fix regression where displayValue crashes

It regressed in the sense that it now uses `displayValue` for the
`defaultValue` as well, but if nothing is passed it would crash.

Right now, it makes sure to only run the displayValue value on the
actual value and the actual default value if they are not undefined.

Note: if your displayValue is implemented like `(value) => value.name`,
and your `value` is passed as `null`, it will still crash (as expected)
because then you are in charge of rendering something else than null. If
we would "fix" this, then no value can be rendered instead of `null`.

Fixes: #2084

packages/@headlessui-react/src/components/combobox/combobox.test.tsx

@@ -412,6 +412,30 @@ describe('Rendering', () => {
         })
       )
     })
+
+    it(
+      'should not crash when a defaultValue is not given',
+      suppressConsoleLogs(async () => {
+        let data = [
+          { id: 1, name: 'alice', label: 'Alice' },
+          { id: 2, name: 'bob', label: 'Bob' },
+          { id: 3, name: 'charlie', label: 'Charlie' },
+        ]
+
+        render(
+          <Combobox name="assignee" by="id">
+            <Combobox.Input displayValue={(value: { name: string }) => value.name} />
+            <Combobox.Options>
+              {data.map((person) => (
+                <Combobox.Option key={person.id} value={person}>
+                  {person.label}
+                </Combobox.Option>
+              ))}
+            </Combobox.Options>
+          </Combobox>
+        )
+      })
+    )
   })
 
   describe('Combobox.Input', () => {

packages/@headlessui-react/src/components/combobox/combobox.tsx

@@ -705,7 +705,7 @@ let Input = forwardRefWithAs(function Input<
   // you don't have to use this at all, a more common UI is a "tag" based UI, which you can render
   // yourself using the selected option(s).
   let currentValue = useMemo(() => {
-    if (typeof displayValue === 'function') {
+    if (typeof displayValue === 'function' && data.value !== undefined) {
       return displayValue(data.value as unknown as TType) ?? ''
     } else if (typeof data.value === 'string') {
       return data.value
@@ -909,7 +909,9 @@ let Input = forwardRefWithAs(function Input<
     'aria-labelledby': labelledby,
     defaultValue:
       props.defaultValue ??
-      displayValue?.(data.defaultValue as unknown as TType) ??
+      (data.defaultValue !== undefined
+        ? displayValue?.(data.defaultValue as unknown as TType)
+        : null) ??
       data.defaultValue,
     disabled: data.disabled,
     onCompositionStart: handleCompositionStart,

packages/@headlessui-vue/src/components/combobox/combobox.test.ts

@@ -465,6 +465,31 @@ describe('Rendering', () => {
         })
       )
     })
+
+    it(
+      'should not crash when a defaultValue is not given',
+      suppressConsoleLogs(async () => {
+        let data = [
+          { id: 1, name: 'alice', label: 'Alice' },
+          { id: 2, name: 'bob', label: 'Bob' },
+          { id: 3, name: 'charlie', label: 'Charlie' },
+        ]
+
+        renderTemplate({
+          template: html`
+            <Combobox name="assignee" by="id">
+              <ComboboxInput :displayValue="(value) => value.name" />
+              <ComboboxOptions>
+                <ComboboxOption v-for="person in data" :key="person.id" :value="person">
+                  {{ person.label }}
+                </ComboboxOption>
+              <ComboboxOptions>
+            </Combobox>
+          `,
+          setup: () => ({ data }),
+        })
+      })
+    )
   })
 
   describe('ComboboxInput', () => {

packages/@headlessui-vue/src/components/combobox/combobox.ts

@@ -676,7 +676,7 @@ export let ComboboxInput = defineComponent({
       let value = api.value.value
       if (!dom(api.inputRef)) return ''
 
-      if (typeof props.displayValue !== 'undefined') {
+      if (typeof props.displayValue !== 'undefined' && value !== undefined) {
         return props.displayValue(value as unknown) ?? ''
       } else if (typeof value === 'string') {
         return value
@@ -874,7 +874,9 @@ export let ComboboxInput = defineComponent({
     let defaultValue = computed(() => {
       return (
         props.defaultValue ??
-        props.displayValue?.(api.defaultValue.value) ??
+        (api.defaultValue.value !== undefined
+          ? props.displayValue?.(api.defaultValue.value)
+          : null) ??
         api.defaultValue.value ??
         ''
       )