## Expression Builder Overview

The business logic editor, also called expression builder is an editor that was introduced in version 3.0 of Justinmind Prototyper. It is a comprehensive front end to define complex expressions used to basically evaluate a result out of some input data. The business logic editor is used mainly for two purposes:

1. To specify a value to set in a 'Set Value' or 'Set Selection' event action.

2. To build the condition that an interaction's conditional block must meet in order to be executed. In this case the result of the expression must be comparative with a boolean (true/false, yes/no) result.

The expression builder layout is fairly simple. It has an area to edit the expression at the top and two panels where you can find all the available functions and data sources.

### Creating expressions

Every expression in Justinmind Prototyper is a combination of Functions and Data sources. To create your own expression, just drag the functions you need from the Functions panel and drop them on the editing area. Do the same with the Data sources you will find in the Data panel. Notice that when dragging the functions or data elements over the expression, its structure will dynamically change showing a feedback of the result.

### Modifying expressions

The editing area is very interactive and you can move elements around by drag and drop and delete elements by clicking on the feedback X that appears at the top-right corner when the mouse is over an item.