Enable astro/tsconfigs/strictest in the preview package (#2834)

shared/lib/array.test.ts

@@ -0,0 +1,12 @@
+import { describe, expect, it } from 'vitest'
+import { requireIndex } from './array'
+
+describe('requireIndex', () => {
+  it('returns the entry at the given index', () => {
+    expect(requireIndex(['a', 'b', 'c'], 1)).toBe('b')
+  })
+
+  it('throws when the index is out of bounds', () => {
+    expect(() => requireIndex(['a'], 5)).toThrow('Expected an entry at index 5')
+  })
+})

shared/lib/array.ts

@@ -0,0 +1,11 @@
+// Indexes into demo-data arrays (people.json, photos.json, …) that pages trust to have
+// an entry at a given position. Throws instead of silently rendering `undefined` if that
+// assumption is ever violated (e.g. a seed file shrinks).
+
+export function requireIndex<T>(array: readonly T[], index: number): T {
+  const item = array[index]
+  if (item === undefined) {
+    throw new Error(`Expected an entry at index ${index}, got undefined (array has ${array.length} entries)`)
+  }
+  return item
+}