fix(compat/findLastIndex): treat NaN fromIndex as 0

src/compat/array/findLastIndex.spec.ts

@@ -40,7 +40,7 @@
   });
 
   it(`findLastIndex should provide correct \`predicate\` arguments for arrays`, () => {
     let args: any;
     const array = ['a'];
 
     // eslint-disable-next-line @typescript-eslint/ban-ts-comment
@@ -107,6 +107,13 @@
     expect(findLastIndex(array, x => x === 2, 4.2)).toBe(4);
   });
 
+  it(`\`findLastIndex\` with \`NaN\` \`fromIndex\` should start from 0`, () => {
+    // Lodash coerces `NaN` via `toInteger`, so `fromIndex` becomes `0` and the
+    // search still covers the first element. The existing falsey test uses a
+    // predicate that never matches index 0, so it does not catch this.
+    expect(findLastIndex(array, x => x === 1, NaN)).toBe(0);
+  });
+
   it('should return `-1` when provided `null` or `undefined`', () => {
     expect(findLastIndex(null, 'a')).toBe(-1);
     expect(findLastIndex(undefined, 'a')).toBe(-1);

src/compat/array/findLastIndex.ts

@@ -55,12 +55,15 @@
  */
 export function findLastIndex<T>(
   arr: ArrayLike<T> | null | undefined,
   doesMatch: ((item: T, index: number, arr: any) => unknown) | Partial<T> | [keyof T, unknown] | PropertyKey = identity,
   fromIndex: number = arr ? arr.length - 1 : 0
 ): number {
   if (!arr) {
     return -1;
   }
+  if (Number.isNaN(fromIndex)) {
+    fromIndex = 0;
+  }
   if (fromIndex < 0) {
     fromIndex = Math.max(arr.length + fromIndex, 0);
   } else {