Eslint-plugin-react: Support for non-required maybe types in require-default-props

[johnryan]

if I have a type like:

type foo = {
   bar?: ?string
}

eslint complains that i have no corresponding defaultProp declaration. However, if i set the default value to null:

static defaultProps = {
   bar: null
}

I get a flow error when passing a string prop: this type is incompatible with null since flow infers the type from the defaultProp first (i believe). It seems like this rule shouldn't apply to maybe types...Thoughts?

[ljharb]

What if you set the default to undefined?

[johnryan]

@ljharb i tried that too i get the error this type is incompatible with undefined

[ljharb]

To me that sounds like a flow bug - maybe types should be compatible with defaultProps. It'd be great if you filed a flow issue and linked it here.

[johnryan]

@ljharb i can file an issue but i'm confused what value this validation provides on a Maybe type...if you initialize it to anything other than null or undefined then it isn't really a "maybe" type anyway since it will always have a value

[ljharb]

Everything in js always has a value - when a maybe type is omitted, it'd be undefined, so at a minimum, that's what flow should allow. Equating undefined and null is also reasonable, if they decide to do it.

[johnryan]

Created an issue in the flow project here facebook/flow#3982

[romulof]

I think it shouldn't require a default prop if the type is optional+maybe.
Think as implicit null/undefined.

[ljharb]

@romulof defaultProps are important runtime information that's needed, and only having a flow type deprives the runtime of that info.

[silvenon]

The official Flow docs never use maybe types for props and recommends using defaultProps if the prop should be optional. If you just set a prop's default value to null/undefined you will get your desired behavior.

I think this issue can be closed.