fix: infinite loop when object has an empty string key (#58)

The key-advance logic in `diffObject` used falsy checks (!keyLeft,
!keyRight) to detect when one side had run out of keys. Because an
empty string is falsy in JavaScript, any JSON object containing "" as
a key would cause the left pointer to never advance, producing an
infinite while-loop that exhausted all available memory.

Fix: replace the falsy checks with strict undefined comparisons
(=== undefined) so that empty-string keys are handled correctly.

A regression test is added to differ.spec.ts covering objects with
an empty-string key on both sides.

Co-authored-by: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: trotzig

src/differ.spec.ts

@@ -1,6 +1,22 @@
 import Differ from './differ';
 
 describe('object diff', () => {
+  it('should not infinite loop when an object has an empty string key', () => {
+    const l = { '': 'before', a: 1 };
+    const r = { '': 'after', b: 2 };
+    const d = new Differ();
+    // Should complete without hanging or throwing
+    expect(() => d.diff(l, r)).not.toThrow();
+    const result = d.diff(l, r);
+    // Both sides must have the same number of lines
+    expect(result[0].length).toBe(result[1].length);
+    // The empty-string key's value change should appear as a modification
+    const leftTypes = result[0].map(line => line.type);
+    const rightTypes = result[1].map(line => line.type);
+    expect(leftTypes).toContain('modify');
+    expect(rightTypes).toContain('modify');
+  });
+
   it('preserve key order', () => {
     const l = { a: 1, b: 2, c: 3 };
     const r = { c: 3, b: 2, d: 4, a: 1 };

src/utils/diff-object.ts

@@ -181,9 +181,9 @@ const diffObject = (
       }
     }
 
-    if (!keyLeft) {
+    if (keyLeft === undefined) {
       keysRight.shift();
-    } else if (!keyRight) {
+    } else if (keyRight === undefined) {
       keysLeft.shift();
     } else if (keyCmpResult === 0) {
       keysLeft.shift();