, Agent, Menu prompts) use {{expression}} interpolation:
Syntax
Purpose
Example
{{expression}}
Variable lookup or expression
Hello {{name}}, you have {{items.length}} items
* {{expression}} — it’s used to indicate where an expression will be evaluated and substituted at runtime.
* Simple variable names like {{name}} resolve to a variable lookup
* Complex expressions like {{items.filter(x => x.active).length}} are fully evaluated
* The double-brace delimiter avoids collision with JSON content in system prompts
* Non-printable results (objects, arrays) are JSON-stringified; null/undefined produce empty\
string
Note: For assistance on writing these expressions, use our new and improved v2 of iX Employee v2.
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