fix: prevent adding non-existent packages

app/components/Compare/PackageSelector.vue

@@ -1,5 +1,6 @@
 <script setup lang="ts">
 import { NO_DEPENDENCY_ID } from '~/composables/usePackageComparison'
+import { checkPackageExists } from '~/utils/package-name'
 
 const packages = defineModel<string[]>({ required: true })
 
@@ -13,6 +14,12 @@ const maxPackages = computed(() => props.max ?? 4)
 // Input state
 const inputValue = shallowRef('')
 const isInputFocused = shallowRef(false)
+const isCheckingPackage = shallowRef(false)
+const packageError = shallowRef('')
+
+watch(inputValue, () => {
+  packageError.value = ''
+})
 
 // Use the shared npm search composable
 const { data: searchData, status } = useNpmSearch(inputValue, { size: 15 })
@@ -76,10 +83,36 @@ function removePackage(name: string) {
   packages.value = packages.value.filter(p => p !== name)
 }
 
-function handleKeydown(e: KeyboardEvent) {
-  if (e.key === 'Enter' && inputValue.value.trim()) {
-    e.preventDefault()
-    addPackage(inputValue.value.trim())
+async function handleKeydown(e: KeyboardEvent) {
+  if (e.key !== 'Enter' || !inputValue.value.trim() || isCheckingPackage.value) return
+  e.preventDefault()
+
+  const name = inputValue.value.trim()
+  if (packages.value.length >= maxPackages.value) return
+  if (packages.value.includes(name)) return
+
+  // If it matches a dropdown result, add immediately (already confirmed to exist)
+  const exactMatch = filteredResults.value.find(r => r.name === name)
+  if (exactMatch) {
+    addPackage(exactMatch.name)
+    return
+  }
+
+  // Otherwise, verify it exists on npm
+  isCheckingPackage.value = true
+  packageError.value = ''
+  try {
+    const exists = await checkPackageExists(name)
+    if (name !== inputValue.value.trim()) return // stale guard
+    if (exists) {
+      addPackage(name)
+    } else {
+      packageError.value = `Package "${name}" was not found on npm.`
+    }
+  } catch {
+    packageError.value = 'Could not verify package. Please try again.'
+  } finally {
+    isCheckingPackage.value = false
   }
 }
 
@@ -138,7 +171,8 @@ function handleBlur() {
           class="absolute inset-y-0 start-3 flex items-center text-fg-subtle pointer-events-none group-focus-within:text-accent"
           aria-hidden="true"
         >
-          <span class="i-carbon:search w-4 h-4" />
+          <span v-if="isCheckingPackage" class="i-carbon:renew w-4 h-4 animate-spin" />
+          <span v-else class="i-carbon:search w-4 h-4" />
         </span>
         <input
           id="package-search"
@@ -149,7 +183,8 @@ function handleBlur() {
               ? $t('compare.selector.search_first')
               : $t('compare.selector.search_add')
           "
-          class="w-full bg-bg-subtle border border-border rounded-lg ps-10 pe-4 py-2.5 font-mono text-sm text-fg placeholder:text-fg-subtle motion-reduce:transition-none duration-200 focus:border-accent focus-visible:(outline-2 outline-accent/70)"
+          :disabled="isCheckingPackage"
+          class="w-full bg-bg-subtle border border-border rounded-lg ps-10 pe-4 py-2.5 font-mono text-sm text-fg placeholder:text-fg-subtle motion-reduce:transition-none duration-200 focus:border-accent focus-visible:(outline-2 outline-accent/70) disabled:opacity-60 disabled:cursor-wait"
           aria-autocomplete="list"
           @focus="isInputFocused = true"
           @blur="handleBlur"
@@ -205,6 +240,11 @@ function handleBlur() {
           </button>
         </div>
       </Transition>
+
+      <!-- Package not found error -->
+      <p v-if="packageError" class="text-xs text-red-400 mt-1" role="alert">
+        {{ packageError }}
+      </p>
     </div>
 
     <!-- Hint -->

test/nuxt/components/compare/PackageSelector.spec.ts

@@ -1,15 +1,25 @@
 import { beforeEach, describe, expect, it, vi } from 'vitest'
 import { ref } from 'vue'
+import { flushPromises } from '@vue/test-utils'
 import { mountSuspended } from '@nuxt/test-utils/runtime'
 import PackageSelector from '~/components/Compare/PackageSelector.vue'
 
+// Mock checkPackageExists
+vi.mock('~/utils/package-name', () => ({
+  checkPackageExists: vi.fn(),
+}))
+
+import { checkPackageExists } from '~/utils/package-name'
+const mockCheckPackageExists = vi.mocked(checkPackageExists)
+
 // Mock $fetch for useNpmSearch
 const mockFetch = vi.fn()
 vi.stubGlobal('$fetch', mockFetch)
 
 describe('PackageSelector', () => {
   beforeEach(() => {
     mockFetch.mockReset()
+    mockCheckPackageExists.mockReset()
     mockFetch.mockResolvedValue({
       objects: [
         { package: { name: 'lodash', description: 'Lodash modular utilities' } },
@@ -18,6 +28,7 @@ describe('PackageSelector', () => {
       total: 2,
       time: new Date().toISOString(),
     })
+    mockCheckPackageExists.mockResolvedValue(true)
   })
 
   describe('selected packages display', () => {
@@ -132,7 +143,9 @@ describe('PackageSelector', () => {
   })
 
   describe('adding packages', () => {
-    it('adds package on Enter key', async () => {
+    it('adds package on Enter key when package exists', async () => {
+      mockCheckPackageExists.mockResolvedValue(true)
+
       const component = await mountSuspended(PackageSelector, {
         props: {
           modelValue: [],
@@ -142,13 +155,16 @@ describe('PackageSelector', () => {
       const input = component.find('input')
       await input.setValue('my-package')
       await input.trigger('keydown', { key: 'Enter' })
+      await flushPromises()
 
       const emitted = component.emitted('update:modelValue')
       expect(emitted).toBeTruthy()
       expect(emitted![0]![0]).toEqual(['my-package'])
     })
 
     it('clears input after adding package', async () => {
+      mockCheckPackageExists.mockResolvedValue(true)
+
       const component = await mountSuspended(PackageSelector, {
         props: {
           modelValue: [],
@@ -158,11 +174,31 @@ describe('PackageSelector', () => {
       const input = component.find('input')
       await input.setValue('my-package')
       await input.trigger('keydown', { key: 'Enter' })
+      await flushPromises()
 
       // Input should be cleared
       expect((input.element as HTMLInputElement).value).toBe('')
     })
 
+    it('does not add non-existent packages', async () => {
+      mockCheckPackageExists.mockResolvedValue(false)
+
+      const component = await mountSuspended(PackageSelector, {
+        props: {
+          modelValue: [],
+        },
+      })
+
+      const input = component.find('input')
+      await input.setValue('nonexistent-pkg')
+      await input.trigger('keydown', { key: 'Enter' })
+      await flushPromises()
+
+      const emitted = component.emitted('update:modelValue')
+      expect(emitted).toBeFalsy()
+      expect(component.find('[role="alert"]').exists()).toBe(true)
+    })
+
     it('does not add duplicate packages', async () => {
       const component = await mountSuspended(PackageSelector, {
         props: {