Add empty-formdata.md

docs/.vitepress/config.mts

@@ -180,9 +180,9 @@ export default defineConfig({
           }
         ]
       },
-      // {
-      //   text: 'Debugging',
-      //   items: [
+      {
+        text: 'Debugging',
+        items: [
       //     {
       //       text: `Why does my logging show an empty/missing value after I've loaded the data?`,
       //       link: '/faq/logging-after-loading'
@@ -191,8 +191,12 @@ export default defineConfig({
       //       text: 'Why can I see my data in console logging, but if I try to access it I get undefined or an error?',
       //       link: '/faq/logging-is-live'
       //     }
-      //   ]
-      // },
+          {
+            text: 'Why is my <code>FormData</code> empty?',
+            link: '/faq/empty-formdata'
+          }
+        ]
+      },
       // {
       //   text: 'Common misunderstandings',
       //   items: [

docs/faq/empty-formdata.md

@@ -0,0 +1,98 @@
+# Why is my `FormData` empty?
+
+## The problem
+
+Let's imagine you've created a [`FormData`](https://developer.mozilla.org/en-US/docs/Web/API/FormData) object:
+
+```js
+const formData = new FormData()
+formData.append('product', 'id-1234')
+```
+
+The specifics don't matter too much, but clearly `formData` should not be empty, it should contain an entry for `product`.
+
+If you try to check the contents of `formData` in the console:
+
+```js
+console.log(formData)
+```
+
+You'll likely see something like this:
+
+```
+▼ FormData {}
+  ▶ [[Prototype]]: FormData
+```
+
+It appears to be empty.
+
+Using `JSON.stringify()` doesn't help either:
+
+```js
+console.log(JSON.stringify(formData))
+```
+
+Output:
+
+```
+{}
+```
+
+You could try outputting the object in a component template using <code v-pre>{{ formData }}</code>, but that still doesn't show the `product`.
+
+## The solution
+
+The `formData` object isn't empty, but none of the techniques we've used so far will show that.
+
+`FormData` exposes the methods [`keys()`](https://developer.mozilla.org/en-US/docs/Web/API/FormData/keys), [`values()`](https://developer.mozilla.org/en-US/docs/Web/API/FormData/values) and [`entries()`](https://developer.mozilla.org/en-US/docs/Web/API/FormData/entries) that we can use to inspect its contents. For example:
+
+```js
+console.log(Object.fromEntries(formData.entries()))
+```
+
+Now we'll get the output we wanted:
+
+```
+{product: 'id-1234'}
+```
+
+Here's a Playground example to demonstrate this in action:
+
+- [Playground example](https://play.vuejs.org/#eNqNks1uwjAQhF/F8iUgUaMWTihF6g+VygGq0qMvabIJoYlt2RugivLutQ2BtGpRb9mZb1ez3tT0Tim2rYBOaGhinSskBrBSUy5iKQySVOryMcKI3BIBO/J0LHt9LrhoTRYpBSLpBUrLpIoxGJAgT66ub0bjwINuliyAFTLrtU3W6Mrz1XLBDOpcZHn6eaZ+YMv3DcTIUi3LmbA0mBPK4Cj0XVM4POxjN7EFQqmKCMFWhIRKw7Suz7s1TTh0mjfX+hv0V66LTf9L2RkRDjsJ6YCisUunecY2Rgp7nNpN5zSWpcoL0EuFuX0UTifEO86LikLu5l5DXcGg1eM1xB+/6BuzdxqnLxoM6C1wevIw0hngwZ6tFrC33yeztDcuLH3BfAV7sMplPGD3lUhs7A7n0z6XSmq0L/tmZnsEYdqlXFBHNp7n1P6hDxdWP8cdsbHv46KhzRf2Fvlu)
+
+## Nope, still doesn't work
+
+If you're still getting an empty object, then perhaps your `FormData` object really is empty.
+
+If you're creating the `FormData` object by passing in a `<form>`, the first thing to try is appending a hardcoded dummy field, just to check that it shows up in the logging. For example:
+
+```js
+const formData = new FormData(myForm.value)
+
+formData.append('dummy', 'foo')
+
+console.log(Object.fromEntries(formData.entries()))
+```
+
+In the example above, `myForm` is a [template ref](https://vuejs.org/guide/essentials/template-refs.html), so we pass `myForm.value` to the `FormData` constructor.
+
+- [Playground example](https://play.vuejs.org/#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)
+
+If we can see the dummy field in the console logging then we know the problem isn't with the logging. The `FormData` really was empty.
+
+The `FormData` constructor will usually throw an error if you pass in an invalid value, but it allows `undefined` to be passed. Try console logging the value you're passing to `FormData`, to confirm it's an `HTMLFormElement` and not `undefined`:
+
+```js
+console.log(myForm.value) // Check it isn't undefined
+const formData = new FormData(myForm.value)
+```
+
+Let's assume the form passed to the constructor isn't `undefined`, what else could be going wrong?
+
+If possible, use the **Elements** tab in your browser developer tools to inspect the `<form>` (or inspect the logged value in the console). Check that it actually contains valid fields, such as `<input>` elements. Then:
+
+1. Ensure each element has a `name`. This is used as the key to populate the `FormData`. An `id` isn't sufficient, it needs a `name`.
+2. Check the element isn't disabled. Disabled elements will be ignored.
+3. For an `<input>` with `type="checkbox"`, it will only be included if the checkbox is checked.
+
+Reference: <https://developer.mozilla.org/en-US/docs/Web/API/FormData/FormData>

docs/faq/index.md

@@ -71,6 +71,7 @@ Only about half the questions have complete answers. Those questions are listed
 
 - [Why does my logging show an empty/missing value after I've loaded the data?](logging-after-loading)
 - [Why can I see my data in console logging, but if I try to access it in my code I get `undefined` or an error?](logging-is-live)
+- [Why is my `FormData` empty?](empty-formdata)
 
 ---