Assignment to `%property%` property (%location%) will evaluate to the right-hand side, not the value of `%property%` following the assignment. This may result in unexpected behaviour.
```
Given a case like this...
```svelte
<script>
let object = $state({ array: null });
function add() {
(object.array ??= []).push(object.array.length);
}
</script>
<buttononclick={add}>add</button>
<p>items: {JSON.stringify(object.items)}</p>
```
...the array being pushed to when the button is first clicked is the `[]` on the right-hand side of the assignment, but the resulting value of `object.array` is an empty state proxy. As a result, the pushed value will be discarded.
You can fix this by separating it into two statements:
`%binding%` (%location%) is binding to a non-reactive property
```
### console_log_state
```
Your `console.%method%` contained `$state` proxies. Consider using `$inspect(...)` or `$state.snapshot(...)` instead
```
When logging a [proxy](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Proxy), browser devtools will log the proxy itself rather than the value it represents. In the case of Svelte, the 'target' of a `$state` proxy might not resemble its current value, which can be confusing.
The easiest way to log a value as it changes over time is to use the [`$inspect`](https://svelte.dev/docs/svelte/$inspect) rune. Alternatively, to log things on a one-off basis (for example, inside an event handler) you can use [`$state.snapshot`](https://svelte.dev/docs/svelte/$state#$state.snapshot) to take a snapshot of the current value.
%handler% should be a function. Did you mean to %suggestion%?
```
### hydration_attribute_changed
```
The `%attribute%` attribute on `%html%` changed its value between server and client renders. The client value, `%value%`, will be ignored in favour of the server value
```
### hydration_html_changed
```
The value of an `{@html ...}` block changed between server and client renders. The client value will be ignored in favour of the server value
```
```
The value of an `{@html ...}` block %location% changed between server and client renders. The client value will be ignored in favour of the server value
```
### hydration_mismatch
```
Hydration failed because the initial UI does not match what was rendered on the server
```
```
Hydration failed because the initial UI does not match what was rendered on the server. The error occurred near %location%
```
### invalid_raw_snippet_render
```
The `render` function passed to `createRawSnippet` should return HTML for a single element
Detected a migrated `$:` reactive block in `%filename%` that both accesses and updates the same reactive value. This may cause recursive updates when converted to an `$effect`.
%parent% passed a value to %child% with `bind:`, but the value is owned by %owner%. Consider creating a binding between %owner% and %parent%
```
### ownership_invalid_mutation
```
Mutating a value outside the component that created it is strongly discouraged. Consider passing values to child components with `bind:`, or use a callback instead
```
```
%component% mutated a value owned by %owner%. This is strongly discouraged. Consider passing values to child components with `bind:`, or use a callback instead
A `$:` statement (%location%) read reactive state that was not visible to the compiler. Updates to this state will not cause the statement to re-run. The behaviour of this code will change if you migrate it to runes mode
```
In legacy mode, a `$:` [reactive statement](https://svelte.dev/docs/svelte/legacy-reactive-assignments) re-runs when the state it _references_ changes. This is determined at compile time, by analysing the code.
In runes mode, effects and deriveds re-run when there are changes to the values that are read during the function's _execution_.
Often, the result is the same — for example these can be considered equivalent:
// the compiler can't 'see' that `sum` depends on `a` and `b`, but
// they _would_ be read while executing the `$derived` version
$: sum = add();
```
Similarly, reactive properties of [deep state](https://svelte.dev/docs/svelte/$state#Deep-state) are not visible to the compiler. As such, changes to these properties will cause effects and deriveds to re-run but will _not_ cause `$:` statements to re-run.
When you [migrate this component](https://svelte.dev/docs/svelte/v5-migration-guide) to runes mode, the behaviour will change accordingly.
Reactive `$state(...)` proxies and the values they proxy have different identities. Because of this, comparisons with `%operator%` will produce unexpected results
```
`$state(...)` creates a [proxy](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Proxy) of the value it is passed. The proxy and the value have different identities, meaning equality checks will always return `false`:
```svelte
<script>
let value = { foo: 'bar' };
let proxy = $state(value);
value === proxy; // always false
</script>
```
To resolve this, ensure you're comparing values where both values were created with `$state(...)`, or neither were. Note that `$state.raw(...)` will _not_ create a state proxy.
