eslint-plugin-fb-flow

eslint-plugin-fb-flow

This is a set of ESLint rules created and published by the Flow team. They are in addition to (not a replacement for) the rules of eslint-plugin-flowtype created and published by the open-source community.

Usage

Add fb-flow to the plugins section of your .eslintrc configuration file. You can omit the eslint-plugin- prefix:

{
    "plugins": [
        "fb-flow"
    ]
}

Then configure the rules you want to use under the rules section.

{
    "rules": {
        "fb-flow/use-indexed-access-type": 2
    }
}

Rules

use-indexed-access-type

You should use Flow's Indexed Access types instead of the $PropertyType<...> and $ElementType<...> utility types.

$PropertyType

Instead of

type T = $PropertyType<Foo, 'bar'>;

write

type T = Foo['bar'];

$ElementType

Instead of

type T = $ElementType<Foo, K>;

write

type T = Foo[K];

Autofixer

This rule includes an autofixer that can fix most cases. It does not handle cases with comments inside the type arguments to $PropertyType and $ElementType.

The autofixer does not remove nested $NonMaybeTypes and output Optional Indexed Access Types. In general $ElementType<$NonMaybeType<O>, K> is not equivalent to O?.[K] as Optional Indexed Access Types are modelled after optional chaining, so have a void in their resulting type. An auto-fixer can't just naively wrap the whole thing with $NonMaybeType as the type of the property at K might be nullable, so doing so would remove that nullability.

use-exact-by-default-object-type

For Flow projects which turn on exact objects by default, this ESLint rule enforces that you use the { prop: type } syntax for exact object types instead of the {| prop: type |} syntax.

This rules includes an autofixer that transforms {| prop: type |} into { prop: type }.

Invalid

type Props = {|
  foo: string,
|};

Valid

type Props = {
  foo: string,
};

type InexactProps = {
  foo: string,
  ...
};