diff breaking changes

9.0.0

5 breaking changes, first published April 13, 2026.

• Removed
  The package no longer supports ES5 environments. Users who need ES5 must stay on version 8. github.com
• Behavior changed formatPatch
  If oldFileName or newFileName is undefined, formatPatch now omits the file headers instead of writing the word undefined. github.com
• Behavior changed parsePatch
  A patch with only a --- header or only a +++ header now causes parsePatch to throw. Patches with both headers or no headers are still accepted. github.com
• Signature changed StructuredPatch.newFileName
  The oldFileName and newFileName fields are typed string | undefined instead of string, so code relying on string-only types may need to handle undefined. npmjs.com
• Signature changed StructuredPatch.oldFileName
  The oldFileName and newFileName fields are typed string | undefined instead of string, so code relying on string-only types may need to handle undefined. npmjs.com

Check your code

These are the changes. To see which of them reach your code, scan a public repository, or have your coding agent call check_code with your manifest or lockfile over MCP. The API reads every change in every version, not only the newest. On Team and up, a fix pull request opens for the changes that reach a connected repository. Nothing merges or deploys.