# Expressions

Learn how to use expressions in ActiveReportsJS.

## Content



There are two types of [report items](/activereportsjs/docs/v6.1/ReportAuthorGuide/Report-Items/Common-Properties) properties in ActiveReportsJS

* ```Static properties``` - their values are known at design time and don't change at runtime
* ```Dynamic properties``` - their values are calculated at runtime. Conditional formatting, page numbering, summary values, and other fundamental features are based on dynamic properties.

The value of a dynamic property at design-time is an ```expression``` which is a string that contains ```interpolations```.

### Interpolation Syntax

Interpolation is a string fragment wrapped ```{}``` that is evaluated at runtime and replaced with the calculation result. For example, you can use the following expression for the [Rich Text](/activereportsjs/docs/v6.1/ReportAuthorGuide/Report-Items/Data-Visualizers/RichText) ```Value``` property.

```html
<p>Thank you for your purchase {name}!</p>
<p>Your order number is: #{orderID}</p>
```

The ```{name}``` and ```{orderID}``` are interpolations that use the [data set field references](/activereportsjs/docs/v6.1/ReportAuthorGuide/Expressions/references#data-set-fields). At runtime, they are replaced with the field values.

The interpolation syntax allows you to use the following features.

* [References](/activereportsjs/docs/v6.1/ReportAuthorGuide/Expressions/references) are identifiers that are associated with various report elements.
* [Operators](/activereportsjs/docs/v6.1/ReportAuthorGuide/Expressions/Operators) are used for calculation and comparison.