Throw an error when the dataLength prop is missing

[karljohansson85]

Previously, missing the mandatory dataLength prop resulted in the component
silently failing to add more content. This commit makes the issue more visible,
as the component will now throw an error after mounting if the dataLength prop
isn't provided.

[rajatsan]

@fooki, it would be great if you could also add a unit test for this change!

[karljohansson85]

@rajatsan Sure, I'll look into it today!

[karljohansson85]

hmm I'm not sure how to test this. the typescript interface Props ensures that dataLength must be specified. If I write a test with dataLength removed, then jest will complain:

error TS2741: Property 'dataLength' is missing in type '{ children: Element; loader: string; hasMore: false; next: () => void; }' but required in type 'Readonly'.

This is only visible in tests though, not when using the library in other projects.

[rajatsan]

You can use // @ts-ignore so that Typescript won't complain when you write that test.

[karljohansson85]

Thanks for your help. I've added a test case now. Let me know if anything else is needed.