svelte/documentation/docs/99-legacy/01-legacy-let.md

title
Reactive let/var declarations

In runes mode, reactive state is explicitly declared with the $state rune.

In legacy mode, variables declared at the top level of a component are automatically considered reactive. Reassigning or mutating these variables (count += 1 or object.x = y) will cause the UI to update.

<script>
	let count = 0;
</script>

<button on:click={() => count += 1}>
	clicks: {count}
</button>

Because Svelte's legacy mode reactivity is based on assignments, using array methods like .push() and .splice() won't automatically trigger updates. A subsequent assignment is required to 'tell' the compiler to update the UI:

<script>
	let numbers = [1, 2, 3, 4];

	function addNumber() {
		// this method call does not trigger an update
		numbers.push(numbers.length + 1);

		// this assignment will update anything
		// that depends on `numbers`
		numbers = numbers;
	}
</script>