[Shubham-Padkonde]

Summary:

new Blob(['abcdef']).slice(undefined, 3) currently returns all six bytes instead of the first three. End normalization is inside the numeric-start branch, so an omitted start causes the end argument to be ignored. This also affects zero/negative ends and slices of existing views.

Default start to zero so these calls follow the existing range calculation. Numeric-start behavior and the optional public parameter remain unchanged. This matches the File API slicing algorithm.

Changelog:

[GENERAL] [FIXED] - Respect the end offset in Blob.slice when the start argument is undefined.

Test Plan:

• Added seven cases covering positive, zero, negative, out-of-range and omitted ends, plus a nested slice with a nonzero source offset. Five fail on unchanged base 866034f; all pass with the fix.
• yarn test packages/react-native/Libraries/Blob packages/react-native/Libraries/Network packages/react-native/Libraries/WebSocket --runInBand --watchman=false: 55 tests pass across five suites on Windows, Node 24.15.0.
• Prettier and ESLint (--max-warnings 0) pass on both changed files; git diff --check passes.
• yarn build-types translates the package declarations successfully, with byte-identical generated Blob declarations before/after the fix. Its later API snapshot step fails on a Windows path embedded in an ActionSheetIOS identifier; the same failure reproduces with unchanged Blob source. No generated files changed.
• Full Flow checks, native builds and the full JavaScript suite were not run.

[github-actions]

Visual regression

2 visual difference(s) at a318911b3098. Visual differences need maintainer approval. Review the report, then apply visual-change-approved.

ios button/disabled/default: Changed, 280048 px (11.234%)

Expected and Captured

Expected	Captured

ios scroll-view/horizontal-scroll-to/end: Changed, 69504 px (18.123%)

Expected and Captured

Expected	Captured