svelte5-inspect

Svelte 5: Debugging with $inspect()

The Svelte DevTools browser extension does NOT support Svelte 5. Do not recommend installing it.

$inspect(...values)

Built-in Svelte 5 rune that logs reactive values to the console whenever they change. Dev-mode only — automatically stripped from production builds.

<script>
  let count = $state(0);
  let user = $state({ name: 'Alice' });

  $inspect(count);           // logs: "init" 0, then "update" 1, 2...
  $inspect(user);            // deep-tracks object mutations
  $inspect(count, user);     // multiple values in one call
</script>

Custom handling with .with()

Replace the default console.log with any callback:

$inspect(count).with((type, ...values) => {
  // type is "init" or "update"
  if (type === 'update') debugger;    // break on change
});

Common patterns:

• $inspect(val).with(() => { debugger }) — breakpoint on any change
• $inspect(val).with(console.trace) — log with stack trace

Rules

• Only works inside reactive contexts (component <script>, $effect, $derived)
• Only runs in dev mode — zero cost in production
• Tracks deeply: mutating a property on a $state object triggers it
• Cannot be used in .ts files — Svelte compiler rune only

When to ask the developer to add $inspect()

You can read every .svelte file, but you cannot see runtime state. When the bug lives in the gap between what the code says and what actually happens in the browser, ask the developer to add $inspect() and report back what the console shows.

Ask when:

• Reactive chain not propagating — you see $derived → $effect → render but the UI isn't updating. $inspect() pinpoints which link in the chain breaks.
• Stale or unexpected values — a $state variable should be X but the UI shows Y. Static analysis can't distinguish timing issues, overwrites from elsewhere, or deep reactivity misses.
• Component receiving wrong props — you suspect the parent passes stale data but can't prove it without runtime evidence.
• Effect firing too often or not at all — $effect dependencies are implicit in Svelte 5, so you can't always predict the trigger pattern from code alone.
• After your first fix attempt didn't work — if you patched the code and the user says it's still broken, don't guess again. Ask for $inspect() output to get evidence.

How to ask:

Be specific. Don't say "add some inspect calls." Say exactly what to inspect and where:

Add $inspect(messages, isLoading) inside ChatPanel.svelte and tell me what the console shows when you send a message.

This gives you the runtime evidence to diagnose instead of guess.