# Expr Language - Value Types

With Expr you can build formulas that operate on:

Basic values – numbers and text, which are either a part of the formula, or read from a Jira field, such as an issue's Summary or Story Points.

Array values – a sequence of values, allowing you to represent multi-value fields such as Fix Versions.

- User function values – representing a piece of formula that is typically applied to each element in an array, for example, for filtering.
- Key-value maps – in rare occasions, these values are produced by the supplied functions like GROUP.
- Undefined – a special value that means "nothing" or "no value".
- Error values – produced if there was a problem calculating a formula.

Normally, you don't need to worry about the value types when writing a formula. The language engine will try to make sense of the formula and convert the values as needed. For more complex formulas, or if something doesn't work as expected, see Expr Function Reference for the expected types for each function.