E0149: A state written where nothing sends it to the page

A ui.State is the browser's: a write is a command the page runs. It is written in an element's on… attribute, where the write is a chain of JS commands the browser runs at once, or in a method of a live component — a handler, a hook, an Update — where the compiler pushes the command with the reply (p.PushJS(p.tab.Set("env"))). Written anywhere else there is no page to send it to: while the markup renders, in a function that isn't a method, in a method of a static component.

Wrong

<!-- tabs.vuka.html -->
<script lang="go">
package app

type Tabs struct {
	tab ui.State[string]
}
</script>

<nav>
  tab = "env"
  <button>Env</button>
</nav>

Right

<!-- tabs.vuka.html -->
<script lang="go">
package app

type Tabs struct {
	tab ui.State[string]
}
</script>

<nav>
  <button onClick={tab = "env"}>Env</button>
</nav>

A state's value before any write is its type's zero value; one that starts elsewhere is set in a live component's @ui.Mount.