# AUR0221

## Error Message

`AUR0221: Trying to assign value to readonly property "{{property}}" through computed observer.`

Where `{{property}}` is the readonly property name.

## Description

This error occurs when Aurelia tries to assign a value to a readonly property via a computed observer. Readonly properties (for example getters without setters) cannot be written to.

## Common Scenarios

* Two-way binding to a getter-only property.
* Binding a writable target to a computed/read-only source.

## Solution

* Bind to writable properties for two-way scenarios.
* If you need a computed-like value that is also assignable, implement a setter or use a separate backing field.