[sen2y]

Fix: Correct isMatch primitive target vs object pattern matching

Fixes #1399

---

🐛 Problem

Two incorrect matches versus Lodash were identified.

Bug 1: Top-level primitive target vs object source (with undefined)

Case:

isMatch('bar', { anyKey: undefined });

Observed behavior before fix:

• Lodash: false ✅
• ES-Toolkit (before): true ❌

Root cause:

• Accessing a non-existent key on a primitive target yields undefined:

'bar'['anyKey']; // -> undefined
source['anyKey']; // -> undefined

These two undefined values accidentally compare as equal, yielding a false positive.

Bug 2: Nested level primitive vs empty object

Case:

isMatch({ value: 'bar' }, { value: {} });

Observed behavior before fix:

• Lodash: false ✅
• ES-Toolkit (before): true ❌

Why it is confusing:

• Top-level: isMatch('bar', {}) should be true (empty object = no constraints)
• Nested: isMatch('bar', {}) must be false (type mismatch vs object pattern)

Lodash intent:

• Top-level empty object is permissive (matches anything)
• Nested empty object still enforces object type (primitive must not match object)

ES-Toolkit previously treated empty objects as truthy matches at all levels.

---

🔧 Solution

1. Reject object-pattern matching against primitive targets

// If source expects object keys, target must be an object
if (!isObject(target)) {
  return false;
}

Fixes Bug 1:

isMatch('bar', { anyKey: undefined }); // false ✅
isMatch(123, { x: 1 }); // false ✅

2. Distinguish top-level vs nested level for empty-object source using stack size

if (keys.length === 0) {
  // Only at nested levels, primitives must not match {}
  if (stack && stack.size > 0 && !isObject(target)) {
    return false;
  }
  return true;
}

Rationale:

• At the top level (stack.size === 0), {} means "no constraints" -> allow any target
• At nested levels (stack.size > 0), {} still implies object type -> primitives must fail

Implementation detail:

• isMatchWith creates a new stack (new Map()) at the entry point.
• On recursion, objects are pushed to the stack (stack.set(...)), so stack.size naturally reflects depth.

---

🧪 Tests

Added to src/compat/predicate/isMatch.spec.ts.

1. Primitive target with object source (Bug 1)

it('returns false when target is primitive and source has object keys (Issue #1399)', () => {
  expect(isMatch('bar', { anyKey: undefined })).toBe(false);
  expect(isMatch(123, { anyKey: undefined })).toBe(false);
  expect(isMatch(true, { anyKey: undefined })).toBe(false);
});

2. Nested empty object matching (Bug 2)

it('returns false when nested target is primitive and source is empty object (Issue #1399)', () => {
  expect(isMatch({ value: 'bar' }, { value: {} })).toBe(false);
  expect(isMatch({ value: 123 }, { value: {} })).toBe(false);
  expect(isMatch({ value: true }, { value: {} })).toBe(false);
});

Result:

• ✅ 25/25 tests passed
• ✅ No type errors

---

📊 Summary of Changes

Modified files:

• src/compat/predicate/isMatchWith.ts (logic updates)
• src/compat/predicate/isMatch.spec.ts (new test cases)

Code added (conceptually):

// 1) Empty-object handling by level (top-level vs nested)
if (keys.length === 0) {
  if (stack && stack.size > 0 && !isObject(target)) {
    return false;
  }
  return true;
}

// 2) Primitive target cannot match object pattern
if (!isObject(target)) {
  return false;
}

Only a few lines, but fully aligns with Lodash behavior.

---

🧭 Behavior vs Lodash

Before (incorrect):

isMatch('bar', { anyKey: undefined }); // ES-Toolkit: true ❌, Lodash: false
isMatch({ value: 'bar' }, { value: {} }); // ES-Toolkit: true ❌, Lodash: false

After (fixed):

isMatch('bar', { anyKey: undefined }); // ES-Toolkit: false ✅, Lodash: false
isMatch({ value: 'bar' }, { value: {} }); // ES-Toolkit: false ✅, Lodash: false
isMatch('bar', {}); // ES-Toolkit: true  ✅, Lodash: true

---

✅ Checklist

• Fix both issues in isMatch from es-toolkit/compat does not behave like lodash-es/isMatch #1399
• Match Lodash behavior at both top-level and nested levels
• Add unit tests covering the two bug scenarios
• Keep changes minimal (constant-time checks only)
• Preserve performance; no extra traversal or allocations

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Project	Deployment	Actions	Updated (UTC)
es-toolkit	Ready	Preview	Aug 17, 2026 12:49am

[raon0211]

Thanks for your work! Let me refactor this a little bit after merging.

[sen2y]

Thanks for your work! Let me refactor this a little bit after merging.

Thanks for reviewing and refining the code! 🙌
It’s much clearer thanks to your help.

While working on this fix, I also discovered some behavior differences between Lodash and compat.
I opened an issue to discuss potential improvements and would love your thoughts when you have a moment: #1504

Thanks again — please let me know if there's anything else I can help with before merging! 😊

[raon0211]

Thanks!

[raon0211]

Thanks!