# AUR0220

## Error Message

`AUR0220: Map/Set "size" is a readonly property`

## Description

This error occurs when a binding or assignment tries to write to the `size` property of a `Map` or `Set`. `size` is readonly in JavaScript and cannot be assigned to.

## Example Trigger

```html
<!-- ❌ size is readonly -->
<template>
  <input value.bind="mySet.size">
</template>
```

## Solution

* Bind to `size` as a read-only value (one-way/from-view), or expose a derived property.
* Mutate the `Map`/`Set` via its methods (`add`, `delete`, `clear`) rather than assigning to `size`.