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@facebook-github-bot facebook-github-bot released this 19 Sep 01:45

Likely to cause new Flow errors:

  • Fixed a bug that caused unsoundness with $ObjMap.

New Features:

  • Flow is now capable of servicing some requests while it is rechecking. This should improve the IDE experience on large codebases.
  • Added $Call utility type.
  • Added $Compose and $ComposeReverse utility types.
  • Added support for spreading mixed into an object type.

Notable bug fixes:

  • Improve results from the find-refs command.
  • Allow null and undefined as React.createElement() config (fixes #4658).


  • Miscellaneous code cleanup.
  • Located error messages related to functions at only the signature, rather than the entire range of the function body.
  • Improved error messages when this types are incompatible.
  • Properly check subtype relationships between callable objects.
  • Fixed a bug that caused mixed not to be properly printed from type-at-pos.
  • Improved error messages regarding incompatible Array type parameters.
  • Preserve some inference information across module boundaries.
  • Support assignments to shorthand method properties in object literals.


  • Added captureStream() to HTMLCanvasElement and HTMLMediaElement.
  • Added HTMLOptGroupElement return type for document.createElement().
  • Added Recoverable and context to the repl module.
  • Fixed return type for AudioContext.createMediaStreamDestination().


  • Various fixes to improve test262 compliance:
    • Correctly disallowed various illegal constructs in destructuring patterns.
    • Allow destructuring in catch.
    • Added \u2028 and \u2029 to the list of line terminators.
    • Allow unicode escape codes in identifiers.
  • Improved parse errors when using private properties outside of classes.
  • Disallowed reserved words as function param names in types (e.g. (switch: number) => void).