At the heart of Svelte is a powerful system of reactivity for keeping the DOM in sync with your application state — for example, in response to an event.

To demonstrate it, we first need to wire up an event handler. Replace line 9 with this:

<button on:click={incrementCount}>

Inside the incrementCount function, all we need to do is change the value of count:

function incrementCount() {
	count += 1;

Svelte 'instruments' this assignment with some code that tells it the DOM will need to be updated.

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Compiler options

result = svelte.compile(source, {

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