svelte/documentation/docs/05-misc/01-debugging.md

title
Debugging

• @debug
• $inspect

Svelte provides two built-in ways to debug your application.

$inspect

The $inspect rune is roughly equivalent to console.log, with the exception that it will re-run whenever its argument changes. $inspect tracks reactive state deeply, meaning that updating something inside an object or array using fine-grained reactivity will cause it to re-fire. (Demo:)

<script>
	let count = $state(0);
	let message = $state('hello');

	$inspect(count, message); // will console.log when `count` or `message` change
</script>

<button onclick={() => count++}>Increment</button>
<input bind:value={message} />

$inspect returns a property with, which you can invoke with a callback, which will then be invoked instead of console.log. The first argument to the callback is either "init" or "update", all following arguments are the values passed to $inspect. Demo:

<script>
	let count = $state(0);

	$inspect(count).with((type, count) => {
		if (type === 'update') {
			debugger; // or `console.trace`, or whatever you want
		}
	});
</script>

<button onclick={() => count++}>Increment</button>

A convenient way to find the origin of some change is to pass console.trace to with:

// @errors: 2304
$inspect(stuff).with(console.trace);

$inspect only works during development. In a production build it becomes a noop.

@debug

<!--- copy: false --->
{@debug}

<!--- copy: false --->
{@debug var1, var2, ..., varN}

The {@debug ...} tag offers an alternative to console.log(...). It logs the values of specific variables whenever they change, and pauses code execution if you have devtools open.

<script>
	let user = {
		firstname: 'Ada',
		lastname: 'Lovelace'
	};
</script>

{@debug user}

<h1>Hello {user.firstname}!</h1>

{@debug ...} accepts a comma-separated list of variable names (not arbitrary expressions).

<!-- Compiles -->
{@debug user}
{@debug user1, user2, user3}

<!-- WON'T compile -->
{@debug user.firstname}
{@debug myArray[0]}
{@debug !isReady}
{@debug typeof user === 'object'}

The {@debug} tag without any arguments will insert a debugger statement that gets triggered when any state changes, as opposed to the specified variables.