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@facebook-github-bot facebook-github-bot released this 16 Aug 07:08
v0.53.0
29ab0af

This release includes major changes to Flow's model for React. The following
links contain detailed documentation on the new model.

Please use the new flow-upgrade
tool to upgrade your codebase to take advantage of these changes!

Likely to cause new Flow errors:

  • We are modifying how you define React class components. The React.Component
    class will now take two type arguments, Props and State (as opposed to the
    three type arguments including DefaultProps that React.Component took
    before). When your component has no state, you only need to pass in a single
    type argument. If your component has default props then add a static
    defaultProps property.

  • Flow used to not type React function refs at all, but now that we are typing
    refs code that used to just work may now have errors. One such error which can
    often be overlooked is that the instance React gives you in a function ref may
    sometimes be null.

  • Flow used to completely ignore the type of React children in many
    places. Intrinsic elements did not check the type of their children (like
    <div>), the type specified by components for React children would be ignored
    when you created React elements, and the React.Children API was typed as
    any.

  • In the past when typing children many developers would use an array type
    (Array<T>) often with the React element type
    (Array<React.Element<any>>). However, using arrays is problematic because
    React children are not always an array. To fix this, now use the new
    React.ChildrenArray<T> type.

New Features:

  • Modeling advanced React patterns, like higher-order components, is difficult
    today because the types you would need are either not provided or
    undocumented. In this release we added a whole suite of utility types which
    are all documented on our website.

Notable bug fixes:

  • Flow used to have a bug where Flow would consider the following code as valid:
function MyComponent(props: {foo: number}) {
  // ...
}
<MyComponent foo={undefined} />; // this is now a Flow error

  • We now allow JSX class components to use exact object types as their props.

  • We now allow member expressions when creating JSX components,
    e.g. <TabBarIOS.Item>.

  • We now allow multiple spreads in a JSX element.

  • We have added a type argument to SyntheticEvents and correctly typed
    currentTarget using that type argument.

Parser:

  • We fixed miscellaneous character encoding issues.

Misc:

  • This release features a major re-architecture in how Flow typechecks modules
    against their dependencies. Earlier, Flow would do a "local" (per-module)
    typechecking pass followed by a global (cross-module) typechecking pass. Now,
    these passes have been merged. This change vastly improve Flow's memory usage
    on large codebases.

  • We found and fixed a couple of subtle bugs in the typechecking engine that
    caused stack overflows in some pathological cases.

  • We made various improvements to refinements. We now recognize vars that are
    only assigned to once as consts, so that we can preserve refinements on them
    through longer stretches of code. Some typeof cases have also been fixed.

  • We now support focus-checking multiple files. You can use it to debug issues
    easier and faster by telling Flow to focus on files of interest.

  • This release also includes lots of improvements to core type
    definitions. Thanks for your contributions!

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