Summary

Fixes react-final-form#1042

Allows using decimal-separated numbers like "5.1.1", negative numbers like "-1", and padded numbers like "01" as object keys instead of incorrectly treating them as array indexes.

Problem

Reported Issue: Keys like "5.1.1" cannot be used in forms because setIn() incorrectly treats them as array indexes and throws errors.

Root cause: The check isNaN(Number(key)) only verifies if a string can be parsed as a number, but doesn't check if it's a valid array index.

Examples of the bug:

Number("5.1.1")  // 5.1 (not NaN) → treated as array index ❌
Number("-1")     // -1 (not NaN) → treated as array index ❌
Number("01")     // 1 (not NaN) → treated as array index ❌

This caused setIn() to try creating arrays for these keys, which failed because:

• "5.1.1" parses to 5.1 (not a valid array index)
• "-1" is negative (not a valid array index)
• "01" !== "1" (canonical form mismatch)

Solution

Created isValidArrayIndex() helper that checks if a key is a valid array index:

const isValidArrayIndex = (key: string): boolean => {
  const num = Number(key);
  return (
    !isNaN(num) &&           // Can parse as number
    Number.isInteger(num) && // Is a whole number
    num >= 0 &&              // Is non-negative
    String(num) === key      // Canonical form (prevents "01" from being treated as 1)
  );
};

Valid array indexes: "0", "1", "42" ✅
Object keys (not array indexes): "5.1.1", "-1", "01", "3.14" ✅

Code Changes

Before:

if (isNaN(Number(key))) {  // ❌ Only checks if it's a number
  // object set
} else {
  // array set
}

After:

if (!isValidArrayIndex(key)) {  // ✅ Checks if it's a valid array index
  // object set
} else {
  // array set
}

Testing

Added comprehensive tests:

it("should treat decimal numbers like '5.1.1' as object keys", () => {
  const output = setIn({}, "5.1.1", "value");
  expect(output).toEqual({ "5.1.1": "value" });
  expect(Array.isArray(output)).toBe(false);
});

it("should treat negative numbers like '-1' as object keys", () => {
  const output = setIn({}, "-1", "value");
  expect(output).toEqual({ "-1": "value" });
});

it("should treat padded numbers like '01' as object keys", () => {
  const output = setIn({}, "01", "value");
  expect(output).toEqual({ "01": "value" });
});

it("should still treat valid integers as array indexes", () => {
  const output = setIn({}, "items[0]", "first");
  expect(output.items).toEqual(["first"]);
});

Impact

✅ Fixes the reported bug

• Decimal-separated keys like "5.1.1" now work correctly

✅ More correct behavior

• Negative numbers like "-1" treated as object keys (not array indexes)
• Padded numbers like "01" treated as object keys (not array indexes)

✅ No breaking changes

• Valid array indexes ("0", "1", "42") still work identically
• Only fixes incorrect behavior where invalid array indexes were attempted

✅ Minimal code change

• Single helper function added
• Two call sites updated

Use Case

This is useful for forms with version numbers, coordinates, or other structured identifiers:

<Field name="versions.5.1.1" />          // Software version
<Field name="coordinates.-1.5" />        // Negative coordinates
<Field name="build.01" />                // Build numbers with padding

Fixes react-final-form#1042

Summary by CodeRabbit

• Bug Fixes
  • Corrected handling of numeric-like string keys so decimal, negative, and zero-padded keys are treated as object properties rather than array indices; valid integer indices continue to behave as array entries.
• Tests
  • Added tests to verify the distinction between array indices and numeric-like string object keys.