Parse and check type arguments on JSX opening and self-closing tags

[weswigham]

Allows, eg,

<MyComponent<number> data={12} />

in .tsx files.

Fixes #6395.

Was there a list of things we were concerned wouldn't parse well? AFAIK, this is actually easy for us to unambiguously parse.

[RyanCavanaugh]

I would find min === max ? min : min + "-" + max clearer

[RyanCavanaugh]

Not sure why the original author didn't just use Infinity / -Infinity ?

[RyanCavanaugh]

I have nitty comments in moved code that can be ignored 👍

[aaronjensen]

This is great! Is there a related issue on Babel? What is the process for ensuring they stay up to date?

[weswigham]

I don't think we have a formal process right now, but pinging @Andy-MS seems like a decent way to check. 😝

[mvestergaard]

@weswigham Really great this finally made it into the language!

Question: When tsconfig is set to "tsx": "preserve", is the generic removed?

[weswigham]

It aught to be. If not, it's a bug.

[andrewbranch]

Is there any sense of in what release this will land?

[weswigham]

2.9 which will be released inside two months, given our bimonthly release cadence 😄

[marty-wang]

It seems that one has to explicitly specify the type, as oppose to that the compiler can infer the type from the value.

What I mean is that in JSX, say I have the component below

interface IProps<T> {
    value: T;
    children: (value: T) => React.ReactNode;
}

export class MyComp<T extends {}> extends React.Component<IProps<T>> {
    public render() {
        return this.props.children(this.props.value);
    }
}

when I use MyComp, in the code below, specify the type number, otherwise the val variable will not have type of number.

<MyComp<number> value={4}>
    {val => {
       // the type of val is number
        return "";
    }}
</MyComp>

However, If I omit specifying the type number, the val variable will be type of {}.

<MyComp value={4}>
    {val => {
       // the type of val is {}
        return "";
    }}
</MyComp>

This seems to be a bug, because compare to Generics in regular class, like below., the type of number does not have to be explicitly specified.

class A<T> {
    constructor(public val: T) {
    }
}

const a = new A(5)
a.val // val is type of number

[andrewbranch]

@marty-wang IIUC, I think what you're seeing is #22636

[marty-wang]

@andrewbranch yes, it is the same issue. In #22636 @mhegazy seems saying it would be addressed in 2.9. However, at least it is not yet in 2.9.0-dev.20180503.
Really hope that it will be fixed by the official release of 2.9

[weswigham]

#23492 has yet to be merged

[marty-wang]

@weswigham sweet! thanks!

[jorySsense]

Question,

What would be best way to pass a generic type to a child component?

interface GenericProps<T> {
  isFetching: boolean;
  items: T[];
  removeItem: () => Promise<any>;
  history: History;
}

class ListPage<T> extends React.Component<GenericProps<T>> {
  render() {
    return <LayoutCard {...this.props} />
      <ListWrapper<T> items={this.props.items}>
        {this.props.children}
      </ListWrapper>
    </LayoutCard>
  }
};

[weswigham]

What you have there aught to be correct.

[jorySsense]

@weswigham i was getting a

Type '{}' is not assignable to type 'IntrinsicAttributes & IntrinsicClassAttributes<ListWithModalConsumer<{}>> & Readonly<{ children?:...'.   Type '{}' is not assignable to type 'Readonly<ListPageProps<{}>>'

[jorySsense]

Oh i guess i should of mentioned that List accepts a generic type

export class ListWrapper<T> extends React.Component<ListWrapperProps<T>> {

  render() {
    const { items, children } = this.props;
    return <ModalConsumer>
      {({ showModal, hideModal }) =>
        <List style={{ width: '100%' }} items={items}
              render={(item: T) => children({ item, hideModal, showModal })}>
        </List>
      }
    </ModalConsumer>
  }
}

[weswigham]

You need to constrained your T to at least have the members required by the component you're invoking. In your example, T has no constraint, so it is overriding the type implied by your addition of the items prop and checking against the {} implied by the unconstrained T. (Or intersect the desired props type with T instead of just passing T)

[jameslaneconkling]

is there a non-jsx equivalent for this? e.g.

given this usage w/ jsx

<MyGenericComponent<SomeType> {...props} />

what is the equivalent using createElement?

createElement(StringInput, props)

// [ts] Type '{}' is not assignable to type 'SomeType'.

[nickserv]

I believe the first generic parameter of createElement is equivalent:

createElement<SomeType>(MyGenericComponent, props)

[jameslaneconkling]

The type signature of createElement is

    function createElement<P>(
        type: SFC<P> | ComponentClass<P> | string,
        props?: Attributes & P | null,
        ...children: ReactNode[]): ReactElement<P>;

so looks like the first parameter is Props

[zero-t4]

nice