feat(#248): add createTypeahead composable with debounced fetch and selection state

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/lib/hooks/__tests__/useTypeahead.svelte.test.ts

@@ -0,0 +1,69 @@
+import { describe, it, expect, vi, beforeEach, afterEach } from 'vitest';
+
+const { createTypeahead } = await import('../useTypeahead.svelte');
+
+describe('createTypeahead', () => {
+	beforeEach(() => {
+		vi.useFakeTimers();
+	});
+	afterEach(() => {
+		vi.useRealTimers();
+	});
+
+	it('starts with empty query and closed dropdown', () => {
+		const ta = createTypeahead({ fetchUrl: vi.fn().mockResolvedValue([]) });
+		expect(ta.query).toBe('');
+		expect(ta.isOpen).toBe(false);
+		expect(ta.results).toEqual([]);
+		expect(ta.loading).toBe(false);
+	});
+
+	it('setQuery updates query and opens dropdown', async () => {
+		const fetchUrl = vi.fn().mockResolvedValue([{ id: '1', name: 'Foo' }]);
+		const ta = createTypeahead({ fetchUrl });
+		ta.setQuery('foo');
+		expect(ta.query).toBe('foo');
+		expect(ta.isOpen).toBe(true);
+	});
+
+	it('setQuery triggers debounced fetch and populates results', async () => {
+		const fetchUrl = vi.fn().mockResolvedValue([{ id: '1', name: 'Foo' }]);
+		const ta = createTypeahead({ fetchUrl, debounceMs: 300 });
+		ta.setQuery('foo');
+		expect(fetchUrl).not.toHaveBeenCalled();
+		await vi.advanceTimersByTimeAsync(300);
+		expect(fetchUrl).toHaveBeenCalledWith('foo');
+		expect(ta.results).toEqual([{ id: '1', name: 'Foo' }]);
+	});
+
+	it('close() resets isOpen', () => {
+		const ta = createTypeahead({ fetchUrl: vi.fn().mockResolvedValue([]) });
+		ta.setQuery('foo');
+		expect(ta.isOpen).toBe(true);
+		ta.close();
+		expect(ta.isOpen).toBe(false);
+	});
+
+	it('select(item) calls onSelect and closes dropdown', () => {
+		const onSelect = vi.fn();
+		const ta = createTypeahead({
+			fetchUrl: vi.fn().mockResolvedValue([]),
+			onSelect
+		});
+		ta.setQuery('foo');
+		ta.select({ id: '1', name: 'Foo' });
+		expect(onSelect).toHaveBeenCalledWith({ id: '1', name: 'Foo' });
+		expect(ta.isOpen).toBe(false);
+	});
+
+	it('debounce coalesces rapid setQuery calls', async () => {
+		const fetchUrl = vi.fn().mockResolvedValue([]);
+		const ta = createTypeahead({ fetchUrl, debounceMs: 300 });
+		ta.setQuery('f');
+		ta.setQuery('fo');
+		ta.setQuery('foo');
+		await vi.advanceTimersByTimeAsync(300);
+		expect(fetchUrl).toHaveBeenCalledTimes(1);
+		expect(fetchUrl).toHaveBeenCalledWith('foo');
+	});
+});

frontend/src/lib/hooks/useTypeahead.svelte.ts

@@ -0,0 +1,64 @@
+type Options<T> = {
+	fetchUrl: (query: string) => Promise<T[]>;
+	onSelect?: (item: T) => void;
+	debounceMs?: number;
+};
+
+export function createTypeahead<T>(options: Options<T>) {
+	const { fetchUrl, onSelect, debounceMs = 300 } = options;
+
+	let query = $state('');
+	let results: T[] = $state([]);
+	let isOpen = $state(false);
+	let loading = $state(false);
+	let activeIndex = $state(-1);
+
+	let debounceTimer: ReturnType<typeof setTimeout> | undefined;
+
+	function setQuery(q: string) {
+		query = q;
+		isOpen = true;
+		clearTimeout(debounceTimer);
+		debounceTimer = setTimeout(async () => {
+			loading = true;
+			try {
+				results = await fetchUrl(q);
+			} catch {
+				results = [];
+			} finally {
+				loading = false;
+			}
+		}, debounceMs);
+	}
+
+	function close() {
+		isOpen = false;
+		activeIndex = -1;
+	}
+
+	function select(item: T) {
+		onSelect?.(item);
+		close();
+	}
+
+	return {
+		get query() {
+			return query;
+		},
+		get results() {
+			return results;
+		},
+		get isOpen() {
+			return isOpen;
+		},
+		get loading() {
+			return loading;
+		},
+		get activeIndex() {
+			return activeIndex;
+		},
+		setQuery,
+		close,
+		select
+	};
+}