Should app context types accept additional properties?

[NickColley]

In #782 we updated the schema to prevent people from adding additional data to app context. However when I look at the type
definitions they seem to allow for additional properties still with [key: string]: any.

Should we remove this from the type definitions?

I'm very much new to TypeScript types so I'm not sure.

• https://github.com/Financial-Times/dotcom-page-kit/blob/master/packages/dotcom-server-app-context/src/types.d.ts
• https://github.com/Financial-Times/dotcom-page-kit/blob/master/packages/dotcom-ui-app-context/src/types.d.ts