Field Value Within A Re-indexed Parent Array Item (keyed By UUID) Temporarily Becomes `undefined` On Preceding Item Removal.

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Field Value Within a Re-Indexed Parent Array Item Temporarily Becomes undefined on Preceding Item Removal

Introduction

TanStack Form is a powerful library for managing forms in React applications. However, like any complex library, it can sometimes exhibit unexpected behavior. In this article, we will explore a specific issue that arises when managing an array of objects with unique React keys, where fields within the item that shifts to take the removed item's place can have their form.Field instance report state.value as undefined for a transient period during the re-render.

Describe the Bug

The issue occurs when an item is removed from the array, and that item is not the last one. In this scenario, fields within the item that shifts to take the removed item's place can have their form.Field instance report state.value as undefined for a transient period during the re-render. This behavior is unexpected because fields, especially those initialized with default values or specific types (like strings or other arrays), are generally assumed to consistently provide a value of that type, not undefined.

Your Minimal, Reproducible Example

To demonstrate this issue, we can use the following minimal, reproducible example:

https://stackblitz.com/edit/tanstack-form-7wb235yh?file=src%2Fcollapsible-item.tsx

Steps to Reproduce

To reproduce this issue, follow these steps:

  1. Setup: The example form contains a sections array. Each item in sections is an object with a unique uuid (used as its React key) and a sectionTitle string field (initialized with a default string). An invariant checks if sectionTitle's value is a string.
  2. Add Items: Click "Add Section" twice to create two distinct section items in the form.
  3. Remove Preceding Item: Click "Remove Section" for the first of the two section items.
  4. Observe Error: The application will throw an error, caught by the Error Boundary, for the remaining section item (which was previously the second). The error message will indicate that the sectionTitle field's value was undefined, failing the invariant check: Invariant failed: Expected sectionTitle field value to be a string! Received: undefined. (See attached screenshot).

Expected Behavior

The state.value of any field (e.g., sectionTitle) within an array item should consistently maintain its correct type and initialized value throughout all re-render cycles. This includes re-renders that occur after removing a preceding item from the parent array and the subsequent re-indexing of items. A field initialized as a string should not temporarily become undefined.

How Often Does This Bug Happen?

This bug occurs every time an item is removed from the array and that item is not the last one.

Screenshots or Videos

To better understand this issue, you can refer to the following screenshot:

https://github.com/user-attachments/assets/8fd533a5-2ab6-4d78-a11f-a6db69a1980a

Platform

This issue was observed on the following platform:

  • Macos
  • Chrome browser

TanStack Form Adapter

The TanStack Form adapter used in this example is:

react-form

TanStack Form Version

The version of TanStack Form used in this example is:

1.11.1

TypeScript Version

The version of TypeScript used in this example is:

No response

Additional Context

No response

Conclusion

In conclusion, the issue described in this article is a specific re-render issue that can occur when managing an array of objects with unique React keys in TanStack Form. This issue can cause fields within the item that shifts to take the removed item's place to have their form.Field instance report state.value as undefined for a transient period during the re-render. To resolve this issue, developers should ensure that their form management code is properly handling re-renders and array item re-indexing.

Recommendations

To avoid this issue, developers can follow these recommendations:

  1. Use a stable React key: Ensure that the React key used for each item in the array is stable and does not change during re-renders.
  2. Use a consistent form management approach: Use a consistent approach to managing forms, including re-renders and array item re-indexing.
  3. Test thoroughly: Thoroughly test your form management code to ensure that it is properly handling re-renders and array item re-indexing.

By following these recommendations, developers can help prevent this issue and ensure that their forms are properly managed in TanStack Form.
Q&A: Field Value Within a Re-Indexed Parent Array Item Temporarily Becomes undefined on Preceding Item Removal

Q: What is the issue with field value within a re-indexed parent array item?

A: The issue occurs when an item is removed from the array, and that item is not the last one. In this scenario, fields within the item that shifts to take the removed item's place can have their form.Field instance report state.value as undefined for a transient period during the re-render.

Q: Why does this issue occur?

A: This issue occurs because of the way TanStack Form handles re-renders and array item re-indexing. When an item is removed from the array, the remaining items are re-indexed, which can cause fields within the item that shifts to take the removed item's place to have their form.Field instance report state.value as undefined for a transient period during the re-render.

Q: How can I reproduce this issue?

A: To reproduce this issue, follow these steps:

  1. Setup: The example form contains a sections array. Each item in sections is an object with a unique uuid (used as its React key) and a sectionTitle string field (initialized with a default string). An invariant checks if sectionTitle's value is a string.
  2. Add Items: Click "Add Section" twice to create two distinct section items in the form.
  3. Remove Preceding Item: Click "Remove Section" for the first of the two section items.
  4. Observe Error: The application will throw an error, caught by the Error Boundary, for the remaining section item (which was previously the second). The error message will indicate that the sectionTitle field's value was undefined, failing the invariant check: Invariant failed: Expected sectionTitle field value to be a string! Received: undefined. (See attached screenshot).

Q: What is the expected behavior?

A: The state.value of any field (e.g., sectionTitle) within an array item should consistently maintain its correct type and initialized value throughout all re-render cycles. This includes re-renders that occur after removing a preceding item from the parent array and the subsequent re-indexing of items. A field initialized as a string should not temporarily become undefined.

Q: How often does this bug happen?

A: This bug occurs every time an item is removed from the array and that item is not the last one.

Q: What platforms have you observed this issue on?

A: This issue was observed on the following platform:

  • Macos
  • Chrome browser

Q: What TanStack Form adapter are you using?

A: The TanStack Form adapter used in this example is:

react-form

Q: What version of TanStack Form are you using?

A: The version of TanStack Form used in this example is:

1.11.1

Q: What version of TypeScript are you using?

A: The version of TypeScript used in this example is:

No response

Q: Is there any additional context that might be helpful?

A: No response

Q: How can I prevent this?

A: To prevent this issue, developers can follow these recommendations:

  1. Use a stable React key: Ensure that the React key used for each item in the array is stable and does not change during re-renders.
  2. Use a consistent form management approach: Use a consistent approach to managing forms, including re-renders and array item re-indexing.
  3. Test thoroughly: Thoroughly test your form management code to ensure that it is properly handling re-renders and array item re-indexing.

By following these recommendations, developers can help prevent this issue and ensure that their forms are properly managed in TanStack Form.