svelte/site/content/tutorial/07-lifecycle/04-tick/app-a/App.svelte

<script>
	let text = `Select some text and hit the tab key to toggle uppercase`;

	async function handleKeydown(event) {
		if (event.which !== 9) return;

		event.preventDefault();

		const { selectionStart, selectionEnd, value } = this;
		const selection = value.slice(selectionStart, selectionEnd);

		const replacement = /[a-z]/.test(selection)
			? selection.toUpperCase()
			: selection.toLowerCase();

		text = (
			value.slice(0, selectionStart) +
			replacement +
			value.slice(selectionEnd)
		);

		// this has no effect, because the DOM hasn't updated yet
		this.selectionStart = selectionStart;
		this.selectionEnd = selectionEnd;
	}
</script>

<style>
	textarea {
		width: 100%;
		height: 200px;
	}
</style>

<textarea value={text} on:keydown={handleKeydown}></textarea>