When you pass an extra value with action.bind(null, value), that value becomes the Server Action’s first argument. The form supplies FormData afterward, so declare the action as (value, formData)—not (formData, value). If useActionState is involved, include its state argument too.
What order do bound arguments and FormData go in?
Function.prototype.bind prepends bound arguments to the arguments supplied when the function is later called. For a form action, Next.js supplies the form’s FormData automatically. The bound value therefore comes first and the form data comes after it.
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const updateUserWithId = updateUser.bind(null, userId)
async function updateUser(userId: string, formData: FormData) {
'use server'
// Validate and authorize before updating the user.
}This follows the signature in the Next.js forms guide, last updated October 6, 2026. The form fields remain available through formData; binding an identifier does not replace or absorb the submitted fields.
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How does useActionState change the signature?
useActionState adds the previous state as the action’s first parameter. When the action also has a bound value and receives form data, account for all three arguments in order: state, bound value, then FormData.
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const actionWithId = updateUser.bind(null, userId)
async function updateUser(
previousState: State,
userId: string,
formData: FormData
) {
'use server'
// Validate, authorize, then perform the update.
}The state parameter is documented in the forms guide. Check the specific wrapper’s documented contract and read its call site alongside the function declaration: an omitted state parameter can shift how later arguments are interpreted.
Should you use bind or a hidden input for an ID?
Choose based on whether the value should be an extra function argument or ordinary submitted form data. A hidden input places the value in the form’s FormData; its value is present in rendered HTML and is not encoded. Binding passes the value as an argument before FormData. Next.js documents that binding works with Server and Client Components and supports progressive enhancement.
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| Consideration | Bound argument | Hidden input |
|---|---|---|
| Where the value arrives | As a function argument before FormData. |
As a field in FormData. |
| Visibility in rendered HTML | The documented comparison is that a hidden input’s value appears in rendered HTML and is not encoded; the guide does not state an equivalent visibility claim for bound values. | The value appears in rendered HTML and is not encoded. |
| Signature effect | Adds a parameter before FormData; state from useActionState, if used, is first. |
Read the value from FormData; it does not add a separate function parameter. |
These approaches affect how data is passed, not whether it can be trusted.
How should you secure a Server Action that receives an ID?
A bound ID or hidden field is input, not proof of permission. Next.js warns that Server Functions can be reached through direct POST requests, so do not rely on a particular UI or form being the only caller. Authenticate the user, authorize access to the referenced resource, and validate submitted data inside the server function before making a sensitive change. Return only the data the UI needs. See the Next.js documentation on mutating data and the use server directive, both last updated October 6, 2026.
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How do you debug a mismatched action signature?
- Start with the declaration. Write down each parameter the function expects.
- Account for wrapper state. If
useActionStateis used, include its state parameter first. - List bound values. Each value passed to
.bind(null, ...)precedes arguments supplied when the action runs. - Put form data last. A form action receives its
FormDatafrom the form submission. - Check the adapter contract. Confirm the wrapper’s documented signature rather than assuming every action setup has identical parameters.
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