Fix: Support type='select' for multiple select defaulting to []

[erikras-richard-agent]

Problem

React-final-form-arrays issue #185 reported this error after upgrading to v4.0.0:

The \`value\` prop supplied to <select> must be an array if \`multiple\` is true.

When users pass type="select" multiple={true} to a Field, the value wasn't being defaulted to [], causing React to complain.

Root Cause

The code in useField.ts only checked component === "select" to determine when to default undefined values to []. However, many users pass type="select" instead of component="select".

Solution

Update both locations where select multiple is handled:

1. Line 125: Initialize undefined values to [] when (component === "select" || type === "select") && multiple
2. Line 197: Ensure value is always an array in getInputValue() when (component === "select" || type === "select") && multiple

Testing

• ✅ Added 3 new regression tests
• ✅ All tests pass (139/139)
• ✅ 99.32% code coverage

Part of

🟠 HIGH: Fix v4.0.0 regressions (select multiple, TypeScript)

This fixes the react-final-form side of react-final-form-arrays#185

Summary by CodeRabbit

Release Notes

• Bug Fixes

  • Enhanced multi-select form field handling to properly initialize with empty arrays instead of undefined values, ensuring consistent behavior across different select element configurations.

• Tests

  • Added comprehensive test coverage for multi-select form field scenarios, validating proper initialization and input value handling for select elements with multiple selection states.

[coderabbitai]

📝 Walkthrough

Walkthrough

This pull request extends the multi-select handling logic in useField to recognize both type="select" and component="select" attributes, ensuring that undefined initial values consistently default to empty arrays for multi-select scenarios. The changes include corresponding test cases validating both patterns.

Changes

Cohort / File(s)	Summary
Test Suite src/useField.test.js	Adds three new test cases for multi-select scenarios: validates that undefined values default to [] when using type="select" with multiple, validates the same for component="select" with multiple, and ensures input.value is always an array for select multiple cases.
Core Logic src/useField.ts	Extends multi-select initialization and value resolution to check for both type="select" and component="select", ensuring initialStateValue and rendered values use empty arrays when multiple is true and initial value is undefined.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~12 minutes

Possibly related PRs

• Fix #1050: useField returns Form initialValues on first render #1060 — Both PRs modify useField's initial-state/value resolution logic for multi-selects, with this PR extending the checks to include type="select" alongside component="select".

Suggested reviewers

• erikras

Poem

🐰 A select with multiple? Oh what a sight!
Type or component, we handle both right,
When undefined knocks, we say "No way, friend!"
An empty array's the proper amend! 🎉

🚥 Pre-merge checks | ✅ 2 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (2 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly describes the main change: adding support for type='select' in multiple select scenarios with proper [] defaulting.

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✨ Finishing touches

• 📝 Generate docstrings

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch fix/select-multiple-type-prop

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